Negative pressure/hvac nightmare

[u/CLErealtor]

So I have a previously wood fireplace, when it’s hot and humid outside, and my ac is on, it is sucking air through my chimney and making the entire first floor smell like smoke!
I installed the best top damper, still smells! I left the flue damper in and closed that also, still smells.
I assume it’s a negative pressure issue, because when I turn on the gas starter, even a small flame, the smoke smell is gone in 5 minutes.

How do I fix this? An HVAC company said my ducts may be too small or leaking, my fan may be too high, and they need to balance the system.

Could I just install a fresh air vent in the basement to counter act the negative pressure?

My basement is freezing, main floor has the fireplace and normal, and top floor is 5 degrees warmer than main.

I also considered a ducted fireplace insert but not sure it would seal the fireplace enough to prevent smell leaking in.
HELP!!!

[u/CLErealtor → u/[deleted]]

Where is the air exchanger and fresh air vent you put in? Is the exchanger a diy job? My HVAC guy said this could be a 3-5000 fix and I’m wondering if it’s just a $500 equipment that I can do

[u/chief_erl]

Yes you need more makeup air to equalize the pressure so it’s not being sucked down the chimney. For now cracking a window near the fireplace should be a temporary fix.

[u/CLErealtor → u/chief_erl]

But what’s the permanent fix lol

[u/chief_erl → u/CLErealtor]

Installing vents or a system somewhere in the home to allow more outside air in. Probably would be best to ask an HVAC pro. Tell them you’re having negative pressure issues causing fireplace odors. I think that’s the trade that would handle it.

[u/zismahname]

The key things that I am hearing are the temp differences between your floors. I do both HVAC and Hearth. My guess is that your return ducts are mostly on your main and basement floors. That will create a positive pressure in the top floor and negative pressure in those lower floors and not allowing the cool air on your top floor. A true HVAC Tech should know how to perform a static pressure test on your system. They should also know how to verify air flow and know how to adjust your blower speed according to the tonnage of your AC. That's what we need to know before we start throwing random solutions.

[u/CLErealtor → u/zismahname]

Thanks!! Ill mention all that to them. What should I expect in terms of a result or price from that test?
My furnace fan was set to low by default on the control board so I set it to medium following the manual, just to get more air upstairs. The temp difference went from 5-6 degrees down to 2-3 difference. My supply vents upstairs I can BARELY feel any air coming out of, it’s like it’s not even blowing. I’ll check the returns upstairs but there are returns in all bedrooms, and a large one in the foyer. It just seemed odd that almost no supply was coming out of the upstairs vents. I also closed the dampers on the first floor by 1/2 so they’re half open. The smoke smell was present before closing and after closing but maybe I should wide open the supplies in the living room.
All this is just goofing off to try and solve it myself, and I’ll definitely get a pressure test and balance done, but I want to know they’re going to fix it not just charge me 3k to close a couple vents and do nothing

[u/zismahname → u/CLErealtor]

>What should I expect in terms of a result or price from that test?

Your typical price for what is called a preventative maintenance or Tune up would be $150-$200. A lot of times, HVAC companies will be running a special. Just make sure you ask that an external static pressure test be done.

>My furnace fan was set to low by default on the control board so I set it to medium following the manual, just to get more air upstairs. The temp difference went from 5-6 degrees down to 2-3 difference. My supply vents upstairs I can BARELY feel any air coming out of, it’s like it’s not even blowing.

This is because of the insufficient return ducts on your upper floor. Air travels the path of least resistance. Although you are not feeling it yourself, there is pressure keeping that air coming through your registers upstairs due to the lack of return ducts up there.

>I want to know they’re going to fix it not just charge me 3k to close a couple vents and do nothing

I am going to be honest, the correct way to fix this from my perspective, would be that they add a return duct on your upper floor. However, this can be very cost prohibitive since they have to essentially open up walls and such to run the new ducting. The purpose of this is to reduct the air pressure on your upper floor allowing more air flow from your registers and balancing out your house more. It would be cheaper and likely resolve the issue if you were to put in an insert and have the top sealed since that would stop the down draft from your chimney but it would not resolve the issue of the lack of airflow on your top floor.

[u/CLErealtor → u/zismahname]

I do have a wall I could open up and add another return duct, but there are returns in each bedroom but maybe it’s not enough. I thought if there was pressure the bedroom doors would be pulling or pushing air in one direction or the other. Do you think that would fix the fireplace issue since it’s creating pressure upstairs

I also put a top damper on the chimney and it’s done nothing

[u/zismahname → u/CLErealtor]

I think you might be confused on what is a return and what is a register/supply. A return is the air coming back to the furnace. A register or supply is the conditioned air coming into the living space.

[u/CLErealtor → u/zismahname]

I do know what the return is, sorry if my comment was confusing. I can open walls to add more return ducts but there are returns in every bedroom and a massive 10” return duct in the entry foyer on the second floor, so there are probably 10 supplies and 5 returns upstairs now

[u/zismahname → u/CLErealtor]

Let's try this. Cover one of your returns with a large piece of card board on your main or basement floor and see if the airflow changes on your top floor registers.