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

Discussion in 'HVAC Heating & Cooling' started by gypsydoctor, Jan 25, 2013.

  1. gypsydoctor

    gypsydoctor Retired Software Engineer

    Messages:
    5
    Location:
    Newton, MA
    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?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    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.
  3. gypsydoctor

    gypsydoctor Retired Software Engineer

    Messages:
    5
    Location:
    Newton, MA
    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: Jan 25, 2013
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,006
    Location:
    New England
    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.
  5. jimbo

    jimbo Plumber

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    8,997
    Location:
    San Diego
  6. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    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 100°F vs. your old oil boiler at 450°F +, 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 140°F 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.
  7. Dana

    Dana In the trades

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    Location:
    01609
    As others have pointed out, the outdoor temps have been very low, and the output temp of a mod-con exhaust is low enough that it doesn't take much time/space for the remaining water in the jet of exhaust jet to condense or even freeze when the outdoor temps are in single-digits. The dew point of typical mod-con exhaust right at the burner is ~125F, but the heat exchanger can only lower it to a few degrees above the temp of the return water from radiation entering the boiler. The water condensing in the heat exchanger lowers the dew point of the remaining exhaust that is exiting the heat exchanger on it's way outdoors. With 130F return water very little water condenses in the heat exchanger (maybe even none, since the "max efficiency" indicator goes out when the return water hits 127F) but it's a low enough temp that it's 10F outside it doesn't take long for that exhaust to hit the ~125F (or is it 127F for this one?) dew point- you'll see quite a bit of condensation forming in the air a few inches to a few feet off the vent cap as it hits the chilly air.

    On a warmer day you'd have cooler radiation return which results in more condensation inside the boiler, with less left in the exhaust products, and a much smaller delta between exhaust temp and outdoor temp, so the exhaust quickly dilutes with the outdoor air as it cools resulting in little to no visible condensation plume.

    Contrary to what Jim stated, the outdoor dew points have little bearing on whether you see a plume or not, but the lower the outdoor dew point the LESS plume you'd see at a given outdoor temperature, since the diluting air is much drier. It's the dew point of the exiting exhaust gas relative to the outdoor temps that is the primary determinant of how much plume you'll see, but when the outdoor dew point is higher, closer to the outdoor temp, the more you get. (When the outdoor temp equals the dew point temp you have fog and rime-icing, and anything coming out the vent will be condensing instantly into a very thick fog.)

    Jim is dead on when he says it's probably oversized, but not necessarily correct about the efficiency consequences. The ALP150 would be a monster-sized boiler for most houses in Newton, unless yours is an uninsulated leaky a 5000' 19th century type of antique. But the high thermal mass of the radiators should keep it from short cycling if the radiation is so oversized that it's able to actually deliver the heat with sub-140F water when it's 10F outside (which is about your 99% outside design temp.) If the burns are typically 5-10 minutes or longer it's a good sign, and properly tweaked you can probably get long efficient low temp burns out of it at the boilers minimum-modulated output even with 100F water, and 90F returns, temps at which there isn't much more efficiency to be had by going lower.

    And yes, lowering the speed on the secondary loop increases the delta-T, lowers the return water temp, both of which would seem in order here. (A 10F delta is pretty small, but not ridiculously small.)

    Key to maxing out the efficiency is to set up the outdoor reset curves to the absolute lowest temps that actually meets the heat load, which can be figured out by programming it ever lower until it doesn't keep up. The particular steps and user interface will vary somewhat from manufacturer, but all outdoor reset controls have adjustable curve parameters. Read up on it in the manual.
  8. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Interesting to note, that a very high stack temperature will result in little more than a mirage, even though all of the water vapor is still venting out the stack, e.g. a good hot wood or oil fired stack.
  9. Dana

    Dana In the trades

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    Location:
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    Yup- when the stack temp is 135F rather than 435F it cools to condensing levels before it's diluted enough in the air to not show a plume. At 135F at mod-con mixtures it's VERY close to saturation already.

    More than just a stack-temperature issue, but an excess-air issue as well. Oil burners & wood burners usually have a fair amount of excess combustion air- it's not as tightly controlled as with a mod-con, which are intentionally controlled to keep the exhaust product's dew point as HIGH as possible, whereas other burners are designed to prevent condensation where it might do some damage- excess air lowers the dew point of the exhaust. Most oil burner venting includes barometric dampers to feed-in dilution air prior to the flue, further lowering the dew point to mitigate against flue condensation. Atmospheric drafted gas-burners usually provide that dilution function with a draft-hood right at the exit point from the unit.
  10. gypsydoctor

    gypsydoctor Retired Software Engineer

    Messages:
    5
    Location:
    Newton, MA
    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: Mar 14, 2013
  11. gypsydoctor

    gypsydoctor Retired Software Engineer

    Messages:
    5
    Location:
    Newton, MA
    At the moment I'm seeing a substantial plume. The outdoor temp is 37F and dew point is 4F. Boiler supply is 121F and return is 109F and it's running in condensing mode according to the LCD display. I'm tempted to conclude that the presence of a plume does not mean that the unit is not operating efficiently.

    Thanks for that suggestion. I have gradually been coming to the same conclusion. By the way, there is a new version of the Alpine manual here: http://cdn.usboiler.net/products/boilers/alpine/assets/AlpineIO.pdf
  12. Dana

    Dana In the trades

    Messages:
    2,840
    Location:
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    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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