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Condensate Tank Steam Blowout

I have a 1890 historic home with a 20 year old Weil-McLean steam boiler. The house heats evenly and easily except for one problem. If the thermostat is turned up more than a couple of degrees at a time, resulting in a long firing period, there is a loud banging and the steam blows out of the condensate tank. I just attached a low pressure gage to check the pressure and found that generally the boiler is running at just a few ounces of pressure until all of the radiators are full and hot. Then the pressure builds up to 1.5 PSI and blows out the condensate tank. Whenever the condensate pump comes on, the pressure lowers for a while and then goes back to where it was.
I have the Pressuretrol set at .5PSI and the cutoff at the lowest 1PSI. The boiler does not necessarily shut down on the blowout and the pressure naturally drops considerably. It is a new Pressuretrol on a new pigtail. Would a Vaporstat stop this problem because it can be more sensitive at lower pressure levels?


  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    Chances are excellent that, if this is two pipe steam, somewhere out there there is a loop seal which is being blown out. It would be well worth checking that.

    And, if so, yes -- you will need a vapourstat.

    Does this happen to be a vapour system? And what kind?

    Also, why is there a condensate tank at all???
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 11,058
    Pictures of your typical radiator showing both ends, please.
    Also your boiler piping and piping coming into the pump.
  • SteamStudent
    SteamStudent Member Posts: 6
    Thanks for your quick reply. This is a one pipe system that also has two long radiators with two pipes leading to them. On one of those radiators I had to close the valve on the vent side to get the entire radiator to heat. The other radiator with two pipes heats just fine with both valves open.
    I don't know if this is a vapor system. All that I know is that it heats extremely well at very low pressure. How can I find out if it's a vapor system?
    The condensate tank looks pretty old so it was probably installed when a replacement boiler was installed some time ago. The original system was piped in 1890 when the house was built and before the town had electricity so I am sure it was originally a gravity feed system.
    The house is 5,500 square feet with several radiators previously removed and several closed in unused rooms. The house was significantly tightened up about 20 years ago. Could the boiler be oversized?
    I will post pictures and will try to measure and report my radiant volume and boiler size soon.
    Thanks for your help!
  • Jamie Hall
    Jamie Hall Member Posts: 23,272
    OK. It's one pipe steam then. So -- not vapour. They are all two pipe. How does condensate get back to the condensate tank? And, related to that, what keeps steam out of the condensate returns? There is a possibility of an F&T or some other similar trap way out in the wilderness somewhere which isn't functioning properly, but you should also look for anywhere in the system where there are water loops or drips to a wet return, if you have wet returns, and if you do have wet returns that they are well below the boiler water line.

    Bottom line is that steam should never get into a condensate line.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England