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Thread: Power Vent fresh air mixing intake question...

  1. #1
    DIY Senior Member Murphy625's Avatar
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    Default Power Vent fresh air mixing intake question...

    My 50 gallon Power Vent water heater has a fan on top that sucks in room air and mixes it with the hot exhaust gases before sending the mixture down the pvc pipe to the outdoors.

    When the unit is not running, I can feel hot air coming from the fresh air intake port. This port has the same diameter as a 3 inch PVC pipe.

    I am concerned that I am losing money via the thermo-siphoning effect and I am wondering if anyone has ever attached a PVC pipe and elbow to that fresh air intake port to stop the heat loss.

    It would require an 8 inch horizontal extension pipe, then an elbow pointed towards the floor, then maybe another 4 to 6 inches of pipe. Not much of a restriction on that short of a pipe.. I could probably also go to a 4 inch if that was thought to be an issue.

    Anyone?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Generally, unless the manufacturer suggests this as an acceptable modification, it should not be attempted. The things are certified to work safely when installed per their instructions. To be safe, you'd have to run that by them, not some opinions that would have no standing with the inspectors or certifying agencies, or more importantly, your ultimate safety.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    Where are you attaching the fresh air port to with your intended duct? And why, exactly?

    The reason hot air is coming out of the fresh air duct is that the air in the center-flue heat exchanger is being heated by the water making it more buoyant than the cooler room air, causing a convection flow, pulling room air into the heat exchanger down by the burner, with warmer air escaping at the air intake of the blower. This is a standard standby loss of all conventional tank heaters. In a cool climate like MI that escaping heat is not 100% wasted, since it accrues to the conditioned space, lowering the heat load of that room 3/4 of the year. If you port it to the cool air near the floor it still doesn't stop the convection from occurring, but will slow it slightly (it would slow it more if you insulate the down-piping.)

    In most installations you get as much or more of standby loss from the near-tank plumbing and distribution plumbing than you get from the convection losses at the tank. IRC 2012 now requires R3 minimum pipe insulation on all hot water distribution plumbing. The cheap 3/8" wall closed cell stuff you get at box stores is only R2. You can order 5/8"-3/4" wall ~R4-ish goods online if you are retrofitting if you can't find it at a plumbing supply house. Start there, since that doesn't affect any of the tank design/engineering.

    If you place too high an impedance on the room-air intake of the blower the exhaust temp may rise to unacceptable levels for PVC vent piping, with potentially catastrophic results. MEASURING the post-blower gas temp prior to any modifications and checking it later would be a worthwhile sanity check if you take that route.

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    Moderator & Master Plumber hj's Avatar
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    That would have absolutely no effect on the air circulating through the water heater. Doesn't it have a motorized damper to stop the air flow.?
    Licensed residential and commercial plumber

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    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by hj View Post
    That would have absolutely no effect on the air circulating through the water heater. Doesn't it have a motorized damper to stop the air flow.?
    Why do you say that? Mathematically, air and water are no different from each other.. People install heat traps (upside down "U" pipe" on the hot and cold lines all the time...

    This is nothing more than the same principle to stop the convection currents from the center flue pipe. (which is where the hot air is coming from)

  6. #6
    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by Dana View Post
    Where are you attaching the fresh air port to with your intended duct? And why, exactly?
    I just want to add about 18 inches of pipe with an elbow pointed down so to create a "heat trap" for the air that is rising out of the center flue tube.

    The reason hot air is coming out of the fresh air duct is that the air in the center-flue heat exchanger is being heated by the water making it more buoyant than the cooler room air, causing a convection flow, pulling room air into the heat exchanger down by the burner, with warmer air escaping at the air intake of the blower.
    Yes, I know. I want to see if I can stop it.

    This is a standard standby loss of all conventional tank heaters.
    And its a significant loss.. That's a lot of warm air coming out of that thing.


    In a cool climate like MI that escaping heat is not 100% wasted, since it accrues to the conditioned space, lowering the heat load of that room 3/4 of the year.
    That is true but I use wood to heat my home so I'd rather try to make it as efficient as I can.

    If you port it to the cool air near the floor it still doesn't stop the convection from occurring, but will slow it slightly (it would slow it more if you insulate the down-piping.)
    It should stop it completely just like an upside down "U" heat trap on the cold water inlet line... I understand there is nothing I can do about the convection going up through the fan and out the PVC vent pipe, but at least I can stop the flow out the fresh air intake port on the side of the fan. Since the PVC vent pipe has a 20 foot horizontal run that drops almost 2.5 inches, that should slow it down on that side considerably.


    In most installations you get as much or more of standby loss from the near-tank plumbing and distribution plumbing than you get from the convection losses at the tank. IRC 2012 now requires R3 minimum pipe insulation on all hot water distribution plumbing. The cheap 3/8" wall closed cell stuff you get at box stores is only R2. You can order 5/8"-3/4" wall ~R4-ish goods online if you are retrofitting if you can't find it at a plumbing supply house. Start there, since that doesn't affect any of the tank design/engineering.
    I know about the losses from the distribution piping.. I have "U" type heat traps on the pipes connecting the water heater and they are insulated so losses there are minimal.


    If you place too high an impedance on the room-air intake of the blower the exhaust temp may rise to unacceptable levels for PVC vent piping, with potentially catastrophic results. MEASURING the post-blower gas temp prior to any modifications and checking it later would be a worthwhile sanity check if you take that route.
    I strongly agree. In fact, I think I might take it one step further and stick a u-tube manometer on there also to see if it develops any pressure drop after I modify it.

    Thank you for the reply and suggestions....

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