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Elevated hydronic system pressure

I have a seven year old home with an in-floor heating system. It has given me little trouble over that time until about one month ago. Normally the system pressure sits around 15 PSI. Lately the pressure gets up to and stays at 30 PSI. The problem this causes is that the pressure relief value on the system is set at 30 PSI, so I have a slow drip from the valve. At first I thought the pressure relief valve was faulty until I saw the value was rated at 30 PSI and the pressure was around 30. My next thought was that I had an issue the the pressure regulating valve that ensures the pressure doesn't drop below 15 PSI. I was thinking maybe it was allowing the higher household water pressure to get into the system. When I closed the supply valve that feeds that valve and then bled some pressure from the system, it climbed again. Next I bled the pressure from the entire system and the static pressure from the expansion tank, then repressurized the tank to 12 PSI (that was the static pressure written on the tank), then opened the feed valve to the regulating valve. Pressure rose to 15 PSI and stayed there for a while. After eight hours the pressure had risen to 30 PSI again. I have two automatic air bleeder valves (one on each of the two manifolds - hot side ) and there's a third one built into the boiler, so I don't suspect air in the system causing this. The boiler is a Baxi Luna HT380 combi boiler used for heat and domestic hot water. Any idea what I'm missing? Does this point to a failed component? Thanks.


  • Jamie Hall
    Jamie Hall Member Posts: 22,916
    I'd bet on the pressure regulating feed valve malfunctioning -- and the shutoff valve to the boiler not shutting off completely. However, I have to ask -- is there a feed to the domestic hot water on this unit?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Scavenger
    Scavenger Member Posts: 4
    Thanks for the reply. :-)

    I was thinking it couldn't be the pressure regulating valve because I closed the supply valve that feeds the pressure regulating valve and the system pressure still went up. That takes the pressure regulator out of the equation, doesn't it? The supply valve is a ball valve. Are ball valves known for leaks? I could put another shut-off on the boiler side of the pressure regulator. It would be convenient to have that anyway for any work required on the regulator down the road.

    There's a separate feed to the boiler for the DHW.
  • Dave H_2
    Dave H_2 Member Posts: 545
    Sounds like the expansion tank to me.

    Dave H.
    Dave H
  • Jamie Hall
    Jamie Hall Member Posts: 22,916
    If there is a separate feed to the boiler for the domestic hot water, does the boiler do the hot water? If so, let's eliminate that for the moment by turning that off, too.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Scavenger
    Scavenger Member Posts: 4
    How do I test the expansion tank. You're suggesting a leak in the bladder (or the valve) so the tank has less capacity to stablize changes in pressure from the heating cycle? Being the heating season it's difficult to turn off the boiler for an extended period of time to see if the expansion tank loses pressure slowly. I can certainly wait a few months until after the heating season and then see if the pressure reduces overnight when there's no DHW need.

    You want me to close the cold water feed to the DHW side? I can certainly do that. Yes, the boiler handles both DHW and radiant.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,696
    If you shut off the pressure reducing valve and make sure the shut off valve is holding , if its the expansion tank it will blow off once . If the pressure goes up and continues to leak , what do you use for domestic water ? Is it an indirect ? If no other plumbing was done just before this problem , like an new sink mistakingly piped from the heating system ...You indirect is leaking between the built in heat exchanger
    I have enough experience to know , that I dont know it all
  • Gordy
    Gordy Member Posts: 9,546
    It's a combi boiler. Check x tank if the bladder is gone. Next would be the HX for the domestic hot water leaking allowing domestic pressure to the heating system. To check that have to isolate domestic supply to boiler then monitor pressure.
  • Scott M_3
    Scott M_3 Member Posts: 32
    I have a Baxi HT330 and had the same problem as you about 5 years ago. I drained the system and repressurized the internal expansion tank but the problem came back after a short time. I read somewhere that these tanks are known for these problems and the best solution is to tee an external tank into the system. I did that and haven't had a problem since. Hope this helps, good luck.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    I suspect a hole in your DHW exchanger causing bleed over into the space heating water. Turn off cold water supply to the DHW heater and if the space heating fluid pressure remains stable, its the HXer in the DHW tank.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Scavenger
    Scavenger Member Posts: 4
    Thanks for all the replies. :-)

    I'll try all the things mentioned to see if it helps isolate the issue. If I get nothing conclusive I'll follow Scott M's advice and tee in an external tank since that fixed his Baxi that was displaying the same issue.