Reason for Differences in Flow Test Results?

8/25/2026

We are working on a school project that needed a 1,000 gpm fire pump due to three standpipes.

We initially did a hydrant flow test and had a flow of 425 gpm with a static of 55 psi and residual of 50 psi when we started the design. By the time it went out to bid and the contractor performed their own test, they flowed 354 gpm with a static of 55 and a residual of 50.

In the first test, we got 1215 gpm at 20 psi, and 1012 gpm in the second test. They went to turn on the pump and were only getting 368 gpm when the building inlet pressure was at 20 psi.

We re-did the flow test to see what changed so drastically and again got a static of 51 with residual of 46 at 375 gpm, which would give us 1004 gpm at 20 psi.

We have confirmed the two valves between the street and the building are fully open, and we only have about 50 feet of 8” pipe, so the pressure drop to the building is negligible, especially at 368 gpm.

We are at a loss for what could be causing such a difference in pressure to the building and such a low flow.

Has anyone ever run into an issue like this or have any suggestions of what else we could check out?

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13 Comments

Pete H

8/25/2026 06:34:15 am

Is there any chance of a separate obstruction in the 8"? Or any other pipe damage?

Dan Wilder

8/25/2026 07:59:15 am

You have a theoretical 1000 GPM based on the assumption that the water supply remains consistent from ~400GPM through 1000 GPM....that is a long way to extrapolate data.

Were the tests performed at the same time of day? Is there any water data from the water utility for system configuration (pump stations on, normal usage charts through the day/night, height of any water tanks/towers in the area, commercial usage spikes, pump station operability)

My first thought is performing the test again and open enough outlets to get to 1000 GPM. And/or flush the underground with a flow meter to verify what the inlet side of the pump is actually getting.

Second, scope the underground line from the flange at the building backwards to verify no obstructions.

Beyond that, it becomes much more involved for a solution.

Anthony

8/25/2026 08:16:45 am

Why are the flow test results so low? Asking specifically why they're flowing such low volumes? Makes me think your feed taps out at ~1500gpm max. Do you even have fire flow high enough for this school? This is a serious infrastructure concern for any fire fighting in the area.

On the reason for why a flow test would be changing by 20%:

Seasonality

Time of day

Water supply construction

Closed valve in a looped system creating a dead end

Development taking water away from the system (literally had a water bottling plant open down the road from an ESFR project between install and initial design once upon a time)

City water supply engineering installing a PRV or flow restriction to protect a subdivision - had this happen in Texas.

Water main break that has not been recorded or identified.

CASEY MILHORN

8/25/2026 08:20:26 am

Dan nailed it. This is why it's always good practice to flow the actual amount needed during your test, and for a sustained period of time (not just for a few seconds). Open up as many outlets as needed to get that 1,000 gpm, and do it as close to the actual connection point as possible. We hade one awhile back that was a 2,000 gpm pump, and we could only get 1,600+/- gpm out of a couple of hydrants. The extrapolated gpm at 20 psi was 2,300 gpm... cutting it very close, but the EOR signed off on it. During commissioning, it was more like 2,000 gpm at 18 psi, and 150% was out of the question of course. No one likes a surprise water storage tank, not good for anyone. Sometimes as the sprinkler contractor, you have to "help" the EOR, just make sure you don't set yourself up to be liable by "helping".

Glenn Berger

8/25/2026 08:27:29 am

As others have said - "Time of day" is a major factor. You did not state the type of area where the project is located. In a city, you have times of day where there is major amounts of water being used.

Jeremy

8/25/2026 08:55:46 am

I've had a wedge ("paddle") fall of the stem of the building OS&Y before. This then restricted flow and we were finally able to find the issue. It was a tricky scenario at first though.

Brian Gerdwagen

8/25/2026 08:58:11 am

Is there a golf course, farm, or industrial plant nearby that may be using a bunch of water? If you're really curious, use a pressure logger on a hydrant nearby for a few days to check the static throughout the day for a week.

danefre

8/25/2026 10:23:42 am

This happened because the project was issued to the contractor without first verifying that the water supply could support 100%—let alone 150%—of the rated flow at 20 psi. That is not a valid flow test.

You never actually achieved 1,200 gpm or even 1,000 gpm—you assumed it based on 2 points on the graph.

Please follow NFPA 291—the requirements are there for a reason. The available water supply must be established through a proper flow test. The graph is only an extrapolation and should not be treated as proof of the available supply.

That being said, we've literally found a 2x4 stuck in the fire service before. I'm still not sure how it got there, but we pulled it out. There are a lot of factors and surprises that can affect the available flow.

Perform flow tests at locations farther upstream until you can isolate the problem segment of piping. Do them at the same time of the day!

On a separate note, is the school that large and tall that it needed automatic standpipes? Maybe try to prove a manual wet standpipe and then your max demand will be maybe a dry OH2 system and you can satisfy that with a 500gpm pump.

Jack G

8/25/2026 11:00:41 am

You have forgotten the most important rules.
Never extrapolate beyond the amount of water you flowed.
Always flow at least the amount of water you need .
Flowing additional hydrants to get the achieved flow, however I d
request a map of the water mains and fire hydrants showing the size and locations supplying your site.
While it’s true there could be an obstruction such as a fallen os& y wedge somewhere there also could be another possibility.
In areas with weak water supplies, the hydrants and mains could be sized to flow a maximum amount of water say maybe 400 gpm.
( city of Trenton NJ had 6 inch looped water mains in the city supplied by 8 inch mains sized somewhat like a deluge system where the pipe sizing controls the flow) some times this is for supplying say 400 gpm in 2 or 3 locations ( simultaneously) but only 400 gpm in one location.
If this is the case your location will only flow that maximum amount of water.
The water supplier can upgrade the pumps but would have to replace the water mains ( Trenton is doing) to get better results.
I would request a map showing the area water mains and the city supply to them.

8/25/2026 11:06:50 am

Also if old cast iron water mains supplying your new service ( examine the coupon) the old mains could have sediment and or corrosion affecting its size ) . There is a chart in NFPA showing the c factor to use on the pipe based on its age.

chad

8/25/2026 11:10:45 am

Its time of day or some other variable affecting the results. Assuming all else equal, others suggestions are a good place to start troubleshooting.

Need the water dept involved as well. If variables are controlled, scoping the line is next.

Sean

8/26/2026 10:50:44 am

All good comments here. If still looking for additional direction to investigate, verify your pump arrangement. Do you have a double check on the suction side of your pump? Is it restricting flow into the pump? Even differences across backflow manufacturers can cause huge losses and obstructions. Do you have OS&Y's and gate valves on the suction side, or butterfly type?
What flow meter are you using on the pump? Is it rated for accurate readings at such low flows? It is 1000gpm flow meter right? Be sure you blead the air out of the tubes.
What equipment is being used to measure actual discharge flow from the test header? Is it the same pitot tubes used from the hydrant flow test? Are you flowing through 50-ft of hose to Hose Monsters? There will be some loss in that. Verify the gauge is working on the pump suction.
Consider removing the pump suction flange, and piping it out the door for a flow test to measure flow capacity. Calculate it like a stream straightener from 291, and use a C of 0.95. See how much flow you can actually get, and watch a static residual gauge on a nearby hydrant. If the results are normal then you should suspect that the issue is in the aboveground backflow and pump assembly. If the results are low, then the issue is likely in the new 8" underground lateral to the pump.

We have seen similar situations a few times. It usually ends up being an issue with the double check valve. Remove it temporarily and run the test. Keep eliminating variables until you are left with the culprit.

Michael Christensen

9/3/2026 01:48:34 am

I had a similar situation on a school that I did a couple years ago. We had the city check the PRV's that were installed. Initially they just checked the settings, eventually they checked the PRV and removed at least a fist-size of rust/gunk. This significantly increased the available fire flow.

Time of year is something to consider and ask about. I am aware of some water purveyors who adjust the PRV settings +5-psi seasonally in the summer time.

For better or worse to get to the bottom, it takes time and eliminating various things. There have been lots of good suggestions trying to isolate the item and suggest what is the issue. I recommend talking to the water purveyor because they commonly have modeled data which could be useful, just make sure that the data they give you is worth it (one time they said that 4,900 gpm can flow thru a 4-inch CI unlined that was over 80-years old)

I hope that helps in your quest to understand what is going on.

Reason for Differences in Flow Test Results?

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