Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Advice on leaving fill valve on, or off, given my setup

Nick246 Member Posts: 11
First of all let me thank everyone here. I'm just a homeowner who is very motivated to learn, in minute detail, exactly how my hydronic system operates with the end of optimazation, safeguarding, and repairing it when i can, and being an educated client of a professional when i can't.

I have three independant hydronic circuits, each with its own boiler, in my 100 year old house with three floors and cast iron radiators on each floor. I'm guessing due to its age and perhaps changing service/setup practices (and probably clearances since all the 3 boilers and 3 hotwater heaters are stuffed together is a small space), all three boilers have their circulation pumps piped in the return side. Only a short length away from each circ pump on each system is the fill valve / pressure reducing valve. Thanks to the pros on here I've come to learn that this will lead to a sub 12psi level on the return side when the circ pump is on, which means that the default pressure on the system wont be 12psi (what the X tank and fill valve are set to), but potentially as high as say 18psi because of the naturally occuring pressure differential across the circ pump. I know relocating the circ pump might be the best idea, but lets forget that for the purpose of this discussion.

My thought is, in order to preserve the proper volume of water in each system, is to fill each system, bleed radiators, and then shut the fill valves Off prior to firing up the boiler and running the circ pump. This would in my mind preserve the proper amount of water in the system and not overfill and thus overpressurize each system and so trip the PRV Valves while each system is in operation.

However... my concern is that my older systems have no low water cuttoffs installed, which makes me tread cautiously toward this open. Yet if i leave the fill valve On, I may trip the PRVs when the as the system reaches higher and higher pressures due to an overfilled condition.

Am I missing anything here. Any thoughts?


  • Nick246
    Nick246 Member Posts: 11
    Btw, my X tanks are on the supply side, right after the boilers
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    You're kind of stuck. With no LWCOs, I honestly wouldn't shut off the pressure reducing feed valves, unless I were really sure I didn't have any leaks. You could verify that, if you liked, by shutting them off, though, and closely monitoring system pressure for a few runs...

    Or, you could lower the setting on them and leave them open. Make sure your boilers are up to the desired pressure -- say 15 psi for the upper floor and 12 for the other two -- and then lower the valve settings to, say 9 and 6 respectively. That way they shouldn't feed when the pump starts (and jack up your pressure) but they would feed if you got a low water situation from a leak. I think I'd try that route.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England