Need Advice on Boiler pressure relief valve

[tmy23]

Greetings and as always thanks in advance for all help. I've a 2 WM boiler system, running into a primary loop then to 3 house zones, managed by a WM management "computer". A couple years ago I sweated a pressure gauge on the supply line after the autofill value but before the boilers/expansion tank so I didn't have to get in an awkward position to check system pressure at the boilers.

Boilers are 20+ years old and when I fired the system up this season, notice one was pushing a little water through the PRV. I have an old victorian, all 3 floors heated and so try to keep the pressure around 23 to get heat to the attic. Noticed no matter how much I refill the system to 23, I'd find a little water in the bucket under the "leaking" PRV and that particular gauge at 20. Incidentally I assume I have some aging gauges because the gauge on one boiler shows 40PSI, no discharge, the "leaking PRV boiler" shows shows about 25 and the gauge I installed right after the autofill is 20. Both PRV's look original as do the gauges.

So I (maybe incorrectly) concluded the PRV is faulty and was going to replace it. Before that question I've a few others. (1) shouldn't pressure be the same across all three locations i.e. pressure in the 2 boilers and the gauge right after the autofill all measure the same? If not is faulty gauges the likely suspect? Is there anyway to determine what the actual system pressure is?

(2) If I end up replacing the PRV (and/or) the gauges for that matter, is there any particular order to the steps particularly refilling? I was planning to isolate the boilers from the primary loop, remove and replace each valve, then refill top down using the autofill until the air remover stopped emitting air, then reconnect the boilers to the loop and restart the system.

Sorry for the lengthy post. THANKS!!!

 

[Jadnashua]

IF one gauge reads 40, and the other 23, I'd investigate further. You can check system pressure with an air pressure gauge on the fill valve of an expansion tank. Once pressurized with water on the wet side, the air pressure will match the water pressure. That will give you another test datum.

THe pressure relief valve (in this case, it's not a T&P valve like you'd have on a water heater) may be prematurely opening as the spring aged over the years, but they're pretty good. But, they aren't designed to regularly be opened. IF they are, after awhile, you tend to get mineral deposits on the shaft, possibly some debris on the sealing gasket, and eventually, it won't seal again, so may be a candidate for replacement. FWIW, the pressure should not get to 40psi if the relief valve is working, assuming it's a 30psi one. If it's a 40psi one, then it's doing its job. With the 3-stories, a 40psi one may give you a little more leeway assuming the boiler is designed to accept one safely. They usually come with a 30psi one installed which handles a 2-story building fine.

Another reason why the pressure may rise is if an ET has failed. WHen you check it's pressure in the system, if any water comes out, it's shot. You cannot, or should not, add air to one unless the system water pressure is relieved, though, and then, assuming you're planning to keep it at 23psi, precharge the tank to 23 psi. That puts the bladder at it's neutral point when cold and maximizes the design volume.