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How much condensation is too much?

tyvektent Member Posts: 3
Have an older (12-15 yr old) Dunkirk BV series 3-section cast iron oil boiler (specifically DPFO-3), setup with an indirect fired DHW tank. It was originally setup with a triple-point aquastat, keeping the boiler hot 24-7 through the summer, as if it had a tankless, which seemed rather wasteful, since it was really using an indirect tank. So we switched it to a high limit only aquastat, thus making it operate in cold-standby mode.

The original instruction manual warns against cold standby operation and calls for the installation of a thermostatic union and bypass to help the boiler get up to temperature faster if you are going to run it this way. Spoke to Dunkirk tech support twice. The first tech, said I should definitely have boiler protection and that I should set it up with a thermostatic mixing valve on the return of a boiler bypass. Second Dunkirk tech said not to worry about it because the water content of my distribution system is low.

My distribution system holds maybe 6 gallons of water and my boiler another 7 so only 13 gallons in all. On a warm day (70 degrees F outside) it takes about 10 minutes of boiler run time to get the return up to 140 degrees f on a heating call from a "cold" start.

So who is right, the tech who says I need a bypass controlled by a thermostatic mixing valve or the one who says not to worry about it?



  • Zman
    Zman Member Posts: 7,577
    How long?

    How long does the cycle usually last after you reach 140?

    The key is that the boiler and the flue need to be up to temp in order to dry the condensate before it shuts down. The length and location (heated space) of the flue also play a role.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • billtwocase
    billtwocase Member Posts: 2,385
    If you have an indirect

    the boiler will have temp on it at all time, and technically won't be a cold start situation
  • tyvektent
    tyvektent Member Posts: 3
    runtime above 140

    It depends on the weather. These days in late October, it may only run a minute or two after getting to 140 on the return, or it may actually have a total run time of only 5 minutes and never get to 140 on the return.
  • tyvektent
    tyvektent Member Posts: 3

    To some degree this is true, the indirect zone starts out a little warmer, but it typically only seems to run 5-10 minutes on a DHW call. Seems to take about 5 minutes to get up to 140 return on a DHW call vs 10 minutes for a heating call.
  • icesailor
    icesailor Member Posts: 7,265
    Cold Starts:

    In my opinion and experience, all cold start boilers will make kibbles and bits when they are running cold. Some worse than others.

    I'd put the triple acting aqua stat back on and set the operating control to 135 to 140 degrees and the high to 160 degrees. If in the middle of the winter, the house doesn't keep up, raise the high limit. It won't use jack schitt for oil compared to when it gets insulated with Kibbles & Bits.

    That's my story and I'm stickin' to it.
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