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Thread: Build external bumpout for internal tankless?

  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.

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    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....

  8. #8
    In the trades Dana's Avatar
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    It might actually be easier to build a mini mechanicals room that is continuous with the thermal & pressure boundary of the house in the attic than as a bumpout.

    Going a bit further, if you have soffit-to-ridge ventilation for the roof deck, putting 2" of rigid foam board on the interior of the rafters and air-sealing it to the upper-floor sealing to form an exterior air-barrier may have utility savings well beyond that of a condensing tankless (or condensing furnace, for that matter), since it kills the stack-effect driven infiltration (== big fraction of your heating bill, unless you've already undergone blower-door verified air sealing.) Putting the HW heater between the attic floor insulation and at least R8+ of roof insulation would be freeze-protection enough for anything in the attic space. If you use 1.5" - 2" (R10-R13) of fire-rated iso you wouldn't even need an ignition barrier, and you'd have sufficient continuous-R to meet IRC 2009 code for condensation control in Portland's climate zone (US zone 4C) even for an UN-vented roof, AND the radiant-barrier effect of the foil facers would keep it cooler in the attic in summers. It's easy to air seal foil-faced goods with FSK tape at the seams, 1-part spray foam at the edges. This would put an attic mounted tankless fully within the pressure boundary of the house, and sufficiently inside the thermal boundary to eliminate any freeze hazard.

  9. #9
    DIY Member justinae's Avatar
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    Thanks Dana for your thorough responses. The update is that I've moved the tub and now the tankless can be mounted on an exterior wall.

    I have narrowed my choices down to the Rinnai RL74i or a electric heat pump water heater. In Oregon there is a $500 cash rebate for installing a heat pump water heater but it only applies to the Airgenerate ATI50 or ATI66. I don't have any idea of the pricing on that unit so I'll check on Monday.

    My only concern with the heat pump system is the cold air in the cold seasons. Since it will be installed in a very small closet I wonder if the cold air can be vented during cold seasons and allowed to circulate during hot seasons. We do have a gas furnace so the extra work that the heating system has to do will be slightly offset by the efficiency of the heat pump.

    What do you think?

    Thanks!

  10. #10
    Master Plumber master plumber mark's Avatar
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    Default why not build your own???

    Hey, I hate to butt in

    ...I would rather just install
    either a 50 gallon electric or go to a tank type gas heater
    and live happily ever after....


    but if you really want to have some fun in your own home

    why dont you build yourself a home made tankless electric
    water heater... you would save time, money and space.
    just like this fellow has done

    ... sweet

  11. #11
    In the trades Dana's Avatar
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    You can't install a heat pump water heater inm tiny closet (even a large closet), since it needs free access to substantial room air from which to extract the heat. They're designed to pull heat efficiently from air that's ~60F or higher, not 40F winter air, so the outdoor venting concept is a non-starter. They're a great fit for cooling-dominated climates, but in a heating-dominated climate like yours it's basically a parasitic load to the heating system, 8-9-months out of the year, and will never be more efficient than your heating system.

    With a gas-fired furnace running at 80% efficiency or better, with well balanced & sealed ducts the heat pump is cheaper to run than an electric tank, but not cheaper to run than a 0.60EF gas fired hot water tank or any gas fired tankless. If your ducts aren't pressure-tested tight, aren't insulated, and run outside of conditioned space the cost of running the heat pump could be on-par with a plain-old electric tank.

    If you're going to spend something like a $1500-2000 on water heating equipment you'd get better efficiency & reliabilty out of a condensing tank type HW heater (Vertex or Polaris similar), and if yours is a showering (rather than tub-bathing) family, a drainwater heat recovery heat exchanger you'd blow away any tankless on efficiency. These are also subsidized in OR, but it may only apply if you heat hot water with electricity. (So maybe you'd install this first or check it out first eh, ;-) ) . It has no maintenance issues or quirks, but roughly doubles the showering time you get out of a gas-fired tank HW heater, and with a 1.5-2gpm low-flow showerhead the "endless shower" experience can be had with a bottom-of-the-line gas fired tank. It does nothing for tub-fills though, since the heat is only transfered from the drainwater to the incoming cold water when both are flowing:



    With a condensing tank + a 4" x 48" or larger drainwater heat exchanger you're effective EF can be GREATER than 1 (more than 100% efficiency), even for a gas-fired HW heater, making it far more efficient than a heat-pump heater drawing it's heat from a hot air heating system, and it's output per hour would be many times that of any residential electric HW heater.

  12. #12
    DIY Member justinae's Avatar
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    This is an excellent post Dana! I'm really intrigued by the idea. It's unconventional, but maybe plumbers just don't like to install them. We are a showering family and even long showers. Though now that we have a kid we do take baths. We're installing an Americast tub which will help reduce heat loss.

    Anyway. I've finally decided against the tankless, mostly because I need to get moving with the remodel.

    I spoke with a rep for Airgenerate who said that they are coming out with a direct vent heat pump water heater soon to combat the issue people are raising with venting cold air into the conditioned space. When I pressed him about the loss of efficiency with a cold supply air he did acknowledge that. Mini split heat pumps here are now able to work down to the upper single digits, but I don't think they are using that technology in the water heater heat pumps. It would be interesting to see an exhaustive study of water heaters here in a heating climate.

    The Polaris sounds great, but whoa is it pricey! The AO Vertex is a bit less. I'll have to see what my pricing is from my suppliers.

    I'll look into the heat recovery for more info and report back.

    Thanks for the great help!

  13. #13
    DIY Member justinae's Avatar
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    One thought with the recovery system you posted is that it seems like you would want to choose to have the preheated water either go back to the shower or to the water heater, but not both. If you're taking a shower you would have competing draws right?

  14. #14
    DIY Member justinae's Avatar
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    Looks like the drain recovery only works on vertical drains. I have a crawl space and the vertical run on my shower drain is tiny.

    Also looks like the incentives are only for 3" or bigger and my drain lines are 2" except at the stack, but there is no part of the stack available to wrap before it goes into the earth.
    Last edited by justinae; 02-13-2012 at 11:08 PM.

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    DIY Member justinae's Avatar
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    What do you think about the Marathon heaters?

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