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Thread: Build external bumpout for internal tankless?

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  1. #1
    DIY Member justinae's Avatar
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    Default Build external bumpout for internal tankless?

    I'm relatively new to this forum and brand new to home ownership. Our new house does not have a water heater. It does have gas already. I installed a used electric 50 gallon tank for temp purposes and weighing the options of tank vs. tankless.

    I've read a lot about the debate on this forum (very helpful!) so don't want to beat that old drum, but I do have a question that doesn't seem to have been asked. I'm thinking that I could buy an indoor unit and build an external bumpout enclosure, with insulation, access door, siding, etc and have a very very short venting out the bumpout roof. My thought is that I could save costs by buying the less expensive non condensing, and save on venting material as well since I'll need stainless steel. Alternatively I could spring for the condensing unit and then just vent right out the sidewall of the bumpout with PVC.

    If that doesn't make sense, what about building some sort of enclosure to protect an outdoor unit from theft/tampering? Perhaps a front door on a bumpout with a cutout for the venting?

    I'm still on the fence with tank vs. tankless. I'd prefer to move away from electric and since I would have to run gas from the meter to a tanked gas heater anyway, why not just do the tankless. I'm a licensed contractor so I feel confident about my capacity to do an install, even though I'm not a plumber, and my plumber will consult and do a final QC for a fee.

    Very appreciative of any advice/insights.

    Justin

  2. #2
    General Engineering Contractor ballvalve's Avatar
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    Very common here to put the heaters on a bump out. Doing one now. No issue with leaks also.

    With the tankless, I would say the fence you are sitting on has razor wire on it and its gonna bite in a few years.

  3. #3
    In the trades Dana's Avatar
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    In Portland I'd stay away from outdoor units. Even though the freeze risk is fairly low and most of the whole-house tankless have electric heaters as freeze protection, it's still a PITA to deal with a frost-damaged heat exchanger. Better to go with an insulated bumpout that stays within the insulation boundary of the house, or at the very least is "earth coupled" with insulation down to a foot below the frost line. There's very little thermal mass in a tankless to keep it warm overnight if it's just it's own little insulated shack.

    Condensing units have the advantage of all being sealed-combustion, direct vent whereas that costs extra on most standard efficiency 0.82EF-ish units. With direct vented units you don't have to make provisions for combustion air, and it isn't an infiltration path for the house or isolated bumpout either, and the urgency for backflow prevention on the exhaust to protect the heat exchanger from freezing up goes down. Add up the whole package, including stack materials, backflow preventers etc, and don't forget any local subsidies that might be greater for a condensing unit and the cost difference may not be as great as it first appears.

  4. #4
    DIY Member justinae's Avatar
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    Grrr. Just typed up whole response and lost it.

    Very common here to put the heaters on a bump out. Doing one now. No issue with leaks also.
    Do you do this for tank models? How do you support the weight?

    Thanks Dana for the tip. I do think I'm sold on internal. I had considered a battery backup for the freeze protection, but I don't want an over complicated setup. The location would be a closet about 7' away from an external wall. I don't know if cost wise it's a wash but I suspect you are right about the cost/hassle of non condensing being about a wash.

    For what it's worth here is my tank vs. tankless breakdown FOR MY SITUATION:

    $450 gas tank, $50 parts, $150 venting (guess), $75 roof vent installation (not guess) = $725 roughly w/o maintenance costs
    $1200 tankless condensing model, $200 venting (guess), $75 roof vent installation, - $150 Oregon rebate = $1325 roughly w/ maintenance costs
    $900 tankless non condensing, $350 venting (guess), $75 roof vent installation, - $150 Oregon rebate = $1175 roughly w/ maintenance costs
    or
    $300 electric tank, $50 materials, = $350

    It's not a scientific method and I think that if I were already setup with gas tanked water heater I would just stick with that, but since I have to bring gas anyway if I want to move away from electric, it makes a little more sense.

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    General Engineering Contractor ballvalve's Avatar
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    I could never understand bringing explosives into ones home, especially a closet, and paying thousands more for that chance to be vaporized and for water to ruin the floors.

  6. #6
    In the trades Dana's Avatar
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    You haven't stated, but just to be sure, if you're heating with a hydronic boiler (either hot water or steam) an indirect-fired tank might a better solution. If you're changing the heating system to gas too, a combi system might work too. Space heating loads in your area are usually under 50KBTU/hr, and a 75K condensing tank HW combi heater might have a lower combined cost than a gas furnace/boiler + tankless, and would have better overall performance (both on efficiency and HW delivery.)

    Sticking a tankless in a closet 7' from an exterior wall can complicate the combustion air picture. With direct-vent you'd have to figure out both the vent and supply locations, and for a standard unit you would need a large grille in the closet door to be able to run a burner that big. A 150-199KBTU tankless is 5-6x the amount burner that could heat a typical Portland home.

  7. #7
    DIY Member justinae's Avatar
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    We are not doing any hydronic heating. House has a gas furnace.

    I would do a direct vent. There is a soffit above the shower I could use to get to the exterior wall. The only consideration is how close will the venting be to the eaves. I need to look at the specs/reqs. The other option would be to install it in the attic, but if I do that I might as well just build the bumpout. hmmmm....

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