Will Backflow Dump When Fire Pump Starts?

4/21/2026

We have a situation where we need to replace a 40-year-old fire pump where the original RPZ is located downstream of the pump. Our current code requires all BFP's upstream of the pump. We are worried that the RPZ will start dumping every time the pump is started.

Does anyone have experience with this that could lend a recommendation?

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

6 Comments

David Kendrick

4/21/2026 08:34:19 am

By design Reduced Pressure Assemblies dump when the downstream pressure is higher than the supply pressure and the #2 check valve doesn’t hold.
This is a good question.
If maintenance is performed correctly the RPZ valve shouldn’t dump under flow and churn conditions.

Jack G

4/21/2026 08:38:57 am

RPZ before a fire pump is a risky deal.
It must be at least 10 diameters before the suction flanged.
Check its cut sheet….most are not sized for 150% flow and will fail during a pump test.
Install a bypass around it .
Must be in an accessible place and where its discharge water won’t harm anything.
Yes it will discharge during a pump test.
Yes it could discharge during pump churn where the casing relief on an electric or the cooling water line on a diesel will flow enough water to destroy the rpz delicate balance and cause it to dump.
The pressure drop to the suction pressure could also be a problem if regular suction pressure during a test is near 20 psi.
Cavitation is always a problem in placing it before a fire pump.
I would tell the twp that it must be installed on the discharge side for all of the above reasons.

William

4/22/2026 04:15:20 pm

Bad advice. You do NOT install a bypass around any backflow prevention assembly for any reason even if the intent is good.

Anthony

4/21/2026 08:52:44 am

I would replace the pump as that is my scope of work. if its a 1 for 1 then any other issues are existing issues. You don't have to take ownership of the RPZ as that is outside of a maintenance or replacement permit. This simplifies your problem.

RPZ after the pump makes low sense from a water safety stand point. Every time the pump cycles on/off there will be some water that bleeds back from the pump to the supply as there is a pressure differential through the impellers. There are no requirements for any of the materials that connect to a fire pump to be potable compliant or safe. Very odd IMO.

Leave the RPZ where it is or replace that later under a similar maintenance or replacement permit down the line.

Robert Morgan

4/21/2026 09:41:47 am

In Illinois, the fire and domestic water to a building is required to be a combined line and must be protected with a backflow device.

sean

4/21/2026 11:24:07 am

backflow protection via rpz is about back pressure not influx of pressure.

the rpz will dump pressure when the downstream pressure is higher than the supply side.

Leave a Reply.

Home
Our Cause

The Blog
The Forum

PE Exam Prep
The Toolkit
MeyerFire University

Pricing
Login
Support
Contact Us

MeyerFire.com is a startup community built to help fire protection professionals shine.
Our goal is to improve fire protection practices worldwide. We promote the industry by creating helpful tools and resources, and by bringing together industry professionals to share their expertise.

MeyerFire, LLC is a NICET Recognized Training Provider and International Code Council Preferred Education Provider.
All text, images, and media Copyright © 2016-2025 MeyerFire, LLC

We respect your privacy and personal data. See our Privacy Policy and Terms of Service. The views, opinions, and information found on this site represent solely the author and do not represent the opinions of any other party, nor does the presented material assume responsibility for its use. Fire protection and life safety systems constitute a critical component for public health and safety and you should consult with a licensed professional for proper design and code adherence.

Discussions are solely for the purpose of peer review and the exchange of ideas. All comments are reviewed. Comments which do not contribute, are not relevant, are spam, or are disrespectful in nature may be removed. Information presented and opinions expressed should not be relied upon as a replacement for consulting services. Some (not all) outbound links on this website, such as Amazon links, are affiliate-based where we receive a small commission for orders placed elsewhere.

Will Backflow Dump When Fire Pump Starts?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES