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Thread: What's in the "clouds" emitting from gas boiler power vent?

  1. #1
    DIY Junior Member rrraven's Avatar
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    Question What's in the "clouds" emitting from gas boiler power vent?

    Last week during the cold snap (of course!) our ancient boiler died, and after a week in a freezing house I got it replaced with a gas Lochinvar Cadet boiler. Our previous chimney venting was replaced with power-venting through the wall for this high efficiency boiler. The contractor I chose said it was not permissible to vent to the side (though the others had planned to) because the neighbor's driveway abuts the house, and freezing condensate might cause a safety slipping hazard. Instead they vented to the front of the house, where billowing clouds now are emitted from below the window obscuring the view and causing the neighbor to complain about the "fumes" which blow over to his front door.

    I'm guessing that most of what's visible is actually water vapor, but that some carbon monoxide and other combustion biproducts are probably also present. Aside from the loss of view through my favourite window, are there any health/safety concerns, for us or the neighbor? The vent is the required distance from the window (18 inches?) but the clouds clearly come right up past it. Perhaps the invisible CO somehow dissipates more quickly than the visible water vapor? Seems like with the move to power venting this must have come up before, but my searches have not found answers.

    Thanks!

    rrraven, Toronto, Ontario

  2. #2
    In the trades Dana's Avatar
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    The CO content is extremely low if the system is up-to-snuff and burning efficiently & cleanly. A high CO content would be an indication that something was wrong with the boiler causing incomplete combustion.

    The primary combustion products from the burning methane with atmospheric oxygen (CH4 + O2) are CO2 & H2O, neither of which is very dangerous to humans unless highly concentrated. There are smaller amounts of NOx, (since N2 is the primary component of the air, and not easily separated from the air) but it's not in concentrations high enough to be concerned about. The vast majority of the N2 present in the air entering the combustion chamber is not at all involved in the combustion, and is still present in the exhaust as N2. By contrast the majority (but not all) of the O2 present in the combustion air combines with the CH4 (releasing chemical energy in that reaction as heat), and the O2 content in the exhaust is pretty low. Unless it's being vented into a very confined space it's not particularly dangerous, since the dilution with outdoor air is very high.

    To lower the amount of H2O in the exhaust, adjust the programmed "outdoor reset" curve to the lowest water temperature on the system that still keeps up with the space heating load. Avoid using overnight temperature setbacks on the thermostat, since recovery ramps from setback require higher water temps in the system. Condensing as much of the moisture in the boiler's heat exchanger as possible has a very positive impact on efficiency (it's the very point of designing condensing boilers), but the strong secondary effect is that the amount of moisture blowing out the vent pipe to form those annoying view-obscuring clouds is also dramatically reduced. When the return water from the radiators/baseboards entering the boiler is above ~125F/52C almost all of the water formed in the combustion goes into cloud formation, whereas when that water is below 110F/43C at least half of it condenses out inside the boiler, delivering the "heat of vaporization" to the heating system water rather than condensing when it hits the cold outdoor air, ticking off your neighbor.

    Consult the manual (or ask the installer, if they seem competent on the subject) on how to adjust the outdoor reset temperature curve for the Cadet series boilers.

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    DIY Member Gordan's Avatar
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    Dihydrogen oxide is quite dangerous - as little as a cup of it is enough to cause death in dozens of laboratory mice!

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    In the trades Dana's Avatar
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    Yeah, but that's a highly concentrated dose, and much higher dose that necessary for killing a mouse (a fully inhaled teaspoon or so would be more than enough to be fatal to a mouse, eh?)

    Most humans would have to inhale a liter or two at that concentration to be fatal, although inhaling a cup wouldn't be very pleasant. You have to burn quite a bit of methane to make a liter, even if it weren't being diluted by the atmosphere.

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    DIY Junior Member rrraven's Avatar
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    Thanks Dana, for that terrific explanation. Very helpful!

    I am still trying to get the manual, which I presume should have "come with" the boiler. The contractor I hired was very knowledgeable, but I guess I shouldn't have been surprised that it was not him but his "guys" that actually did the installing, and they were not able to answer questions. I thought he was coming back for the final check, but apparently that is going to be yet more "guys"!

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    DIY Junior Member rrraven's Avatar
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    Gordon, now you have me worried. Our new boiler also leaks onto the boiler room floor, and I am pretty sure it is dihydrogen oxide! Now I am concerned for our mice!

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    In the trades Dana's Avatar
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    Quote Originally Posted by rrraven View Post
    Gordon, now you have me worried. Our new boiler also leaks onto the boiler room floor, and I am pretty sure it is dihydrogen oxide! Now I am concerned for our mice!
    Most installations also include a condensate pump to dispose of it properly in a drain rather than letting it drip on the floor endangering the rodents. It's not pure H2O, it is mildly acidic (dilute carbonic acid at a pH comparable to red wine), and will degrade concrete over time if you just let it puddle. It's not strong enough to attack most drain plumbing, and if it's a drain that's also used by say, a clothes washer, the dilution factor is large.

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    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Most of our condensing boilers are piped to a nearby floor drain and all emissions can be considered non-hazardous. Low NOx, low SOx, low to no CO and out the drain a carbonic acid concentration with a pH of 4. Unless you are afraid of CO2 you have no worries. The slip hazard is only present in rare cases and where the installation manual is ignored...OK it happens.

    Now for the vapor plume. With all the options for venting a condensing boiler (the Lochinvar Cadet is one of the most versatile for venting) your situation is intolerable. Make him move it.

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