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Thread: Power-vented Gas fired boiler won't start when its cold out

  1. #1
    DIY Junior Member johngoldfinger's Avatar
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    Default Power-vented Gas fired boiler won't start when its cold out

    Hi -

    In my house I have a one-pipe steam heating system with a gas fired boiler. It is power vented. The problem I'm having is that when it's really cold overnight (say maybe 30s and below), the boiler will not come on. When I wake up I find that the house is cold below the thermostat temp, the thermostat is on, the power venter is blowing, but the boiler is not firing. The hot water heater is also attached to the power venter and if I run hot water, the heater will eventually fire. The thermostat was recently replaced by an HVAC guy as an attempt to fix the problem, but no dice.

    Last year I did have a problem with the pressure tube on the power venter not being clear or loose; this was causing similar symptoms of the venter blowing but the furnace not firing. He came out and blew in the tube and tightened it and the furnace came on. However, the problem has reappeared, and other techs have looked at that pressure tube on the venter and think it's OK.

    Does anyone have any ideas what might be going wrong here? I'll note also that when the day heats up, like after the sun comes out, the furnace will start working.

    Thanks for any help.
    John

  2. #2
    In the trades Dana's Avatar
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    Can you start with some basic information like the boiler's make, model, & vintage?

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    DIY Junior Member johngoldfinger's Avatar
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    I can indeed ... It's a Hydrotherm model SG-160 Gas fired Boiler with 160,000 BTU/HR input, output 128,000 btu/hr. I've been told that this model was manufactured between 1974 and 1979; I don't know exactly how old my unit is.

    Let me know if any other info would be useful and I can get it.

    Thanks
    John

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There are likely multiple safety interlocks on that boiler. Low water pressure cutout, vent flow (usually a vacuum switch), high limit switch, and maybe more. If it won't turn on, it could be one or more of the interlocks not closing, or a fault in the control logic board, or a relay. The schematics can be greek to the uninitiated, but it will show them. With that and some understanding, it's usually fairly straightforward to figure out what's causing the issue.

    But, considering it doesn't work when it's cold, and then starts once things warm up a bit, it might be that the low-pressure interlock is telling you something...water expands and contracts with temperature. If the boiler system pressure is a little low, it may not turn on, but as things warm up a little, the water expands, the pressure rises, and then it will work. First thing I'd check is the system's water pressure and maybe the state of the expansion tank which may be shot, dumping a bit of water when the boiler is on.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    The manual doesn't indicate a forced-draft option, but it's conceivable that a draft inducer was retrofitted to make it work with an out of spec flue for atmospheric drafting or if the water heater was backdrafting or something. The flue inducer is not original equipment, and has been tied into the controls. I can't know for sure how they rigged it to come on when the water heater burner fires- there are ways, but I won't speculate.

    If that manual matches your version of the boiler it originally came with (and hopefully still has) an automatic flue damper that will open up when the thermostat calls for heat (it usually takes a handful of seconds for a small electric motor to crank it open), which is "proven" with a set internal contact switches in the damper module before the gas valve will open up to the burners. If those contact as dirty or if any connection in the wiring harnesses are corroded it could easily be a temperature-sensitive intermittent problem. I'm not sure if that explains why getting the draft inducer going by running hot water affects it, but it's conceivable that the heat of the hot water exhaust may warm the critical contact or component enough to start functioning again. The harness may have been modified to use the same signals for opening the damper to start the draft inducer (or not), hard to say.

    Go through the manual and check out the wiring harness to the flue damper and the quality of those connections. Even a cold-solder on the printed circuit board in the damper assembly can cause this sort of problem, but will eventually fail-hard.

    If the Hydrotherm in the manual looks nothing like the beast in the basement, many details may be off. But if it has a draft spillage hood the draft inducer is surely a retrofit, and how they hacked it in can only be guessed at.

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