How to size an RPZ / Backflow Preventer?

Jason Quinn

When used in hydronics, is an RPZ size based on the connected BTU's of the boilers,system pressure, something else?  Does anyone have a reference I may use?  Thanks in advance. 

Mark Eatherton

Size it to....

the required flow. 1/2" is typical for a residential system.



If you don't make a solid connection between the potable water and the non potable space heating water, you don't need any protection required by code (sill/drain cocks).



That will also avoid long term unseen water damage.



Make sure you have low water and or low pressure cut offs.



ME

Jason Quinn

Thanks, ME

By required flow, do you mean the flow of the heating system or the flow required to fill the system?

EricAune

RPZ

I think Mark is saying the required flow needed at the point of connection.  Operating parameters of both sides of the RPZ/backflow device must be considered as well.  Check the specifications of the device and compare it to the installation.



Most boilers will only need a 1/2" line for make-up.  Can't think of the last 3/4" or larger RPZ I tested/serviced connected to a boiler.  The larger boilers I see are usually in churches and apartment buildings though.