differential pressure bypass valve schematic
August 25, 2014

How does a differential pressure bypass valve work?

We get calls about selecting, applying and setting differential pressure bypass valves (DPBVs). Why, where and when are they needed, and how do they work?

The DPBV is a 2-way, self contained proportional valve with an integral differential pressure adjustment setting. The valve inlet is piped to the supply line and the outlet is piped to the return line. Caleffi's 519 Series prevents excessively high head pressure (a result of the branch piping 2-way zone valves closing) by allowing flow from the supply piping directly to the return piping. The 519 Series will open to start bypassing flow when differential pressure reaches the adjustment setting. Head pressure of a fixed speed pump will increase in proportion to the number of 2-way zone valves that are closed. DPBV's are not needed when using a pressure-controlled variable speed pump. Excessive differential pressure between supply and return lines is undesirable and can deadhead a pump, prevent zone valves from fully closing, cause circuit imbalance, and can result in corrosion, water noise and water hammer. A DPBV can be installed at the end of a zone valve piping circuit, or at the beginning, after the pump discharge.

Caleffi’s Technical Brochure 01007 explains more and describes how to adjust the DPBV set point.

Blog post comments
Mark Olson
September 05, 2014

Some manufacturers instruct as follows to set a diff pressure by-pass valve. Make good sense.
a) Close off the DPBV fully
b) Open up all zone valves. Fire boiler and pump(s).
c) With your hand on the by-pass loop, slowly begin opening the DPBV. As soon as flow begins through by-pass, you will feel the heat instantly.
d) Back off the know a quarter turn. You are now set.

ted sanetra
March 12, 2017

thank you for your profesional knowledge

September 14, 2017

Does anyone know if the caleffi 1" and 3/4" brass differential pressure bypass valves are compatible for use in potable water systems?

September 18, 2017

The Caleffi 519 DPBV is not made of low lead brass so, no, by law it is not compatible.

July 15, 2022

By closure of 2 way valve why the head pressure of fixed pump will increase as the zone gets closed the flow is reduce actually the flow rate shall increase i.e the pump curve shall go towards right.


In reply to by dipak

August 03, 2022

If using a fixed speed pump sized for the system flow rate but divided into multiple zones, when all zones are calling the pump and pressure will meet the design. When the zones start to close the pump cannot adjust so pressures may go up, flow rates or velocity may increase.   That is the purpose of the differnential bypass valve to help bypass the zones and keep pressures equal or closer to equal.   With the technology of variable speed pumps they can auto adapt to help maintain proper flow, pressure and velocity with changing zone demands.