Difference of Main Drain vs. Hydrant Flow Test?

10/17/2025

How does the main drain test differ from the hydrant flow test in analyzing a water supply?

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8 Comments

Anthony

10/17/2025 07:40:04 am

Several items come to mind: Flow test tells you how much water is at the street/at that hydrant. Main drain test will tell you how much water is flowing out of the base of riser with the limitation of the size of your drain. Main drain is also a check that the city supply and required base of riser pressures are still present within 10% of design conditions.

The difference between calculated street flow and calculated main drain flow curves can tell you a bit about the supply pipe into the riser. You could use a flow test from a main drain test and get supply pressures and flow but again its based on the size of your drain which is always smaller than your supply these results can be dubious. It can be useful to diagnose problems in supply piping or validate calculations.

There was a blog post about the advantages of flowing more water a few years back which is useful here I think:

https://www.meyerfire.com/blog/advantage-of-flowing-more-water-in-a-flow-test

TL;DR: the more water you flow the more you know about the system. DATA points!

Dan Wilder link

10/17/2025 08:06:53 am

They both do it however one big restriction is the amount of water flowing at the visualized residual. That point is critical as it defines the known (0GPM at static pressure to Flowing GPM at residual pressure) from the theoretical (and graph line to the right of the residual flow and pressure).

The post Anthony linked visualizes this well. There is also the impact of all the valves to the point of discharge for a main drain and I would equate that to opening the hose bib at your house to determine the city supply at the hydrant in front of your house.

https://www.meyerfire.com/blog/the-trouble-with-estimating-main-drain-flow

Glenn Berger

10/17/2025 08:17:46 am

Validation of the water supply conditions into the building is achieved with the main drain test.

Jose Figueroa

10/17/2025 11:30:16 am

I remember my early days in Europe, where fire hydrants were seldom private. Conducting a water test on a public hydrant was nearly impossible. The only option available was to attach a pitot gauge to the 2-inch drain test and estimate the available water supply for the sprinkler system.. 😄😄😄

Mark Harris

10/17/2025 01:37:17 pm

A reasonably current hydrant flow test should be used for system design. The main drain test when comparing from prior years can be helpful for seeing if water supply is deteriorating. Main drain test also confirms valves are open after work was performed. Had a situation once where the street valve was left hand close and the city said it was open. But found out very quick it was not with the main drain test.

Jack G

10/17/2025 03:04:11 pm

Differences :
A hydrant flow test measures the available water from a municipal main for fire protection purposes.
A main drain test evaluates “ the water supply “ to a fire sprinkler system “by simulating a fire”
A hydrant test is for the public water supply s capacity during an emergency, while a main drain test checks “ for deterioration” in a specific buildings water supply by comparing static and residual pressures to past years tests ( relative to elevation adjustments. )

Randy Kimbro

10/20/2025 07:06:44 am

One important item missing in this conversation is the required annual forward flow test. As an AHJ, we still run into many fire protection technicians who think this is a 5-year test. It is not! The annual test requires a flow at the minimum flow rate of the system demand (including hose allowance),

https://www.meyerfire.com/blog/new-hose-valve-requirement-for-forward-flow?mc_cid=e113fbe8c6&mc_eid=d43952e107

NFPA 25 (2020) 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.

This forward flow is measured via pitot gauge and the resulting GPM is then compared to the hydraulic plate to ensure the flow meets or exceeds the engineered GPM requirements.

Bottom line: the hydrant flow test, main drain test, AND the forward flow are all important parts of the puzzle and important data points for sprinkler evaluation.

James Art, FPE

10/22/2025 01:27:38 pm

A very important part is to REVIEW and evaluate the results.
and compare to previous.
Glen mentions "validation".
Rare, but I have discovered several
serious impairments, based on 2" drain results.
In one case, at a National Laboratory, there were 3 risers side by side, but the flowing results for one were lower.
This had been just recorded for several years,
but I wondered why?
When we ran a camera thru the pipe we found a key valve partly closed. It had been paved over.
Another reason to use Indicating valves,
and to abide by the codes to "Exercise the Valves" annually.
The crew closes each valve completely, while counting the turns, and then re opens. Checks the tamper alarm at 3 turns at the same time. (Also can be considered "Exercising the Maintenance Crew" :-)

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Difference of Main Drain vs. Hydrant Flow Test?

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