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Why might this system be over pressure constantly?

My mother's oil heat system is constantly over pressure, leaking about a half gallon per hour out the pressure relief valve. At this point I can rule out the pressure reducer, pressure relief valve, and the expansion tank, because I've replaced all of them as well as a bunch of old pipe within the past 10 days.

At this point I really only can think of two possibilities :
1. The hot water tank has a leak between the DHW and the heating loop, bringing in pressure from the higher pressure water supply.
2. There's something wrong with the furnace.

It seems unlikely to me to be the former, but later today I can try cutting a valve in so that I can shut off the DHW pressure supply without also shutting off the pressure reducer and maybe learn something from that.

I have no idea how it could possibly be the furnace. The temperature the controller reports does overrun the 193F it's set at to about 211F, but I'm not sure if that's in any way abnormal and at 30 PSI there's no way it's making steam so I don't think that explains anything. The heat exchanger probably hasn't been cleaned out in several years, but I also don't see how that has anything to do with this.

I'm well beyond my supply of ideas here and would appreciate some input.


  • Jamie Hall
    Jamie Hall Member Posts: 22,928
    And you are quite correct -- there's nothing to add water to the boiler other than the feed, which you have replaced, and the indirect -- which leaves the indirect as the only possibility (even so, I'd shut off the automatic feed and observe it for a while...).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • lookingforideas
    lookingforideas Member Posts: 3
    Thanks for the sanity check. I did get that valve put in and I can confirm it's definitely the indirect tank and not anything else.

    It's a masterfully designed unit. Failed about 2 months after the warranty ran out on it.
    Dave in QCAGGross
  • Dave in QCA
    Dave in QCA Member Posts: 1,785
    I would have to say that is incredible design engineering..... or absolutely terrible, depending where one stands. For sure, I would not buy a second one from them.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
    EBEBRATT-Ed Member Posts: 15,329
    a hot water boiler has a minimum of 12 psi on it. The water will not boil at that pressure until the temperature reaches 243. High temperature commercial systems run up to 400 deg.

    I am not saying he should run his system that high but 200-210 isn't uncommon
  • kcopp
    kcopp Member Posts: 4,393
    What model/ mfg?
  • lookingforideas
    lookingforideas Member Posts: 3
    Late update :

    The tank is an HTP SuperStor SSU-45. When I made this thread, we were 3 days from leaving on a 2 week vacation. Before we left, I turned off the supply to the indirect tank, and drained the pressure in it down. That way, the pressure would be coming from the boiler side, and it would max out at the normal boiler pressure of ~18PSI. When we got back, I turned the suppy on, expecting to see the same pattern of a slow pressure rise and about 4 gallons of water in the 5 gallon bucket I put the overflow hose in the next morning. I went to check it next morning and... no leaks, and boiler at 18 PSI when hot.

    That was 7 weeks ago. It's not leaked at all yet.

    All I can figure is that maybe somehow with the water slowly pushing through from the boiler to the DHW side, the deposits from our hard water sealed it up. I don't know, and it's annoying because I don't know when it will leak again and it turns out there may actually be a lifetime warranty on this thing, but I can't well replace it when it's not leaking. Oh well, problem for another time.