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Thread: What are the conditions that cause an exhaust plume from my mod-con boiler?

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    Retired Software Engineer gypsydoctor's Avatar
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    Default What are the conditions that cause an exhaust plume from my mod-con boiler?

    I've just gotten my new Burnham Alpine 150 boiler going in the last few days and I'm trying to get it set up properly. My installer's attitude is pretty much that the standard settings are just fine and I'm happy to experiment with them on my own. I'm in Newton, Mass and my antique house has old-fashioned cast-iron radiators. The outdoor temp since I have installed has been in the 5-20F range.

    I have several questions, but let's start with the "smoke" plume, which is more than I expected. It seem to me that if the unit is condensing, there should be little moisture in the exhaust to cause a plume. At the moment I have a setpoint of 139F and return temp maxing out around 130 degrees. Each cycle tends to run about half way indicating "Max efficiency" but seems to drop that indication when the return temp hits about 127. I'm still getting a pretty good plume.

    What conditions contribute to the amount of plume?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The dew points the last few days have been below zero, so any moisture left will create a condensation cloud. May be normal. Although, it does sound like you have a much bigger boiler than required. If it is keeping the house comfortable with the outside temps we've been having with only 139-degree water, it's probably too big, maybe by a factor of two and likely more, which means it will never reach max efficiency. If properly sized, it's likely when it's near zero F out, it couldn't keep the house warm unless the water temp was higher.
    Jim DeBruycker
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    Retired Software Engineer gypsydoctor's Avatar
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    Thanks, Jim.

    I had the unit installed in a big hurry because my oil tank sprung a leak at the worst time. Let's just say the planning was not done optimally. I suspect your guess of "too big by a factor of two" is pretty accurate.

    I changed the "Central Heat Maximum Modulation" from 5500 to 4850 RPM, the value for the 105 MBTU model. The manual says to change this setting if the unit is too big for the amount of radiation. It did not seem to make much of a difference. Do you think this affects things much? I suppose it affects the resolution of the modulation since the minimum speed is fixed. Maybe I will try a lower setting.

    I have a three-speed Grundfos circulator on the secondary loop (one zone) set to medium. How does this setting affect the return temp? Should I use low speed?

    Do you know how the burner meters the gas flow to match the air supply as the fan changes speed?

    Mark
    Last edited by gypsydoctor; 01-25-2013 at 08:35 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If the water goes through slower in a secondary loop, it should return cooler since it has more time to release heat and there's less heat going in. I do not know how the gas valve modulates to maintain a proper air mix.
    Jim DeBruycker
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    Plumber jimbo's Avatar
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    Extraordinary weather causes strange things to happen, as : http://pittsburgh.cbslocal.com/2013/...y-power-plant/

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    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    The more it runs the more you save and the more comfortable you will feel.

    A modulating, condensing boiler (ModCon) is the perfect match for cold cast iron radiated houses. Here in Minnepolis we regularly cut fuel bills in half. The plume is by-and-large, water vapor, and a normal byproduct of any fuel conversion, likely at 100F vs. your old oil boiler at 450F +, which may very well "burn" with a clear flue discharge but in reality is much dirtier with excessive SOx, NOx and particulates never found in a gas-fired condensing boiler.

    Most of the time your radiator will run below body temperature and only in the coldest weather get to say 140F depending on original construction and subsequent updates in insulation, window and weather stripping.

    You should have the unit set up by a trained professional used a combustion analyser before playing with controls.

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    Retired Software Engineer gypsydoctor's Avatar
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    Quote Originally Posted by jadnashua View Post
    If the water goes through slower in a secondary loop, it should return cooler since it has more time to release heat and there's less heat going in.
    Since there are primary and secondary loops, wouldn't slowing the secondary loop just let more of the flow of the primary loop get back to the boiler, increasing the return temperature?

    Aside from the efficiency issue, I find that things are much quieter with the circulator set at Lo speed. At Medium and Hi speeds I get noise through the floor into the room above, and also some water noise from the radiator in the bedroom.
    Last edited by gypsydoctor; 03-14-2013 at 09:15 AM.
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    In the trades Dana's Avatar
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    Quote Originally Posted by gypsydoctor View Post
    Since there are primary and secondary loops, wouldn't slowing the secondary loop just let more of the flow of the primary loop get back to the boiler, increasing the return temperature?

    Aside from the efficiency issue, I find that things are much quieter with the circulator set at Lo speed. I get noise through the floor into the room above, and also some water noise from the radiator in the bedroom.
    At minimum fire you're kind of stuck with that truth (another reason why you don't want to oversize the sucker 3x like this one probably is.) But if it's in a modulating region it will vary. Either way, you have to design the correct flow for both loops to optimize efficiency.

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