Forward Flow Test: Flow & Pressure, or Just Flow?

8/31/2026

We are having a debate between our office and the field.

What numbers are we looking for at the initial forward flow test?

The office claims we are looking for the full flow possible from the riser testing arrangement, with a minimum flow rate equal to system demand. Pressure is to be noted but not have a requirement. (The pressure is to be noted for future tests to assess system degradation.)

The field claims we are looking for Flow and Pressure of the demand of the sprinkler system. Achieve flow and throttle the discharge to achieve the sprinkler system minimum required pressure for system acceptance.

NFPA 13, 2019 Edition, Section 16.14.5.1 and A.16.14.5.1 address this "tests at a minimum flow rate of system demand including hose allowance where applicable" and the appendix "this test is only concerned with testing at the proper flow rate."

What are your thoughts?

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14 Comments

Mike Morey

8/31/2026 08:14:44 am

It's like the inverse of a main drain test, you're only concerned with the flow. The arrangement of the forward flow testing apparatus is not analogous to what was calculated. The pressure readings during a forward flow test are only useful for future comparisons, not comparisons to hydraulic calculations.

M. Newell

8/31/2026 08:16:42 am

You are only looking to make sure the backflow can supply the demand flow rate.

Theoretically, the degradation in the main drain test or even the 5 year flow test as required per NFPA 25 would be more about degradation of the water supply from a pressure point of view.

This is language from the CMTC: Forward flow test performed in accordance with 6.10.2.5.2:

NFPA 24: 6.10.2.5 Backflow Prevention Assemblies. [24:10.10.2.5]
6.10.2.5.1 The backflow prevention assembly shall be forward
flow tested to ensure proper operation. [24:10.10.2.5.1]

Backflow device
forward
flow test

When means to test device was opened, was system flow demand created?

NFPA 25 gives criteria for once it goes through acceptance testing.

13.7.2 Testing.
13.7.2.1* All backflow preventers installed in fire protection
system piping shall be exercised annually by conducting a
forward flow test at a minimum flow rate of the system
demand.

Dan Wilder

8/31/2026 08:21:47 am

For the initial test, NFPA 13 has this info.

28.2.3.4
Statis & residual pressures recorded after system pressure stabilizes.

While not specific, this means to me, a fully open valve and the appendix section also indicates flow is not intended to be measured.

That info goes on the AGTC (28.2.3.4.1) & the General Information sign (28.6.2).

Any ITM activities fall under NFPA 25, specifically 13.2.2 (2026 Ed.) and when there is a >10% reduction in the full flow pressure compared to the original, that is when the investigation needs to occur.

Neither test has mention of measuring GPM as a requirement.

8/31/2026 08:28:26 am

and that was for Main drain, not backflow.....ugh...

Backflows require the GPM to be measured per 13-28.2.5.2

25-13.7.2.1 - Forward flow test and Minimum flow rate of the system demand or at the maximum available (13.7.2.3 when connections do not allow the full flow test).

Pretty much the opposite of a main drain, where they want the valve to be exercised at the flow rate (exclusive of pressure) and pressure is a secondary item, likely because the main drain already addresses it.

Bruce Edward Seiler

8/31/2026 08:34:05 am

I believe you are referencing the 2019 Edition of NFPA 13. The sections you reference above are for the Main Drain Test which does only require pressure and not gpm.

The question above is for the forward flow test of the backflow preventer.

8/31/2026 08:39:57 am

Don't tell me how to read a question incorrectly only to realize after I hit Submit and see the actual question.....

LOL...ugh...but I do appreciate the heads up!

Shawn S

8/31/2026 08:22:03 am

The full forward flow test is only concerned with a volume (GPM) of at least the system demand can be achieved. The standard is fairly clear on that issue. I have seen AHJ's use the full forward flow test as a validation of the BFP friction loss, or the validation of the water supply. It is simply an exercising of the BFP. Being on the AHJ side, there remains a fair amount of misunderstanding of this issue on both sides of the table.

Glenn Berger

8/31/2026 08:37:34 am

It sounds like both sides are correct. Initially the look is for pressure and flow at the riser, which can/will be used as future reference for supply side changes in flow.

8/31/2026 08:38:21 am

I would agree with the comments above stating GPM is the only requirement for documenting the information. When we first started completing this requirement, we did record the pressures as well, but changed it to only record the gpm. We do still take the static and residual readings to confirm system demand can be met. We use that information as our new test point and plug that into the calculations. We had one project that had the gpm, but didn't have a residual comparable to the hydrant flow test. A valve on the underground main was found to be partially closed causing the problem. So not required by the standard, but good information to compare to your calculations if you are performing the initial installation.

Ricardo

8/31/2026 08:39:00 am

For testing you need both. How else are you going to confirm the flow? The basic equations for hydraulic flow require pressure as one of the values used to determine the flow of a system, outlet or sprinkler. Simplified the equation is Q=k*Square root(p). Q being the flow, k is the derived K constant of the sprinkler and p being the pressure.
If you've ever done sprinkler calculations by hand (computers do that for you) you quickly find pressure is very important to understand the amount of Q from a sprinkler and the whole calculated area. Even the computer printouts provide pressure information so the sprinkler plan reviewer can perform hand calculations if they feel the numbers just don't quite make sense.
Even NICET's testing requires the input of pressure to determine flow.
SFPE, ICC and Sprinkler societies provide classes on designing sprinkler systems, which include hydrant testing information, which require a significant amount of information to determine if a system is capable of supplying the necessary water before determining if a fire pump and or tank is needed. (usually to increase the pressure of a system)

danefre

9/1/2026 08:40:42 am

While there’s certainly nothing wrong with recording the static/residual pressure during the test, you don’t need it to determine or confirm the flow.

You’re conflating several different hydraulic concepts. Both the sprinkler flow equation and the pitot flow equation are ultimately derived from Bernoulli’s principle, but they are used for very different purposes.

When you’re testing flow from an open outlet, the flow is measured using a pitot device. The pitot measures velocity pressure, which is then used in the pitot flow equation to calculate GPM.

You’re referring to static/residual system pressure, which is a completely different measurement and is not the pressure used in the pitot flow calculation. Applying the sprinkler equation (Q=K\sqrt{P}) to a forward-flow test is therefore not appropriate.

As you mentioned, there are plenty of classes on these subjects. I’d recommend revisiting the distinction between static pressure, residual pressure, and velocity (pitot) pressure.

Jack G

8/31/2026 01:09:53 pm

A forward flow test of the BFP has long been required by 25 fating back maybe to 2002.
This test is to provide flow data to see if the BFP will fully open in a fire situation.
It also indicates that the minimum flow is to be the calculated sprinkler demand plus any applicable hoses. ( inside and outside hose streams )
If there aren't any hose racks, or hose valves or hydrants after the BFP, then the minimum flow and maximum flow should be recorded.
The mechanical test of the checks to ensure they are working is a separate test using the test cocks.
In the good ole says, BFP s came with separate 2.5 valves. One, two, or three depending on size.
How to flow:
A 2 inch drain test is usually not adequate to flow enough water to test a BFP.
SO 2.5 valve(s) are generally used.
I don't like testing thru the fdc as it requires flipping the check( shutting the system down) or bypassing the check and taking readings thru a siamese with clappers.
So a permanent method like a test header with hose valves is ideal. If there is a pump, testing the BFP thru it is ideal.
While not required I take static and residual pressures while flowing. I add a calculation point so I can check system demand flow an do a supply calc to test full fire flow.
I use and like the additional information.

8/31/2026 03:43:06 pm

If you don’t record the pressures while you flow, I feel the use of the tests is like an extrapolation , you have a max and minimum flow and extrapolating between the two points without pressure readings is useless information for an AHJ.

9/1/2026 03:26:39 am

Coming from the an AHJ side, UFGS 21 13 13 is clear about what must be recorded. The intent is to flow the calculated system demand flow and record the pressure drop across the backflow preventer. The recorded pressure drop must correspond with the backflow preventer manufacturer’s product data at the specified flow rate. This information is typically provided on a metal placard attached to the backflow preventer. The maintenance activity needs to know the expected pressure drop so it can be replicated during future testing. This provides a consistent reference point for verifying that the backflow preventer continues to operate as intended.

Forward Flow Test: Flow & Pressure, or Just Flow?

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