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Condensing during cold starts?

dh1989 Member Posts: 22
Is there any danger to running a cast iron boiler with a high limit only aquastat if it is not used for domestic hot water? Without a low limit control the circulator starts as soon as the heat call beings. During the heating season there are frequent enough heat calls that the boiler stays hot, but my concern is the spring and fall months where it may be sitting all day and then fire up from near ambient. Under these circumstances, is condensing a concern as the boiler heats up, and if so how is this typically addressed?


  • Paul48
    Paul48 Member Posts: 4,469
    What type of system? Baseboard, cast-iron radiator?
  • 776v63
    776v63 Member Posts: 61
    Are you referring to condensing of the flue gas?

    This occurs at temperatures that are out of spec for a standard, non-condensing oil fired appliance. That is to say, the burner would be adjusted to ensure the flue gas temperatures were not low enough to permit condensing. It is my experience that 350 degrees F is generally considered the lowest recommended temperature.

    To answer your question more directly, you shouldn't have to worry about flue gas condensation as long as your burner has been set up correctly.
  • billtwocase
    billtwocase Member Posts: 2,385
    Also, what boiler do you have? Some that have coil or blank plates with a gasket don't like going cold. They leak
  • Hilly
    Hilly Member Posts: 427
    I know icesalior used to mention it a lot... Cold Starts from his experience required a lot more cleaning to keep the efficiency up. With that said it does depend on the boiler. I know the last couple of Buderus I installed the manual said that condensing wasn't an issue if you could get up to 130-140F (I cannot remember the number) within X minutes. This way there was no issue. So I gathered at that point that it's prolonged runtime at low temperature that is of most concern. I may stand to be corrected.

    So in short cold starts are fine but the system needs to be setup properly to protect the boiler.
  • Jason_13
    Jason_13 Member Posts: 304
    The condensing of cast iron boilers happens at most start-up of the firing cycle. After it starts to warm up the boiler dries out in the flue passes. The time is determined by the size of the boiler and radiation type and water volume. You never want to shut down a cast iron boiler before the sections dry out or the acids will continue to affect the iron.
    This is usually controlled by a simple boiler bypass and higher boiler delta-T.
  • dh1989
    dh1989 Member Posts: 22
    I was referring to flue gas condensation. Stack temperature and drying of the flue passes answers my question. It does not short cycle from a cold start as it is set up. It also does not leak when cold and uses no water (no pressure loss with the fill shut off over time).

    System is a Utica Statfire with hot water baseboard.
  • enalkarion
    enalkarion Member Posts: 7
    At my parents house, my Dad thought he was saving money by turning the boiler off in the summer unless they needed hot water. (thermotron2) summer/winter hookup ie. hot water coil, now the flange the coil mounts to is rotting away, I had to make an emergency repair by cutting all the welded studs off (which were snapping as I tried to take them off to replace the gasket) and drilling, and cutting away sheet metal to get access to bolt it all back together. now they need a new boiler. I wouldn't be too worried about the flue gasses condensing as much as some boilers do not react well to the extreme expansion/contraction that occurs. if its a summer/ winter hookup and its warm out just lower the aquastat settings. ( I may be rambling and a little off topic lol)
  • Paul48
    Paul48 Member Posts: 4,469
    I'd only worry about running for extended periods with cold return temperatures in a high volume system, such as a gravity conversion. Baseboard systems will quickly take the return temperatures out of the range that could cause flue gas condensation.
  • hot_rod
    hot_rod Member Posts: 22,271
    A poll taken a number of years ago showed that most agreed that within 10 minutes the non condensing boiler should be warmed up above dew point. A few factors determine that exact number, typically 130- 140F return boiler temperature.

    If you have a high mass system as Paul mentioned you can install boiler protection functions, mixing valves, or control devices to limit pump operation.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream