Build external bumpout for internal tankless?

Discussion in 'Tankless Water Heater Forum' started by justinae, Feb 8, 2012.

  1. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    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. ballvalve

    ballvalve General Engineering Contractor

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    3,261
    Location:
    northfork, california
    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. Dana

    Dana In the trades

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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. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    Grrr. Just typed up whole response and lost it.

    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.
  5. ballvalve

    ballvalve General Engineering Contractor

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    3,261
    Location:
    northfork, california
    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. Dana

    Dana In the trades

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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. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    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. Dana

    Dana In the trades

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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. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    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. 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 [​IMG] [​IMG]
    Last edited: Feb 13, 2012
  11. Dana

    Dana In the trades

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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:

    [​IMG]

    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. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    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. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    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. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    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: Feb 13, 2012
  15. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    What do you think about the Marathon heaters?
  16. Dana

    Dana In the trades

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    With drainwater heat recovery you want it to feed BOTH the hot water heater AND the cold feed to at least the shower (but the whole-house is still OK, if you don't mind room-temp water coming out the cold taps.) The heat exchanger presents some restriction, to flow, and some are better than others in that regard (PowerPipe is the least restrictive) but the shorter they are the least restrictive and higher the flow. (I have a 4x48" and don't have any problem getting 6gpm+ through the thing, and that's with some half-inch distribution plumbing and a pressure reducing valve at the meter.)

    They make some pretty stubby ones that fit in crawlspaces, and creating a wide section to manage a 3" or 4" version doesn't diminish the drain function but enhances the heat exchanger efficiency. The fatter and longer they are, the better the efficiency, and the labor to install a tiny one is about the same as installing a large one, so if you do it, get the fattest & tallest one that fits. Even a 4" x 24" PowerPipe delivers better than 30% return, and a 4" x 30" returns 40% @ 2.5gpm, according to Natural Resource Canada's third party testing. (At lower flows the return is even higher.) Even if it's not subsidized by the state, if you can fit even a 2x24" in there it will extend tank capacity measurably, and have a return on investment for families with long-shower addictions. It has to be on the drain, not the stack, but it can be located anywhere downstream of the shower. The closer it is to the shower it is the better, but not dramatically better, and even 50 feet away is fine, taking only small hit in measured efficiency.

    A split-system heat pump water heaters (mini-split style) work where the tank-top versions won't, since it has the capacity & controls for de-frost. By not presenting a load to the heating system it's true efficiency in a heating dominated climate is higher, independent of it's EF rating.

    The Vertex gets good reviews and tests very well on efficiency. The ~75KBTU/hr burner makes for excellent recovery times (it's 2x the burner of typical 40 gallon conventional tanks), and is enough burner to run continuous 1.5gpm showers at Portland's summertime incoming water temps (but maybe not all winter) even without a drainwater heat recovery heat exchanger. Marathon's are great for an electric tank heater, but like all electric tanks (including the heat pump variants) recovery times are poor and the first-hour gallons ratings are quite low compared to even the smallest burner gas-fired tanks of similar volume, so you have to go with a big 'un to go electric. Heating with a gas fired condensing water heater will be literally half the cost (or less) of heating hot water with electricity in most areas. (What are your gas & electric rates?), so even if you're paying $500-800 more up front, the payback on the difference for a typical 4- person family water usage would be under 10 years, maybe under 5 years.
  17. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    Just got official rates this morning direct from suppliers:

    Electrical: $.11039/kwh
    Gas: $1.09/therm

    Here is a matrix I made. The numbers tell a better story than the theory. It's interesting that an 80% efficient water heater is the best bang for buck.

    EDIT: Ignore the first matrix. I added 5 and 10 year and can't figure out how to delete it.

    Attached Files:

    Last edited: Feb 14, 2012
  18. Dana

    Dana In the trades

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    The numbers for the Airgenerate are misleading, since they show the extra natural gas burned in your furnace spent supporting the load 8-9 months out of the year.

    It's not too surprising that an 80% tank heater is the better financial performer at buck-a-therm fuel rates. If that goes to a buck-fifty the Vertex will suddenly start looking a lot better.

    But there's more to it than raw net-present-value of utility savings. In a multi-person multi-showering household first-hour performance counts. A tankless takes that hands-down, but not necessarily with multiple simultaneous flows. The Vertex would be second, but has higher peak flow and fewer quirks.

    The EF rating of the tankless also exceeds as-used efficiency (by at least 0.08) since A: They lose a lot of efficiency to flue purges and ignition cycles on short draws and B: you end up using more hot water due to ignition delays, etc.

    The venting costs on the 0.82 tankless are likely to be 1.5-2x what you have there, and for the condensing tank it'll probably be less. The gas piping to the tankless units will also be measurably more expensive than to the gas fired tanks too- you're looking at 1-1/4 gas piping compared to 3/4" for the tanks.

    And if you're using more than 63 gallons/day per the EF test, (which would be the case for the long-showering folks) the payback of going with a condensing unit pulls in by quite a bit, and the effective as-used EF of all tank heaters goes up a few notches. You probably won't hit 100gallons/day for an average but 80 is pretty common for a 4- person household.

    BTW: Internet pricing for the Vertex with the 76K burner is running around $1600 these days- does it really cost another $498 to get it to your house? I think you probably priced the Vertex with the 100K burner (the GDHE rather than the GPHE) which is way more burner than you need. At 76KBTU/hr input and 95% combustion efficiency your getting about 72,000BTU/hr out of it, which is good for an endless 2 gpm shower with a 72F rise. That means even with a smokin' hot 108F shower you're good down to ~36F incoming water temps, as long as you don't have any other big draws while showering. With a 1.5 gpm low flow shower you can even take some other pretty big gulps off the 50 gallons without flinching- it more than keeps up with the shower alone. The 100K burner may make sense if you have real gusher-showers with sidesprays or a monster tub to fill, or if you're using it as a combi-heater to run the radiant floors in a McMansion, but it's another $500 YOU probably don't need to spend. Subtracting that $500 out of your matrix puts it closer in net-cash to the lower-end gas tanks, but you get a LOT more first-hour gallons out of it. (Given current hourly rates of divorce attorneys it might pay for itself in the first month by never running out of hot water just before SHE steps in! ;-) )
  19. justinae

    justinae New Member

    Messages:
    49
    Location:
    Portland, OR
    Good stuff Dana. Some of those costs are estimates, others are based on my contractor pricing. In terms of use we do have the guilty pleasure of long showers, though we've agreed to curb it. Other than that we aren't huge users. We've lived with a regular 50 gallon electric heater for quite a while and never had issues with running out or anything so I think we're fine there.

    I think you might be right about the GDHE pricing. I'll have to call my supplier back and confirm.

    I should have a decision in the next few days. I'll be running the pipes tomorrow and will need to get something connected.

    The other concern with gas is that the heater will be installed in a 2' x 2' closet. I called one manufacturer today and asked if that size closet with upper and lower louvers would be enough for combustion air and he said yes.
  20. Dana

    Dana In the trades

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    If you made out OK with a 50 gallon electric you'd be in HOG HEAVEN with the small-burner Vertex, (or even a bare-bones 40 gallon gas-fired tank.)

    Also in-re the Airgenerate, I had meant to say "...they DON'T show the extra natural gas burned in your furnace..." (self-editing isn't my strong suit, eh? :) )
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