Boiler pressure slowly rising

samurainstein · 8:42PM

Hey Heating Help,

I recently drained my hydronic heating system (first timer here). That went well overall with help from other posts. Now I'm refilling the system, and experiencing a slow rise in pressure at the boiler pressure gauge, while the system sits cold and turned off.

I've read in multiple places that the PRV often goes bad, so I'd like to get some professional input before I go run out and buy one. It's a Watts S1156F PRV. I opened the lever a bit when refilling the system to let water in. Sounds like it might not be reseating anymore? The pressure goes up gradually as it's just sitting (noticeable over 10 minutes). When I close the valve coming into the PRV, the pressure stops rising.

Other notes, I did tap on the expansion tank, and it does sound hollow at the top. There are 2 zones. Zone 1 is series loop (1 radiator directly to the next and so on). Zone 2 is parallel (supply and return lines have multiple tees that go off into a single room). The baseboards are base ray cast iron, and after filling, I went through and bled each one (top floor first) until water came out.

Assuming the PRV is the cause of the pressure increase, sounds like I should replace with a Watts SB1156F?
As for replacement, seems like it's a union on one end, and a FIP on the other. Can I simply unthread and swap it out?
If I need to do actual plumbing work (IE: modifying connections), can I use pex here?

EdTheHeaterMan · 8:55PM

OK the first thing you need to realize is that water expands as the water temperature rises.

What temperature was the water when you added it to the system? (maybe below 58°)
What is the water temperature now? (Maybe 70°)
That might be 12° warmer and that water expanded.

The next thing is that the Watts 1156 can fail in the open position after using the fast fill feature. You may want to try using the fast fill feature and letting the valve SNAP closed to see if that will correct the problem. Sometimes the failure is that some small spec of debris is lodged on the valve seat and SNAPing the valve closed may dislodge the debris.

After doing that you can lower the pressure by letting water out of the boiler drain until you get it to 12 psi and watch it again. If it continues to rise then there are other things to check before firing the parts cannon at the problem.

samurainstein · 9:36PM

Thanks for the quick reply @EdTheHeaterMan

I followed your instructions.

Let water out of the boiler down to 12 PSI
Opened the supply to the PRV
Opened the PRV all the way and gave it a good snap shut. Did this twice.
Let it sit for 15 mins, and it got up to 30 PSI.
Closed the supply to the PRV, and it stopped going up.
Drained it to 12 PSI again (supply to PRV still shut). It's sitting steady at 12 PSI.

Water temp says 70 on the meter (but that's as low as it goes). Anything else you think I should check?

EdTheHeaterMan · 9:40PM

When you leave the manual valve open the pressure increases. When you leave the manual valve closed the pressure is stable.

Your 1156 is passing. You can try taking it apart and cleaning it or you can replace it.

As a professional I don't have time to clean it and see if it works. My time is expensive. I just recommend replacing it and tell you the price before so you can approve it. Since it is your own time, you can try to clean the valve seat and strainer. This video shows you how to take it apart to get to the strainer and valve seat. The important part is at time stamp 2:38 to the end.

https://www.youtube.com/watch?v=TybXQDe1YrY

samurainstein · 9:59PM

I think I'll go the replacement route, cause this seems rather old as is. Since there's no shut off on the side of the PRV going to the boiler, it sounds like I have to drain the whole system to take this off? If so, while the system is drained and the PRV is off:

Should I put a shut off on the other side of the PRV?
While I'm at it, should this have a backflow preventer as well?
Seems like I should replace with a Watts SB1156F?
If I need to do plumbing work (IE: modifying connections), can I use oxygen barrier pex here?

hot_rod · 10:17PM

You should have a backflow preventer on a boiler fill valve. Here is an example of a combi unit. Brass construction, BFD, Fill, and gauge. Plastic knob on bottom is a shut off valve.

Looks like you have a tee above the expansion tank connection. That would be a good place to tie the fill valve in.

Those two yellow handle ball valves may isolate the boiler and expansion tank connection?

HydronicMike · 10:41PM

Put another shut off valve downstream of the boiler feed. Then if you have to replace/repair it, you don’t have to drain the entire system.
I exclusively use the one Hot Rod pictured, but I’ll add a union on the downstream side, and valves on both sides.

Boiler pressure slowly rising

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