My 1880s 3-story house has a circulating hot water heating system, with cast iron radiators on all floors and a 20-year-old gas boiler in the basement. The system slowly loses pressure. I'm wondering if I should be concerned, whether I should pursue repairs soon (or safely defer until later), or whether I should simply keep adding water to the system.
We bought the house over the summer. When I tried to bleed the 3rd floor radiators a couple of months ago, the system had a vacuum at the top and pulled air in! I tracked a problem to the pressure reducing valve (Watts model 1156F); the valve was full of dirt & sludge, but once I cleaned it I could then add water to the system from the street.
I brought the system up to 15psi, bled the radiators - so far, so good. After the boiler turned on, the system pressure got up to ~20psi. A week or two later, I noticed that the pressure had dropped. There's a slow leak from the pressure relief valve, although it states that it's set to 30psi and the system pressure has not raised that high. I have not noticed any other leaks.
I have tracked the system pressure and the water volume leaking from the pressure relief valve over the past few weeks. I've added water via the fill valve (through my newly-cleaned pressure reducing valve), but I don't know exactly how much. Over one four-day period, the pressure dropped from 20psi to 8psi, and the pressure relief valve leaked about a half gallon. I opened the fill valve for ~30-45 seconds to increase pressure from ~8psi to ~15psi. Graph below.
Replacing the pressure relief valve seems like an obvious first step. Should I do so soon, or wait until spring? Is it likely that there's another leak elsewhere? What else should I consider?
Thank you in advance for the advice!