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  1. #1
    Mike Guest

    Default Flue pipe condensation

    Looking for advice on how to eliminate condensation in a flue pipe for a 40 gal. residential hot water heater. Current configuration is as follows: 3" galvanized flue pipe approx. 32" connects draft hood of tank and connects to a 3" I.D. heavy pipe (maybe steel or cast iron) which penetrates finished interior ceiling, through unheated attic crawl and the roof. Pipe extends approx. 24"-32" above roof line. All pipe is vertical. Condensation forms in heavy pipe and pours down onto tank in significant amounts. Can flue pipe be re-done to avoid this problem and what materials/configuration would solve the problem. Told customer we may have to replace tank with power vent type out the sidewall of the house.

  2. #2
    Mike O'Handley Guest

    Default Re: Flue pipe condensation

    Hi Mike,

    The 3" flue from the draft diverter should connect to a B vent before it passes through that ceiling and roof membrane. I've never seen anyone connect one of these to a cast iron vent. How long is the total run of vent? Is it within manufacturer's specs for length? If longer than the maximum allowable length, it may be necessary to replace the unit with one with a power vent, in order to push that moisture-laden exhaust out of the exhaust vent quickly enough so that it hasn't time to condense in the vent.

    Recommend you contact the specific manufacturer for that water heater and get some additional input.


    Mike O'Handley

  3. #3
    Mike Guest

    Default Re: Flue pipe condensation

    Thanks Mike for your speedy reply. Total run is 7' vertical. Contacted manufacturer and they would not advise on flue because of local codes, etc. Contacted local building dept. and was advised to replace tank with power vent type as proper flue materials to conform with code were hard to find and expensive. Thanks again for your valuable insight.

  4. #4
    Ken Wiggers Guest

    Default Re: Flue pipe condensation

    The mass of the "heavy duty" pipe is too much for the flue gases to heat up and thus you get the condensation. I am of firm conviction that naturally aspirated combustion appliances should be banned -- there is too much risk in having them not vent properly. Power venting is the best way. We found that a house that we built several years ago would be depressurized to 3.5 pascals by running one bathroom exhaust fan. It only takes 1 to 2 pascals to make a water heater backdraft. The running of all exhaust appliances (bathroom fans and clothes dryer) depressurized the house to about 37 pascals.

  5. #5
    The Gasman Guest

    Default Re: Flue pipe condensation

    Better late than never, but, I wonder why anyone would want to ban naturally aspirated gas appliances, which are prone to spillage due to depressurization problems, by putting in power vented units? That wouldn't alleviate the depressurization problem, it would make it worse, wouldn't it? Or am I missing something here?

    Personally, I prefer balanced flue, sealed combustion systems, when I'm looking at a potential problem with natural infiltration air.


  6. #6
    Ken Wiggers Guest

    Default Re: Flue pipe condensation

    I should have said "sealed combustion appliances" rather than "power venting." Sealed combustion appliances are in a sense "power vented." I suggest "banning" naturally aspirated combustion appliances because of the inherent uncertainty about the certainty that they will exhaust combustion gases properly. Vehicles could not be sold (and if sold there would be a massive recall) that had the degree of uncertainty (of venting properly)that naturally combustion appliances have. Sealed combustion appliances would remove the "uncertainty." Sealed combustion units are preferable because the air consumed for combustion is derived from the outdoors -- not the inside. A power vented unit would be preferable to a naturally aspirated combustion appliance -- the flue gases from the combustion appliance would be vented with certainty. A power vented (excluding the sealed combustion units) would tend to depressurize a house -- probably a better option than having combustion gases enter the house. Depressurization can enhance radon entry and other undesirable gases, pesticides, moisture, etc. into the living space. A very airtight house can be slightly pressurized easily to keep undesirables from the soil from entering the house.

  7. #7
    The Gasman Guest

    Default Re: Flue pipe condensation


    Hate to disagree with you sir, but, there aren't too many sealed combustion, power vented, domestic water heaters, gas ranges, or dryers the market yet. So for the time being, we're pretty well stuck with naturally aspirated units in those area's.

    Of all of the power vented and high efficiency sealed combustion systems I have encountered on the market today, there isn't one I can rubber stamp as something I would trust to work reliably for a reasonable period of time, as compared with an old fashioned naturally aspirated or conventionally vented gas appliance. I can point out hundreds of conventional gas appliances that have been around for 20, 25, 30 years with few service problems. I can't point at one sealed combustion, power vented system and say the same thing.

    Well, perhaps the Lennox Pulse? No, can't use that as an example, they don't make them anymore. Maybe the ICG " Ultimate" nope, out of production. Clare? No, out of business. The list of power vented systems that have hit the markets and disappeared is staggering when you really look at it.

    Ban the conventionally vented gas appliance? I don't think so. Ban the high efficiency garbage that's out there? Perhaps, until they get their act together. When are we going to stop embracing technology for the sake of technology and start using some good old horse sense?

    Ken, this is my opinion on a serious reliability issue in the HVAC industry at the present time. I've been in this business for over 25 years and I've worked for a number of gas appliance manufacturer's, so I do know a little bit about the R&D side of things. If you want to read more about my views on power vented gas appliances, you can click on the link below.


    Power Vented Central Heating Systems:

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