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Community and Q&A

Deciding between power vent and chimney water tank

user-4053553 | Posted in Energy Efficiency and Durability on

My current water tank is rusting, rental company screwed up and it rusted prematurely, when i replaced my chimney vent furnace with high efficiency i was told the “orphan” water tank may have back drafting issues which did not happen, but the water tank anode was already gone and it was rusting internally. I figured when the water tank was getting old 5-10 years from now i would go with dual pipe power vent but it died prematurely.
want to get off renting because owning is much cheaper in the long run, but there is a $500 differential installed between power vent and conventional. Since money is tight i am wondering over a new tank’s expected lifespan how much heat will go up the chimney if i go with conventional over power vent, will the lost heat (at 95%) cost more then $500?
The chimney is 6 inches wide (metal/”mineral” coming out the roof) and runs up the center of the house.

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  1. GBA Editor
    Martin Holladay | | #1

    The problem of "orphaned" water heaters connected to oversized flues is a real problem. Here is a link to an article with more details: When a Flue is Too Big."

    Here is a link to an article that gives you an overview of water heater choices: The Water Heater Payoff.

    There is no simple answer to your question about fuel savings and payback -- the answer depends on your fuel cost and how much hot water your family uses. The bottom line, however, is that atmospherically vented water heaters have lots of disadvantages and potential problems, including backdrafting, and that you have the wrong type of flue. This type of water heater is obsolete. You should get the power vent model.

  2. Dana1 | | #2

    The orphaned water tank issue both a potential backdrafting issue, but it also degrades the ceramic liner & chimney masonry with acidic exhaust condensation. The flue is many times oversized for the heat input of the exhaust, which leads to much lower stack velocity, and when combined with the large surface area of the oversized flue for a condensing surface, it just accumlates. The condensation occurs primarily during the cooler seasons, but the acid erosion goes on year-round, since it doesn't go away once it's in the ceramic & mortar.

    When you get the power vent water heater you can then seal up all of the old chimney penetrations-brick 'em up, or use tight fitting metal air barriers sealed to the masonry with mortar. That will eliminate a 24/365 stack effect infiltration driver, reducing your net heat load.

  3. user-4053553 | | #3

    Thanks for your replies :)
    @ Martin, i will read your links when i get a chance later today
    @ Dana My chimney has no mortar, its metal, runs up the middle of the house so is always room temperature until the attic and the water heaters are not condensing, though of course if money were not an issue i was originally planning to go with power vent when this one died in the future (if it were not defective and died early).

  4. user-2890856 | | #4

    Alan ,

    Is this a single B Vent flue that used to have 2 appliances venting through it ? If so Dana is right on the money whether it be masonary or metal (worse) .
    If the B Vent served 2 appliances it should have been sized for 2 appliances , removing one will leave a condition whereas the one remaining cannot raise the temp enough to NOT CONDENSE and draft properly , leaving you at risk . Room temperature , if in fact that is the case is no hedge against a dangerous situation .

  5. Expert Member
    Dana Dorsett | | #5

    If the flue is stainless the flue will survive, but an oversized B-vent is a disaster! With an oversized stainless flue you'd still have some risk of condensate running down the interior of the flue and finding it's way out somewhere, and the risk of backdrafting doesn't go away.

    There is no value to keep using an inappropriately sized flue of any type- it's just another liability.

    Any atmospheric drafted combustion equipment is also a liability not worth owning if you intend to air-seal the place to 3ACH/50 (the IRC 2012 code-maximum air leakage), which is usually worth it.

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