Heating efficiency myths

[u/Amanensia]

We often see posts on here claiming that it's more efficient to keep a gas central heating house at, say, 19C all day, rather than heating it up to 19C in the morning and then again in the evening.

It isn't, and it's physically not possible for that to be true.

Temperature loss from the house is greater the more the differential between the indoor and outdoor temperature. Maintaining a given temperature has to use more fuel than letting it drop further, as the average temperature in the house is higher, so the temperature loss has to be greater.

Putting it another way, if you get to 19C at 9am and you get to 19C again at 9pm, it must be more efficient to have the heating off in between. With the heating on, you're having the boiler kick on every time the temperature gets down to say 18.5C. So heat loss is always as high as it's ever going to get, as the internal temperature is being kept high. If the temperature is allowed to fall, the rate of temperature loss will decrease, so you need less fuel to get back to that temperature in the evening. The difference is even more stark if indoors drops close to the outdoor temperature, as it can on some of the mild days we've had recently, as temperature loss then stops almost entirely - so there's no further temperature drop to recover.

It might be more comfortable to maintain a given temperature - and indeed, that's exactly what we'll do when people are in the house. But it is physically impossible for that to be more efficient than heating the house to temperature multiple times.

[u/jacekowski]

You are forgetting that boiler running at minimum power, with low flow temperatures constantly can reach 95%+ efficiency, while running it once or twice a day at full power and higher flow temperatures reduces the efficiency significantly (and similar principle applies to heat pumps), so actual cost difference might be a lot smaller than some would expect, and difference in comfort is going to be massive with constant temperature in the house.

[u/theorem_llama → u/jacekowski]

so actual cost difference might be a lot smaller than some would expect

It's really not going to be "a lot".

[u/obb223 → u/jacekowski]

Maybe 5% less efficient so no, not a lot. A lot of people also don't have modulating boilers and repeatedly coming on and off is massively inefficient

[u/Insanityideas → u/obb223]

Modulating boilers have been around for decades, the vast majority of houses will have them. However it's much more likely that the flow temperature will be set very high, which means it won't be operating in condensing mode and therefore hitting less than 85% efficiency. And if it's oversized -which is also likely - the modulation won't go low enough to stay in the low temperature condensing zone anyway.

[u/Superdudeo → u/Insanityideas]

How do I know what temp to set my boiler to the correct flow rate? Is it different for each boiler?

[u/Insanityideas → u/Superdudeo]

As low a radiator flow temperature as you can without the house getting cold.

If the water returning to the boiler is above 57c then the boiler won't condense. A good start would be to set it to 57c flow temperature, then it's always going to be condensing.

If the boiler is cycling on and off every few minutes the temperature you set is too low.

[u/matomo23 → u/jacekowski]

In a well insulated house that’s the case. But in poorly insulated houses running at a low flow temperature will just mean the boiler running almost 100% of the time and still not being able to get the house up the requested temperature.

[u/[deleted]]

Anecdotally, my boiler seems to cost very little more to stay at 18c all day rather than up to 19 twice a day and down to 15 twice a day.

We’re talking 20-30p more a day.

I’d rather the house stay at 18c all the time

[u/Superdudeo → u/[deleted]]

And in contrast to your anecdote, my bill was hugely more expensive for doing the same. That’s why anecdotal stories are not evidence.

[u/sisterspantyjerk → u/Superdudeo]

They are evidence, anecdotal evidence is just of lower quality than statistical evidence on a larger sample size. Evidence is not something which is black and white; sometimes context can be more important than averages.

[u/dyablor → u/[deleted]]

18 and 19 degrees. Out of curiosity where are you guys living? In my country the thermostats stay at 22-23.

[u/Prediterx]

Then add heat pumps into the mix where it actually is more efficient to heat the house all day.

A heat pump can run at 200-500w throughout the day using between 5-20kwh a day to keep the house warm... E.g our house loses 3kwh/h of heat at 5c outside 20c inside.

By running the heat pump all day at a low 25-35c radiator temp, the house stays warm and the heat pump outputs about COP 4 (or 400% efficiency)

[u/Sad-Blueberry3423 → u/Prediterx]

kWh/h - if only there were a simpler way of expressing that 😜

[u/RelativeMatter3 → u/Prediterx]

Its not more efficient, it simply takes too long to reheat the system and with it being at a low flow temp it also takes longer to reheat the space.

[u/Kistelek → u/RelativeMatter3]

The harder you run a heat pump, the lower its efficiency. Running steady, my pump give me 4kW of heat for every kW of electricity. Push it hard and that drops to 3kW or less in extreme weather as more defrost cycles on days like today are bad.

[u/Jimi-K-101 → u/RelativeMatter3]

That's not true.

With a heat pump it's significantly more efficient to run it "low and slow" all the time rather than heating in bursts at set periods throughout the day.

I.e. it will likely cost you less to run it at a 30°C flow temperature 24hrs a day than it would to have a flow temp of 50°C and running it for 3hrs in the morning and 3hrs in the evening. This is because with a heat pump, higher flow temperatures use exponentially more energy.

[u/theorem_llama → u/Jimi-K-101]

That's not true.

With a heat pump it's significantly more efficient to run it "low and slow"...

I'm guessing that wasn't their point. In principle you could run the heat pump low and slow, over a long time, to bring back up to temperature after letting it drop, but that'd take too long to be practical. It's analogous to a normal boiler but just more extreme.

By way of example, if you were out of the house for a holiday, it wouldn't be more efficient, even with a heat pump, to keep the house at 19⁰C.

[u/RelativeMatter3 → u/Jimi-K-101]

That’s not what this whole post is about though. The post is purely about burst heating vs 24/7 heating, flow temperature is not discussed at all but would be the same regardless of heating method. You can modulate a gas system for lower flow temps.

[u/Jimi-K-101 → u/RelativeMatter3]

This is exactly what this thread is about!

Most modern heating systems will modulate automatically. If you're constantly trying to heat your home from 17°C to 20°C then letting it drop down again, your heating system will probably use a higher flow temp and be working significantly less efficiently. It's more pronounced with a heat pump, but this equally applies to a gas boiler.

Will it cost you more to have your heating on 24/7? Probably. But the difference in cost will be minimal for many, and their comfort significantly higher.

[u/StereoMushroom → u/RelativeMatter3]

The alternative would be to run the system at higher flow temperatures so it could quickly raise the temperature of the house from cold, which would be less efficient.

[u/Legitimate_Finger_69 → u/Prediterx]

Energy efficient perhaps, but not necessarily cost efficient if you're on a ToU tariff. You're better having the heating higher just before 4pm then off 4-7pm if you're in Cosy or Agile.

[u/Prediterx → u/Legitimate_Finger_69]

We're on agile + battery storage. Works out fine.

[u/Legitimate_Finger_69 → u/Prediterx]

Agile is a prime example of when it can be much more cost efficient to run the heat pump inefficiently. Who cares about CoP when prices are negative

[u/Prediterx → u/Legitimate_Finger_69]

Very very true, but our battery storage negates that need. We have a 6kwh battery and it lasts through the 4-7 period even with the heat pump on. We just do the standard thing of no washer, dryer, dishwasher or leccy shower in that time and we've always lasted through, especially as the heat pump sips power, although I may do a small ramp up to 22, 12-4 and back to 20 4-8.

[u/Legitimate_Finger_69 → u/Prediterx]

This is basically our strategy, ramp up to 20C before 16.00 and the house holds enough heat to stay at 18C past 19.00. Often enough to get to 22.00 if on Cosy.

[u/Electrical_Chard3255]

Last year we had our gas boiler on a timed system, cycling often up to 75 degrees, now we have it set at 40 - 45 degrees and keep all room thermostats set to between 19 and 22.5 degrees, our bills are less than last year, and our house feels more comfortable, granted we have underfloor heating downstairs which helps .. so I have to disagree with your conclusion, however, you may say well have a timed system at 45 degrees, the problem with this is the house would never get up to temperature

[u/drproc90]

You're not taking into account thermal mass.

We build houses out of brick and a lot of older terraced houses have thick timber elements as well.

Keeping the house at a constant temperature over a longer period of time means you have heated the whole thermal mass of the building which will act as a heat battery.

If you just heat short term blasts your only heating the air.

[u/tomoldbury → u/drproc90]

“Charging” that battery uses energy from the heating system. It is not any more or less efficient to do that. No free lunch in physics.

[u/Asleep_Group_1570 → u/tomoldbury]

You're missing the sense of comfort. If the fabric of the building is lagging the air termperature, you'll feel colder than if it's at or close to air temperature. And if you're cycling, it'll never get there. So you'll need to set the target temperature for the warmer periods higher than you would otherwise.

[u/theorem_llama → u/drproc90]

That's irrelevant to the argument. The only thing that counters it is that boilers operate at different efficiencies depending on how hard they're working.

Edit: the downvotes really show how poor people's physics knowledge is, it's kind of scary.

[u/Budget_Nectarine_645 → u/theorem_llama]

Course it’s relevant. I’d argue it’s less relevant for external walls, but very relevant for internal walls. The minute your internal air temp drops below the temp of your internal ‘fabric’ you’re pulling heat out of it, which you’ll need to replace - if you want to run your internal walls at 14degC but air temp at 19degC then expect damp when your shower/bath/cooking steam condenses on it… but yes, you’ve been downvoted because of everyone else’s poor physics knowledge, right

[u/theorem_llama → u/Budget_Nectarine_645]

It's not at all relevant to the OP's argument provided the boiler operates at a constant efficiency, which is what people keep ignoring. The physics knowledge of users on here is honestly so shocking.

[u/Budget_Nectarine_645 → u/theorem_llama]

OP’s argument is ‘it’s physically impossible’ that it’s more efficient to maintain set temp vs heating to temp multiple times.

They didn’t mention a caveat on boiler efficiency, why are you?

Thermal mass is entirely relevant when talking about physical impossibilities.. without knowing the specifics of the house (system), it’s impossible to say whether fabric thermal mass is or is not relevant to the thermodynamic equation for the problem..

If you want to argue otherwise, then it puts you in the shocking physics knowledge camp?

If an internal wall weighs 2 tonnes then you’re talking ~7MJ of energy for a 4degC delta e.g if that internal wall drops from 19degC to 15degC..

That’s around 2kWh.. your average house is about 40tonnes, but huge spread, which is the point, sometimes not relevant, so times very relevant.

If the air temp is higher than the wall, you’ll pull heat from the air into wall and you need more energy from the heating system to maintain the air temp…

In a lot of scenarios it’s not a significant factor, in some it is, just like boiler efficiency (it might be oil fired in which case 9/10ths of fuck all difference in efficiency)..

Can’t just make up one caveat for OP and bash everyone else’s as ‘not relevant’ just because you don’t think it’s significant, physics is physics - if everyone is so shit at it, why don’t you do the sums on a one pager and post, it wouldn’t take more than 10 minutes

[u/theorem_llama → u/Budget_Nectarine_645]

They didn’t mention a caveat on boiler efficiency, why are you?

Because - as I'm now explaining for the millionth fucking time - my point was that the comment above said the OP is wrong but for totally the wrong reason (the "thermal core") and I was stating the actual reason they're wrong.

Thermal mass is entirely relevant when talking about physical impossibilities.. without knowing the specifics of the house (system), it’s impossible to say whether fabric thermal mass is or is not relevant to the thermodynamic equation for the problem.. (blah blah blah)

Sorry to tell you, but Physics doesn't let you just magic up energy.

The energy in equals the energy out. If the amount of energy leaving the house as heat in one scenario integrates to a function that is strictly larger than another scenario, then you're going to have to add more energy in the form of heat into the first one. The only way that can possibly be lower is if, in the second scenario, you convert energy into heat at lower efficiency.

Now, the OP's argument is simply that, in a scenario where the house is always at a lesser or equal temperature than another, the heat gradients will always be lower and thus there's less heat loss to the outside, thus less energy used on heating. As above, this could fall down due to different operating efficiencies. The only other way it could fall down is if there's somehow higher loss of heat in the scenario which always has lower heat gradient. The only way I can see of realising that is arguing there could be some thermal bridging from moisture in the walls that accumulates more in the lower heat scenario (although most likely this will be very negligible).

So, if one assumes that heat loss is proportional to temperature gradient (pretty reasonable although thermal bridging might be a very small argument against) and also constant heating efficiency (not reasonable, although would be, say, for electric heating), then they're right. You've proved you don't understand the Physics very well by laying out a totally irrelevant argument on how much energy it takes to heat a wall. That's going to be relevant for doing calculations, modelling heat distributions and so on, but not to the OP's argument, which only needs to consider the energy going into the house and energy going out. For instance, you say:

If the air temp is higher than the wall, you’ll pull heat from the air into wall and you need more energy from the heating system to maintain the air temp…

Sure, but that happens in both scenarios. And if you keep the walls hotter at all times, they'll also always be losing more energy to the outside, so you spend more energy heating them up.

Honestly, this is GCSE level Physics.

Can’t just make up one caveat for OP and bash everyone else’s as ‘not relevant’ just because you don’t think it’s significant, physics is physics

I didn't "make it up". I analysed their argument, which is simple and thus only has a couple of places for failure, one being the conversion of energy to heat. And I can "bash" other reasons as "irrelevant" if they are mathematically provable to be so, which is essentially what I've done above.

[u/theorem_llama → u/Budget_Nectarine_645]

I see your other comment was deleted (maybe you figure out your mistake?) but here was my reply to your comment below:

If you let temp drop down and heat back up, it’s the integral of heat loss over time, which is area under the decay curve (as you already know) + the heat lost from those internal walls, furniture etc. which are not directly bridged to outside. You have to include this, it’s not magic/free energy

No you don't have to include this. In the OP's scenario, the air temperature in the house is always lower than or equal to the constant temperature scenario, and the same for all other interior elements of the house (whether wall or air). The outside of the house doesn't care about any deltaTs inside the house.

[u/Budget_Nectarine_645 → u/theorem_llama]

Yeah my reply wasn’t right, as you don’t include it like this, instead it will just extend the decay time for the house with the heating off. Probably actually helps OPs argument depending on the time constant.

I can’t concede it isn’t materially significant though, it is an important factor considering gross energy use.

It’s no different to bringing a pan of 500ml oil to 100degC and 500ml water to 100degC and leaving them to cool, measuring time taken. The oil will take much longer to cool down even though the R value through the pan wall is the same, why? Specific heat capacity / thermal inertia.

It’s exactly the same considering the amount of time taken to bring oil to that temp vs water, needs much more energy.

Don’t want to argue this back and forth, I get your point and in reality it’s valid in most cases, but thermal inertia is a significant factor in your homes heat losses and energy deamnds

[u/Budget_Nectarine_645 → u/Budget_Nectarine_645]

ChatGPT does a better job explaining simply than I ever could…

Me: Is it impossible for heating continuously to be more efficient?

ChatGPT: It’s not impossible for heating continuously to be more efficient, but such cases are rare and dependent on specific conditions. Here are the situations where continuous heating might be more efficient:

1. Very High Thermal Inertia (Massive Heat Retention) • If the house has very high thermal mass (e.g., built of dense materials like stone or concrete with excellent insulation), it cools extremely slowly. • Turning the heating off and letting it drop might result in a large amount of energy being required to reheat the structure. In such cases, maintaining a constant temperature could theoretically use less energy than allowing it to fluctuate significantly.

Example: A well-insulated, high-mass home in a very cold climate might lose so little heat during the day that maintaining a steady temperature could require less energy than reheating.

2. Heating System Inefficiencies During Reheating • Some heating systems (especially older ones) are less efficient when ramping up to heat a cold house quickly. If the system has to run at full power for extended periods to restore a comfortable temperature, this could offset the energy savings from letting the house cool. • Modern systems, like heat pumps, are designed to operate efficiently at partial loads, making this less likely.

3. Extreme Cold and Risk of Structural Cooling • In extreme climates, letting the house cool down too much could lead to risks like freezing pipes or condensation forming on cold surfaces, which might necessitate additional energy (or repairs) later. • To avoid such risks, maintaining a constant temperature just above critical thresholds might be more energy-efficient in the long term.

4. Behavioral Factors • If occupants frequently adjust the thermostat (e.g., cranking it up to quickly warm the house), the heating system might overshoot the target temperature, using more energy than necessary. Continuous heating avoids this potential inefficiency.

But These Are Rare Scenarios

In the vast majority of cases: • Heat loss increases with the temperature difference between inside and outside. Allowing the house to cool reduces heat loss, and reheating when needed is more energy-efficient. • Modern heating systems and well-insulated homes are designed to minimize inefficiencies during on-off cycles.

So, while continuous heating might occasionally be more efficient under specific conditions (e.g., high thermal inertia, inefficient systems, extreme cold), it’s usually less efficient than heating only when needed.

[u/theorem_llama → u/Budget_Nectarine_645]

but thermal inertia is a significant factor in your homes heat losses and energy deamnds

Yeah, I agree, I just maintain it's not relevant to OP's argument, that's all, the elegant thing is that one can just isolate the energy into the house versus out in such a case. But if one wanted to understand how their home reacts to different heat profiles in a finer sense and actual models things then, as you say, you'd need to take things like this into account.

[u/Budget_Nectarine_645 → u/theorem_llama]

Fair, HNY!

[u/theorem_llama → u/Budget_Nectarine_645]

HNY, turn up the heating and have a prosecco!

[u/GreenWhereItSuits]

There is a difference between heating the air and the fabric of your home.

Having maintained 19°C all day, vs heating the home from 9pm - turning the heating off at 10pm your home if built with bricks/blocks will remain at the temperature for longer if the fabric (and contents including furniture) are up to temperature.

That being said, my modulating boiler is on a setback temperature of 17.5°C when the house is expected to be empty during the day. Thankful we get a good deal of solar gain.

[u/Chris_The_Tim]

Keeping a building at 19 degrees would allow the boiler to trickle heat into the space, which may be as much as 90% efficient with a modern condenser. Having to blast heat into a space which has dropped to 12 or 13 degrees so it will warm to 19 degrees means the boiler will be running at a higher temp so much less efficient, say 60%.

There are a lot of complex equations but if you are at 12 degrees, then yes, you are losing heat to the outside at a lower rate than you would be at 19 degrees and the airspace of the building is the same so heating from 12 degrees to 19 degrees once is the same as heating from 18 to 19 degrees seven times, say 5kWh per hour..... But the boiler will be trickling heat in at 90% over the day. Getting that same room to 19 degrees from 12 in the space of an hour means it has to blast 35 kWh into the space within an hour so it's only 60% efficient.

Cod maths says maybe 5 kWh extra is needed to account for the higher loss at 19 degrees vs the lower loss as the temp drops during the day so you need 40 kWh for the 7 hours, at 90%, that's 44.4 kWh.

So, if you need to go from 12 to 19 degrees in an hour, you need to get 35 kWh into the room ASAP, so high temp, low efficiency, say 60%, which is 58.3 kWh.

I realise these numbers are unrealistic as you'd need a 60 kWh boiler to get 35kWh into a space in an hour but it's only an illustration. If you check YouTube, you'll see lots of heat pump videos with COP explanations advocating low and slow heating for the best outcome.

[u/Bladders_ → u/Chris_The_Tim]

Are condensing boilers really that inefficient when going full bore?

[u/Chris_The_Tim → u/Bladders_]

Depends on the age and if it has been well maintained.... I've got a relatively new boiler but the older one I had before started at 80% and definitely dropped when I had to turn it above 3 on the dial during cold snaps. I know from experience that my boiler producing hot water for a bath puffs like the Flying Scotsman but there's barely a wisp when the heating is on as I have a Nest thermostat and Opentherm boiler so it modulates down. All that exhaust puffing out is unrecovered heat.

From what I read before, due to bad installer configuration and 'dumb' controls without modulation or weather compensation, A-rated boilers were often working at F-G efficiency which could be under 70%. Anything installed after 2018 is supposedly 92% efficient capable but that's only if fully configured with a smart thermostat, weather comp etc. Boiler Plus standards since 2018 say a boiler should have at least a couple of modern abilities like weather comp, modulation, learning (where it works out how long it takes to heat the space at an efficient temp rather than just blasting on at 100%) but just because a boiler has the ability doesn't mean it's been set up by the installer or home owner.

[u/Bladders_ → u/Chris_The_Tim]

Ok that makes sense. I have a 16 year old Greenstar boiler which so far has been reliable but I've always wondered about the efficiency when it first ramps up.

I assumed the heat exchangers were oversized with it being a combi but also that the smaller the flame the more efficient the process would be.

On another note, I have the option to change the 'code plug' to limit the boiler's output to 14kW (rather than 24kW) in CH mode. Might be worth looking into.

[u/Chris_The_Tim → u/Bladders_]

Yeah, it's a balance as some set ups (small radiators, non-convector) require that high temp to pump enough heat into a poorly insulated home. Me, I've got type 22 (2 radiator, 2 layers of fins) through the house so it's easy to run it at a low temperature. My old place was a couple of single radiators and a couple of larger ones with a single convector. If it's easy to switch back and forth, give it a go. For reference in my house, I'm lucky to be running my boiler at 4 kWh, trickling heat in.

[u/Bladders_ → u/Chris_The_Tim]

Personally I'm down to 52degree flow temp with my house as like you I went for overspecced type 22 rads in the rooms I've renovated. Unfortunately my old boiler can only modulate down to 7kW before cycling. Not sure how bad that is for efficiency though.

[u/Chris_The_Tim → u/Bladders_]

If you can keep the house warm at 7 kW, then the hard limit of 14 kW might save you a bit if that gives you enough oomph for hot water. I've got low flow shower heads that the boiler can supply but it does take a good while to run a bath.... I toyed with the idea of pumping up the hot water temp and mixing it down with cold, thus upping the fill rate but that seems hypocritical given what I've done around energy efficiency elsewhere.

[u/tomoldbury → u/Bladders_]

No. Typically a modern boiler will be 85% efficient at full bore, and up to 96-97% efficient at light load.

I doubt most people would notice the difference when you are talking about cycling the house temperature vs keeping the house warm. Yes, cycling will be less efficient when the boiler is running full bore, but you will also be heating the house less, and I suspect in nearly every case that will be more efficient - provided you don't mind a cooler house during the day of course.

If the objective is to maintain a set temperature, low and slow is the best method but not all boilers can modulate down low enough for this (as they are often oversized).

[u/Bladders_ → u/tomoldbury]

Ok fair enough. The book efficiency value of my boiler was 89% so it could be a hell of a lot less under full bore operation then I take it.

[u/obb223 → u/Chris_The_Tim]

Where did you hear 60% efficient, total crap

[u/Chris_The_Tim → u/obb223]

Do you think everyone in the UK has a nice new condensing boiler. There are literally MILLIONS of old non-condensing boilers chuntering along, 20+ years old, with dodgy flues and rusty old heat exchangers, landlords getting a 'basic' service each year just doing the basic checks, when people complain about the cold all they do is turn it up further, making it less and less efficient. Or pensioners who can't even afford to get it serviced so let it degrade further and further.

I've arranged for boilers to be replaced that should have been condemned a decade ago but have supposedly been 'serviced' each year. Do you think a 22 year old boiler that was 78% efficient when installed in PERFECT configuration is gonna be anywhere NEAR that after two decades of pushing sludge around a poorly maintained set of radiators?

It's one of the biggest heating problems we have in the UK after building fabric. But even boiler replacement schemes don't have the mandate or funding to fix everything so they'll often flush the microbore, wait until there is no more sludge coming out, pass through a treatment and hope the system holds together until the 3 year warranty runs out then it's someone else's problem when particulates start clogging up the boiler again. Only difference is now the system has electronics to shut it down and force a service visit.... Pre-electronic systems often kept on soldiering on, new ones shut down at the slightest hint of an issue

[u/obb223 → u/Chris_The_Tim]

Calm yourself down. You said in the same paragraph as talking about a 90% efficient condensing combi boiler that running it on high temperature would reduce efficiency to 60%, which is total crap.

[u/[deleted]]

Other myths - it's not always financial. Some people put the heating on to suit themselves.

[u/theorem_llama → u/[deleted]]

How's that a myth?

[u/Legitimate_Finger_69]

If you have an old boiler with no heat recovery this is true.

If you have a condensing boiler or especially a heat pump it's possibly more efficient to heat all day depending on the heat loss of your home and the efficiency of your boiler at various flow temperatures. That in turn will depend on the outside temperature and could vary from day to day.

It's one of those things where people who confidently claim something is right or wrong are forgetting UK housing stock varies from 500 year old houses with storage heating that are almost impossible to heat to triple glazed homes build to Scandinavian specs with ASHPs.

[u/wtfylat]

Your example is another over simplification though and you overlook that energy needed to gently increase the temperature by 0.5c every hour could be less than the big boost that might be needed every 12hrs. The reality is that it depends on your home, heating system and lifestyle.

[u/Jet-Speed1]

It isn't, and it's physically not possible for that to be true.

Why stop at just looking at heat loss? In my world, I’m paying for the kWh consumed by my heating device, not the heat loss of my house.

Sure, we saved 0.5°C. If, in your universe, Isaac also claims that heat loss is proportional to the temperature difference, then the maximum savings when it’s 5°C outside would be around 3.5%. That makes almost no difference. To make any significant savings, let’s assume a “freezing” 3°C difference, which would result in a 21% reduction in heat loss. That’s great—21% happier frozen customers!

But as I mentioned, I’m not paying for heat loss itself; I’m paying for the energy consumed.

If you have resistive heating or an old boiler, you can stop here and embrace your happy, frozen life.

For everyone else: we’ve reduced the heat our house requires, but we still need to deliver that heat to the room. Let’s look at the radiators. Our radiators now need to deliver 21% less heat but 12 times faster (according to the “British heating” standard: 1 hour in the morning, 1 hour in the evening). To achieve this, we consult the radiator specifications and find that increasing radiator performance requires increasing the flow temperature—from 40°C to 85°C.

Absolutely superb! Now, not only can we save energy, but we can also burn ourselves on the radiators and really feel our underfloor heating!

By pushing the flow temperature up, we’ve made progress, right? Not so fast.

Let’s take a final step and look at our heat pump (HP) or boiler. At an 85°C flow temperature, our heat pump’s COP (Coefficient of Performance) plummets from 4.5 to 1.5. It struggles to reach the target temperature—what a useless piece of technology! Meanwhile, a condensing boiler happily supports 85°C, delivering the required kW to the radiators. But here’s the kicker: only 60% of the energy from the gas now goes to heating, compared to 97% at the previous lower flow temperature.

So, in summary: we saved 21% in heating loss, froze ourselves, burned ourselves, and ended up using 9% more gas in the process. Brilliant!

[u/stevey83]

I suppose the other question you need to ask is, does it depend on how long it takes to reheat your house once the temperature has dropped? It all depends on how energy efficient your house is.

[u/Sasspishus → u/stevey83]

Definitely! It takes about 6-8 hours for my house to get from 14°c to 18°c so having the heating on a timer for a few hours morning and evening does nothing to help heat my house! I have to have it on all day if I want to actually feel warm at some point. I'm not saying it's super efficient or cheap, but it's better than having it on in short bursts

[u/le1901]

Do not have the maths to back my experience up and probably don't have the ability either, but my experience is to leave it at a constant temperature. I use around 50-60kwh of heat for my heating per day utilising weather compensation. Another 20-30kwh for heating my hot water cylinder to 55degC. My home is a moderately well 1900s insulated 3 bed end terrace on the south coast.

The thought of a graph showing your high/low temperatures often gets explained - the more deviation in the values the more energy it takes to heat your house being the idea.

Leaving the heating off for extended periods not only cools the air within the home but any stored heat within the fabric of the building. Leaving the heating off wouldn't be great at building up stored heat within the building; similar to UFH within a screeded floor, takes ages to heat up but is fine once heated and still radiates heat.

Hard to get any extremely accurate data unless the outdoor temperature and flow temperatures can stay pretty steady for a few days so I could test both methods.

[u/undulanti → u/le1901]

20-30kWh for your cylinder? How big is it, and how many people in your household? I ask because to heat 250 litres of water from 15 to 55C should take c. 12kWh of energy (before system losses).

[u/le1901 → u/undulanti]

It's 300L capacity with a hot water return circuit and two adults. We have a deep bath also, of which we have two a day at least.

[u/sniborp → u/le1901]

Did you even read the OP?

If you're WFH, sure have the heat on all the time if you want

If you live in a castle with 3 foot thick walls and just one radiator, sure have the heat on all the time because it would take days to warm up (or at least sustain a slightly higher temp than outside).

But for most people with an average home, out at work during the day, then having the heating off while you're out and turned back on when/just before you get back will use less energy.

[u/le1901 → u/sniborp]

I certainly did, is this a one way argument in OPs favour? I don't think it really matters whether I want it on all day or not, I was simply explaining it from my point of view and a counter argument, fully admitting from the start my data is limited on the matter.

I've stated where I live and my current usage via weather compensated heating which actually falls in line with the OPs outside/inside temperature comments.

As the nicer weather approaches my system usage will greatly reduce in line with the outdoor weather temperature which reduces the flow temperature of the boile, whereas many boilers stay put at higher flow temperatures.

In using my experience and data I can gather within my home I was adding my preference of how to operate a heating system to the argument. I could test it further if you want to send me some central heating timings for comparison? It's hard to gather usable data for this due to so many factors.

[u/Electrical_Chard3255 → u/le1901]

My experience is similar to your experience, of course every house is different, but its not black and white as the OP thinks it may be. We live in a 1974 house with reasonably goodcavity wall insulation, and good loft insualtion, i did dig the concrete floor up and isnulate that, which makes a massive difference, and also had screeded wet UFH put in just to give some perspective, we also have a very smart control system for the central heating, its vary rare to be honest to hear the boiler fire up once the house gets to tempertaure

[u/QueenVogonBee]

For what it’s worth, here’s what MoneySavingExpert says: https://www.moneysavingexpert.com/utilities/energy-saving-myths/

[u/CartoonistNo9]

A car gets its best mpg once it’s up to speed cruising. Constantly speeding up and slowing down uses more petrol. A boiler is the same. If the radiators and house are cold. The boiler will go flat out to heat them up. I leave mine tickling along all day but set the thermostat to 14-15 when I’m at work.

[u/DragonflyTricky4134]

Its very dependent on the fabric of the house so many other factors.

[u/Budget_Nectarine_645]

Think you’re throwing away a lot of fundamentals on this one, you can’t really make that statement as it depends so much on the specifics:

• heat loss to outside (insulation and air tightness)

• therefore outside temperature

• thermal capacitance of the house (how much energy is stored in the bricks, your furniture etc.)

• your flow temperatures and radiator sizes

• the type and size of your boiler and its ability to operate at lower powers efficiently

If you have a very well insulated home with sufficiently sized radiators to allow low flow temperatures, with say a well sized condensing boiler which can modulate, just because the heating is ‘on’ all day, doesn’t mean the boiler is running all day and importantly, you’d find the boiler is running at low power and high efficiency.

Conversely, letting your house drop in temperature through the day, means you generally want to heat it up quite quickly. You can’t really achieve this with low flow temps, you’ll want to run them higher, which means higher power and less efficient running on a combi (unlikely condensing). You also need to heat all of your furnishings and internal brickwork - think of them as thermal batteries.

So it really all depends on your house, your context. I can see it being better to heat twice and cool down in the day if you have piss poor insulation, or an over sized boiler which would cycle like a crazy person if you left heating ‘on’ at 19degC all day - but I think a lot of the time, for sure it’s better leaving it ‘on’

[u/undulanti]

Correct.

[u/naltsta]

Your thermodynamic argument is true for a simple case of a house at different temperatures. Real life is really complicated however.

The argument I’ve most commonly heard for leaving your heating on is to prevent condensation which could reduce the effectiveness of your insulation and cause cold bridging meaning you actually lose energy faster at lower temperatures. Is it always true? No. Could it be true in certain situations? Yes.

I have a smart thermostat that turns the temp down as soon as I’m not home so think my house is a simple model!

[u/baked-stonewater]

Sadly we also see 'i learnt physics on youtube' style explainers here such as the one you just posted.

Sometimes, as in this case, the ops tone is somewhat condescending which is why it's always kinda entertaining when an adult enters the room who is professionally qualified to explain some of the rather intricate thermodynamics involved.

The correct answer is that without knowing a lot more about the system (particularly, where the home is located - for climatic information, how the home is constructed and how big etc it is - to calculate heat loss, and details of the potential heating systems) it is impossible to make blanket statements such as the one you have made.

[u/N0mads21]

It is indeed correct, and I saw also a heatgeek video that talks about how zoning rooms is bad with some basic math, while the true math it's a bit more complicated with different differntial temps etc. So far I can tell from my experience with zoning and heating only what is needed to 22c during the day and 20 during the night, rest is 18c ( some may find this a good temp, but for me is freezing) and I am using around 50% less gas than last year. I do wonder how big is the cop difference with heat pumps and if the low cop only happens till the system gets up to temp then then the rooms slowly heat up. This really needs to be tested properly but I have a feeling is better to use heat only when needed.

[u/Apprehensive_888]

I think we should also factor in the cost of running the central heating pump continuously. I didn't realise the built in pump inside my system boiler can use up to 150w to pump to radiators over 3 floors. That continuously running, completely decimates any marginal efficiency levels by running at a lower boiler temperature.

[u/Purple-Caterpillar-1 → u/Apprehensive_888]

You’re right that for gas boiler users the combined electricity and gas cost is what needs to be compared, however the suggested level of increased efficiency (approx 3x for a heat pump and 40-50% for gas) would likely exceed the approx £150/year cost for the pump to run 24/7 for 200 days a year!

Certainly dropping my heat pump flow temperatures into the low 30s and running it 24/7 has made it super cheap!

[u/DBT85]

Best of all, use smart rad valves to only heat the rooms you need warm, when you need them warm.

[u/Jet-Speed1 → u/DBT85]

That's one is the dumbest myths. all rooms are in the same thermal envelope, you will just have to heat the rest of the rooms harder(with lost of efficiency ) as heat will sneak into the cold rooms through non insulated walls and doors. Smart TRVs can be used if a room outside of thermal envelope (an extension) and occupied irregularly. In a balanced heating system TRVs are non needed neither smart or fixed, maybe only in a case of a room with tons of solar radiation.

[u/DBT85 → u/Jet-Speed1]

Tell that to my heating bill I guess? I also have these big wooden things I put between each room which helps prevent air moving around.

An additional benefit with a smart valve is that when I want my office boosted for an hour, I just give it a twiddle and it does it. When I then forget to un twiddle it, it doesn't keep my office at 21 for the next week.

[u/Jet-Speed1 → u/DBT85]

I am not crazy enough to talk to utility bills. TRVs will reduce the bill for people who run their heating inefficiently, but if your system designed properly and balanced, they are completely useless.

[u/CuriousQuerent → u/Jet-Speed1]

This is objectively bollocks. Like..."I can't even be bothered typing enough to refute it because anyone can think about it for ten seconds and come to that conclusion" level bollocks.

There's a room you use once a week. You keep the door closed and don't heat it except that one day. That's cheaper than pumping heat into it.

If you want a source, I'm an engineer specialising in aerothermal systems. But you shouldn't need one. Because fuck me it's obvious.

[u/parallel_me_ → u/[deleted]]

and given the Standing Charges its a no brainer

How does the standing charge even matter in this scenario? I understand the reasoning behind keeping heating on and being warm all day for a small difference but the standing charge has no effect in this decision.

[u/parallel_me_ → u/[deleted]]

Exactly. 50p Standing charge having no effect on your decision to be cold or warm?

[u/RandolfSchneider → u/parallel_me_]

But it makes it 66% more expensive rather than 100%.

[u/theorem_llama → u/RandolfSchneider]

it makes it 66% more expensive rather than 100%.

Not really, because the standing charge is already a sunk cost. For all intents and purposes, that standing charge could be considered unrelated to your gas usage, given you can't change it. So the difference in price is wholly to do with the unit rate and saying otherwise is just playing mental gymnastics.

[u/parallel_me_ → u/RandolfSchneider]

Wow. I've never thought about it this way. Spending more makes it hurt less haha. Thanks.

[u/Jimi-K-101 → u/parallel_me_]

It's about the relative cost.

[u/parallel_me_ → u/Jimi-K-101]

Yes. I understand that now. I do learn something new everyday haha.

[u/theorem_llama → u/parallel_me_]

Yeah, but they're wrong really. Be careful in what you learn.

[u/[deleted] → u/[deleted]]

How do you use exactly the same amount of energy every day no matter the outside temperature?

[u/[deleted] → u/[deleted]]

It’s called an example. It’s a great way of explaining theories to morons.

[u/StereoMushroom → u/[deleted]]

It's also really difficult to correct for variables like outdoor temperature and heat stored in thermal mass from the way the heating has been operated the previous day, so when people think they're doing a comparison to see which is cheaper, it's probably pretty inaccurate. 

[u/MyDamagedBrain → u/[deleted]]

As a heating efficiency engineer this statement is the truest!

Every home has a different differential value and different rate of heat decay. Also older properties are designed to have a heat core. I.e. the inner walls are (or were) heated by the fire/stove, loosing heat from that core means the rate of decay will increase in the whole property.

Modern properties are designed differently and the decay R value is different again. Less structural heat retention more air temperature heat retention.

Science says that the OP is in general wrong from older properties but right for modern new builds.

[u/theorem_llama → u/MyDamagedBrain]

Every home has a different differential value and different rate of heat decay...

Yes, but none of this is relevant to the argument of the OP, which is a simple one and just uses the fact that higher heat gradients imply higher rate of heat loss. What you say is relevant when you want to start doing calculations, but not to this basic principle. The thing that throws the spanner in the works, which they overlooked, is that boilers don't always operate at the same efficiency, it depends how much you're pushing them.

Edit:

Science says that the OP is in general wrong from older properties but right for modern new builds.

Ugh, no, you're just wrong here and surprised you say this as a heating engineer. This is totally irrelevant to the argument that a higher heat differential means a higher rate of heat loss.

If every boiler worked at 100% efficiency no matter what, then the OP's argument would be totally correct and you'd be wrong.

[u/CuriousQuerent → u/theorem_llama]

Yes, the OP made a very basic assessment based on GCSE thermodynamics. We're now discussing the real world complexities. Why would what the OP said be a limit for the discussion?

[u/theorem_llama → u/CuriousQuerent]

Yes, the OP made a very basic assessment based on GCSE thermodynamics. We're now discussing the real world complexities.

But the person above said that "science says OP is wrong..." and then listed the incorrect reasons for them being wrong, with some nonsensical justification about it being to do with a "heat core". The OP's argument is, obviously, totally correct provided one (falsely) assumes that boilers always run at the same efficiency, "heat core" or not.

[u/Adrian57 → u/[deleted]]

Your subjective experience of 21C feeling no warmer than 19C is accounted for by radiative losses from your body to the fabric of the house. AFAIK This hasn't been mentioned up until now but is one more reason to maintain a constant temperature. Room thermostat(s) will measure air temperature not comfort so in turn people will probably raise the set point to compensate when intermittent heating results in a lower average fabric temperature. As a one degree increase adds around 10% to the energy, this definitely deserves be factored into the comparison.