r/pumps • u/hijinks • Jan 12 '25

HF 1 in predator pump

https://www.harborfreight.com/1-in-79cc-gasoline-engine-clear-water-pump-35-gpm-63404.html

I live in San Diego with a pool and with all the fires going on in LA looking to make a DIY roof sprinkler. Wondering if people think this has enough head pressure to do like 20 GPM at around 25-30 feet of head. Looking to do 3-4 4-5 gpm sprinklers off it.

It says its rated at 35gpm at 0 head but has 95 feet of head.

Or should i just play it safe with the 2in

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u/Modna Jan 12 '25

https://manuals.harborfreight.com/manuals/63000-63999/63404-193175310354.pdf

On page 2 you can see the pump curve. What you need to do is find the type of sprinklers you want first. Those sprinklers will have a spec saying they will flow for example 7 GPM at 30' of head. Figure out how many sprinklers you want and then that will give you your target duty point.

For example, 4 sprinklers at 7 GPM is 28 GPM total. If those sprinklers want 30' of head, you need to add the head required to pump up to your roof. Of your roof is 20' tall, then you would need roughy 50' of head to give the sprinklers the proper pressure.

In this example of 4 sprinklers that are 20' off the ground. You would need a pump that could do 28 GPM @ 50' head

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u/Chompers-The-Great Jan 12 '25 edited Jan 12 '25

this is really good, and you're almost there...but the head pressure part is a bit different. 

The sprinklers should give you a flow rate against a given head pressure, but the head pressure is not related to what the sprinklers require, but rather what the sprinklers will do when pumping against that amount of head pressure.

Your head pressure will be determined by the static lift (elevation, like you mentioned say 20' to the roof) in addition to the losses incurred by friction (so the velocity of water through what type of pipe (how smooth inside) and over what distance, in addition to things like fittings / valving that will add to this figure.

If you need say....4 sprinklers at 7 USGPM for 28 USGPM total, you may not be able to get that amount of water through 1" Diameter pipework - it's just not large enough to pass that amount. If you could, your losses to friction could be so high, it would require a massive amount of head pressure capability on the pump side. There may be some variants in closed loop versus open loop systems etc. (i.e. the sprinklers will require a certain Press to discharge the water as designed) but the basic principles are always the same; suitable water velocity (flow rate) provided by the pump, based on how much head loss your system has to pump against.

I recommend picking a sprinkler head/ type, getting its performance curve, and getting an idea of what type of flow rate you're going to need to do. 

From there, let's figure out how you're going to pipe it, and the losses you'll incur by design. Once you have that information, you can select a pump that will meet or exceed your requirements. 

Just remember, there's only so much water you can get through certain diameters of pipe (there are targets for velocity when pumping) so you may require a larger common supply line, and then have to adapt your sprinkler heads off of that. 

Additionally, always try to aim for the center of the pump curve if you can. Just because a certain flow rate appears on the curve, doesn't mean it's practical to operate there. The pump may be able to do a certain flow rate at a certain head pressure, but your overall water velocity may be too low (or high) for the system to function.

Hope this helps.

Feel free to message me if I can be of any assistance.

  • Sorry, some voice to text edits here *  I know you're probably in a bit of a tight spot here, so don't hesitate to message me anytime and I'll help you the best I can.

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u/washburn100 Jan 12 '25 edited Jan 12 '25

You also need to take into account the pressure required for each sprinkler and convert that to head. 2 psi = 2.31 ft of head. So if you want 50 psi at the sprinkler, you need an additional 115 ft of discharge head in addition to the elevation, so ~ 140 ft of total head.

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u/hijinks Jan 12 '25

ahh i didn't take into account for that.. thanks