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direct vent water heater

apiersma | Posted in General Discussion on December 15, 2010 11:11am

I just installed a direct vent water heater.  It is not a sealed combustion water heater, and the blower on top of the unit sucks in indoor air to vent out the exhuast through PVC pipes outside.  I feel like it must be forcing in significant amount of outdoor air to make up for it.  If it matters, I also have a 95% efficient furnacne with intake and outtakes coming from outside.

Can I just create an outdoor intake by running pvc to the inlet to the blower? The insatall manuel doesn’t mention this, so I wonder if the blower can handle the extra piping. I currently use 2″ piping and am just under the permitted 40ft, but if I went to 3″ PVC I can go up to 120 feet.

 

http://www.searsoutlet.com/40-gal-Tall-Natural-Gas-Hot-Water-Heater-33204-/d/product_details.jsp?md=ct_md&cid=1279&pn=1&ps=10&pid=17722&mode=buyUsedOnly

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  1. DanH | Dec 16, 2010 12:04am | #1

    Well, you should generally have an outdoor air feed to the area of the combustion appliance.  But given that a water heater uses relatively little combustion air, obtaining a tight fit with the outdoor air feed is unnecesary, in addition to being hazardous.

    1. apiersma | Dec 16, 2010 10:00am | #3

      Sorry, I think I have given you all a poor description.  I am not concerned with the pull of combusiton air.  My water heater blower pulls in a lot of indoor air into the vent directly, not the combustion chamber.  I simply want to know if I can can pipe in outdoor air into that vent intake to lesson negative pressure  in the house.  Its an old 1950 cape cod that will always be a bit drafty so the less negative pressure the better.  Does that make sense?

  2. rdesigns | Dec 16, 2010 09:56am | #2

    You have a power-vent WH, not a direct-vent WH. The difference between it and a conventional gas WH is the exhaust blower on top, which is what allows the vent to be made of pvc instead of metal. It accomplishes this by pumping in enough cool air (dilution air) to reduce the temperature in the flue to the point where it won't melt PVC.

    This means that a power-vent WH needs more outside air--the usual amount of combustion air AND a lot more dilution air. Conventional WH's also need dilution air, but they get thru the draft hood on top by natural convection, not from a powered exhaust blower.

    In your situation, as Dan stated, there is probably a source of outside air ducted into the furnace/WH room, and it used to supply both the furnace and WH with combustion air. Since you went to a direct-vent, sealed combustion furnace, the entire amount of outside is now available to the WH. I would say, let it stay the way it is unless there's more you haven't yet told us.

    1. apiersma | Dec 16, 2010 10:08am | #4

      Thanks

      I was wondering about the difference between power vented and direct vented.

      There is no source of outside air ducted into the basement beyond leaky rim joists. I have been slowly sealing them up to gain effciencies. I should add that we have a fireplace upstairs that we like to use and since the install of the WH my house has been a bit smokey.  My thought is to simply draw the air directly from the outside to reduce fireplace backdrafting and avoid cold air pulling into the basement mixing with warm air before some of it being pulled back out though the power vent of the WH.

      1. rdesigns | Dec 16, 2010 12:27pm | #5

        If you can run a 4"

        If you can run a 4" combustion air duct into the WH enclosure, that would help. First, by delivering the air close to where it's needed, you will reduce air infiltration in other places where you want more warmth. Also, it will help the fireplace slightly, by alleviating the de-pressurization caused by the power vent. That alone, however, may not be enough to keep the fireplace from smoking under certain atmospheric condidtions. But, at least with a fireplace, you immediately know if there's enough draft, and you can open window a little, if needed.

  3. Clewless1 | Dec 16, 2010 09:17pm | #6

    We used to discuss this issue back in the 90's when trying to comply w/ energy codes. The conclusion was that if you direct connect ducting to a device, you will void the UL listing of that device since the connection you propose has not been UL tested by the manuf. If the manuf. offers a 'conversion kit' of sorts, then they probably have had it UL tested.

    While on the surface your modification may appear benign, I'd take care.

  4. User avater
    BossHog | Dec 17, 2010 08:09am | #7

    I wonder if you'd be better off to pipe makeup air into the return air vent for your furnace. That way it would make a trip through the filter & the furnace before being spread around the house.

    1. rdesigns | Dec 17, 2010 09:21am | #8

      That's a good idea if the WH enclosure has a grille or some easy way for the air in the rest of the house to get in to the WH.

      Also, I would make that a 6" duct connecting the outside air to the return, and have a manually-operated damper on it so that it can be adjusted to meet the need.

      The downside is that anytime the furnace blower runs, summer or winter, the return is drawing in outside air quite powerfully, and the amount of air may be far more than what the WH actually needs for combustion/dilution. On the positive side, however, indoor air quality is likely to be improved, and there's less chance for his fireplace to backdraft.

      1. apiersma | Dec 17, 2010 04:28pm | #9

        what could go wrong ?

        By connecting a 3" PVC pipe to pull outside air into the blower , which then blows it right back outside with the exhaust gases.  Pressure would be neutral, and assuming i don't work the blower too hard with a long pipe and lots of elbows, I should be fine. Correct?

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