Yes, a standard boiler. And yes, one of them does get hot so it sounds like those are the potential problems. Thank you so much for this advice! Are those relatively straightforward things to fix?
I have a Viessman boiler that I noticed had been losing water pressure but relatively slowly (with no noticeable effects). After a while I topped up the pressure and it continued to work just fine. More recently I noticed the low water pressure and despite planning to increase it again, I went...