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Boiler Pressure Issue Has Plumber Scratching His Head
I'll start at the beginning and try to be as thorough as possible (but I'm not super handy, so bear with me!).
I have a 13-year-old Peerless boiler (series MI/MH) along with a Peerless Indirect-Fired Water Heater (60 gal). Both have been working fine since installation (house has 4 zones, plus the water heater). 3 zones are baseboard, 1 zone is cast iron radiators.
Last week I heard a ton of swishing in the pipes when the heat kicked on (this happened in a couple of zones). I checked the boiler and the pressure was at 0. And I noticed some residual water stains on the PRV pipe (but they could've been old). Plumber came, we bled the zones and he replaced the fill valve and expansion tank (which was filled with water). Next day, the pressure rose to about 25 and I saw some dripping from the PRV (just a few drips an hour). When the pressure got to 29, I let some water out of the system and called the plumber back.
He wondered if the Extrol expansion tank he put on the day before was bad, too. Sure enough he took it off and the tank started spraying water (which he said he never saw before). So he replaced it again with another brand-new Extrol.
When he left, the pressure was about 18.
Two days later, the PRV is still slowly leaking and the pressure keeps creeping up. It was at 27-28 again today and rising so I let a little water out of the system. During this time, I've been keeping the thermostats in all the zones really low (60), because the boiler pressure seems to rise when the boiler is working.
The plumber seems dumbfounded. I've seen some posts where the culprit could be the Indirect Water Heater. Do my symptoms sound like it could be the water heater? I'm just confused as to why the pressure was at 0 to begin with. And now, why is the pressure constantly building?
Thanks so much for any help you can provide.