A basic question about water supply for hydronic system –

Rob Main

I am a backflow tester who is also a long-time plumber, but only vaguely familiar with heating systems. It seems that every boiler I’ve ever seen has had a pressure reducing valve on the feed line that drops the supply pressure to about 10-20 psi.
Hydronic heat has become much more popular in this area lately, and at the same time the requirement for an RPBA backflow preventer to be installed on the boiler supply has been more consistently enforced. So now the typical installation includes both a PRV and an RPBA on the feed line.
“RPBA” stands for “Reduced Pressure Backflow Assembly” and it reduces pressure by an inconsistent and non-adjustable amount. Static pressure drop is generally around 10 psi, and with flow the loss is greater. The end result is that it usually takes 15 psi or more at the inlet to get any flow at the outlet.
This usually is not a problem if the RPBA is installed upstream of the PRV, but I often find the opposite. A typical installation is just as likely as not to have a PRV set at 15 psi installed ahead of the RPBA, which in turn drops the pressure to zero or close to it. The installers use the fast-fill bypass on the PRV to fill the system and bring the pressure to about 20 psi, and don’t realize that their valve arrangement effectively shuts off the water supply.
Here is where my knowledge fails me. This seems like a problem to me, but I don’t really know enough about it to be sure. I would like to offer advice to the installers (and to the inspectors who don’t realize the water is “shut off”), but I don’t know enough about heating to know if this is really improper.
Am I right in presuming that there should be a positive pressure at the outlet of these valves? If so, is the setting critical? What is an acceptable range—10-20 psi? My feeling is that the RPBA should go in first, and then the PRV can be set precisely to deliver the desired pressure. Adjustment to a desired setting would be difficult if not impossible with the RPBA after the PRV.
Is this important? Does it matter what the supply pressure is? Would just cranking up the pressure setting on the PRV until you get some flow at the outlet be an adequate fix, or is a precise setting important. I am eager to get some feedback from some people who understand the systems.

Thanks!

Rob -
On-Site Testing
Bellingham, WA

JohnNY

to answer your question

The PRV goes after the backflow device.

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Big Ed

Vent Type

A lot of installers have put the vented type after the prv for the reason of preventing the vents dripping problem. But if your right about the pressures needed it makes sense it belongs before the prv... In our area we are allowed to use a ventless bfp on residental boiler..

Rob Main

Thanks for good answers - another question

I appreciate your responses confirming that the PRV goes after the backflow preventer. By the way--yes that is the vent type (required here). Can someone educate me about the required supply pressure? If you are installing a hot water heating system, what pressure do you set on that PRV? If you're inspecting the work of someone who does it differently, is there an acceptable range for the supply pressure?
Thanks again!

John Starcher_4

Being a backflow guy.....

....I'm sure you know that 1 PSI will lift water 2.31', right? With that in mind, your system pressure should be set high enough to assure sufficient "lift" to the height of the highest part of the heating system, with a couple of psi left over at the top for good measure!

Most prv's are factory set at 12 psi, which is usually sufficient for most two story homes. If the home is taller, and the heating system extends to the upper floors, then more pressure would be needed. A system for a single story home, with the boiler on the main level, could realistically be set much lower than the factory setting and still work just fine.

Most residential boilers are rated at 30 psi maximum working pressure, and their relief valve will discharge at that point.

Hope this helps.

Starch

Rob Main

The light has come on!

Excellent answer, Starch! I get it! People like me seem to somehow turn off our common sense when we're looking at something we think we don't understand. It seems obvious now that there has to be at least enough pressure to push the water to the highest part of the system, and the mwwp minus a safety factor would clearly be the upper limit. I'm not going to start designing systems now, but at least I understand enough to explain what "my" part of it is doing to the rest of it. Thank you very much! ...and thanks again to all who responded.

John Starcher_4

Happy to help!

Glad you found the answers you were looking for. That's what this place is all about!!!!!

Starch