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Thread: Replacing electric with NG.... need some advice..

  1. #1
    DIY Senior Member Murphy625's Avatar
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    Default Replacing electric with NG.... need some advice..

    My ranch style home in the country with well water has a 40 gallon electric water heater. Even though we have a special "reduced rate" electric meter for it, it still costs north of $30 to $40 a month to run it. I'm tired of the high cost and want to switch to a gas unit... I'd go tankless but I think my well water would shorten the lifespan.

    I do have a NG forced air furnace (we heat with wood so its only a back up unit).. but I don't know if I can plug into the vent stack on that.. not sure how to calculate how big the stack needs to be... Any advice on this?

    So, while I would prefer to just get a run of the mill 40 gallon water heater and stack it in with the furnace stack, I'm thinking that I'm going to be required to get a power vent type.

    Some local guy has a Lochnivar DVN041 for sale that he says was only used for 2 weeks for $250.. Suggestions?

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    I would not use the existing furnace chimney. When two fixtures are combined, the stack is enlarged.

    I like idea of the fan assist water heater that will vent separately.
    I think that's where things are going anyway.
    And $250 for a barely used water heater with power vent would be a screaming deal.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Sometimes, good deals aren't so good! I'd want to know why it was only used 2-weeks, then taken out. It may be that they found it was too small for their needs, or the exhaust fan noise (most aren't that loud) bothered their sleeping , or it had a slight dent in the outer casing (not an issue) and the customer complained. Then, it should be fine. If it leaked, or had electrical problems, I'd not want to touch it. As Terry says, it may be a screaming good deal.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    The combined BTU rating of the furnace + HW heater determines the lower limit on the stack diameter, but if you're only running a ~30K-BTU water heater into a stack capable of handing 300KBTU/hr you're guaranteed to have flue condensation in a MI climate. This can be fine if the flue has a stainless liner, but you'd be well advised to also check for spillage/backdrafting at the heater's dilution hood when you have all the bath & kitchen fans and the clothes dryer running.

    If the furnace is a mid-efficiency induced-draft type you could also run into code & backdrafting issues if hooking up an atmospheric drafted heater to the flue, even if the flue isn't grossly oversized for the water heater alone.

    Power drafted HW heaters vented out the side of the house won't have these issues, but be sure to pay attention to code on vent clearances, and keep it well above the historical maximum snow depth levels.

    On a 1-story house it's usually pretty easy to just retrofit a B-vent stack properly sized for the burner, assuming you don't jump on that Lochinvar deal. See this for guidance.. Observe clearances to combustibles on the chases, but DO take the time to air-seal the chases (using the appropriate sheet metal for the collar around the flue). Where it goes through attic insulation give it a wrap with R15 rock wool (Roxul is carried by the big box store chains these days) tied with steel wire as a thermal barrier between the B-vent and the rest of the attic insulation. (Rock wool won't burn or melt no matter what, whereas fiberglass & cellulose can.)

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    DIY Senior Member Murphy625's Avatar
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    Thanks!
    The water heater goes into the basement of a late 70's brick ranch.. I'll have to inspect the wall thickness where the 6 inch furnace stack goes up to see if there's enough room for another stack..
    Might be best to just stick with a power vent eh?

  6. #6
    In the trades Dana's Avatar
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    It's legal to wye or tee into the 6" stack with the 3 or 4-incher from the water heater, provided the minimmum height specifications between the furnace vent output and the water heater connections are met, as well as the minimum total stack height. See figure 6, page,13, and table 4a, p.14. But most hot water heaters won't meet the min-BTU requirements for a 6" stack. Unless you're using the furnace regularly you'll have an "orphaned water heater" problem, with a cold stack, and unpurged mildly acidic gas exhaust condensate collecting in the flue all winter, ruining the stack or chimney.

    With a stainless and narrower liner/stack it might work, but the cost uptick of the installation would far exceed that of going with a power vented unit, so yeah, power vent is the ticket. If MI or your local utility offers any subsidy (or you have higher than average gas rates) you might even consider a power vented condensing unit like the Vertex, but without subsidy standard efficiency power vented units are probably going to be the ticket.

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