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Thread: Electric tankless

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    DIY Junior Member peeler's Avatar
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    Default Electric tankless

    I'm trying to decide between electric whole house tankless and a conventional
    electric tank. Gas isn't an option because of venting difficulty with the particular building I have. My use will only be sporadic (it is an office, so only hand washing - 3 to 4 lavs total). In this situation, would the tankless make more sense do to light duty use? Equipment would be in the basement, which gets cold. I'm in the Southern US. This is a total building retrofit, so I have to get a system now. I was looking at Stiebel Eltron or Seisco for the tankless.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Handwashing only uses a small amount of water...I think I'd consider a small 4-6 gallon tank in each lavatory. Recovery rate is fairly slow, but should be plenty for hand washing unless it is something like a theater where there could be hundreds of people through quickly. These can be run off of 110vac. An electric tankless needs substantial power, then you have the losses to get it to the lavs, so people probably wouldn't get hot unless you add recirculation which adds even more to the complexity and cost. I'd keep it simple with a small tank in each lav, or maybe one, if they are back to back. Close to the point of use, so fast warm water.

    Alternately, if I was going to use a tankless, I'd consider installing it in the bathroom, or nearby in a closet to minimize the response time to get hot water there.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Plumbing Designer FloridaOrange's Avatar
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    A single tankless that is rated to serve "3 or 4" lavs or multiple tankless for each sink will take much more power than a tank type water heater.
    Matt
    Semi-professional plumbing designer
    Enjoying life in SW Florida

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    In the trades Dana's Avatar
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    Quote Originally Posted by FloridaOrange View Post
    A single tankless that is rated to serve "3 or 4" lavs or multiple tankless for each sink will take much more power than a tank type water heater.
    The tankless will take more instantaneus power, and it'll likely use more kilowatt-hours to boot (less efficient, as-used.)

    A hand washing might take less than a quart of water, but with the heater located remotely the user ends up purging a gallon or so to get the temp up to comfort level, abandoning most of it in the distribution plumbing, whereas with a 2-3 gallon mini-tank under the sink there'll be less than a cup of tepid water between the faucet & the tank to purge. Standby losses on mini-tanks tends to be ~30Watts, and can likely be reduced to under 20W by insulating all near-tank plumbing to R4 or higher and reducing the storage temp.

    From a user point of view minimizing the wait for hot water counts too.

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    DIY Junior Member peeler's Avatar
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    Thanks for the replies. As far as the individual models, I'm afraid that may get too pricey. I'm almost done with this project and I'd have to run 240 circuits to
    these locations. This is an historic building and I don't have cabinetry to hide things in (pedestal sinks) so equipment in the bath isn't feasible. I do have easy access in the basement under the first floor baths, but its not heated down there. I will point out that all the lavs, regardless of which floor they are on are fairly centrally located and all at the middle to rear of the building.

    I'm starting to wonder if using a traditional tank (30 or 40 gal?) and putting it on a timer to turn down at night, and even setting the max. temp to hand washing levels as opposed to shower levels makes good sense?

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    In the trades Dana's Avatar
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    Quote Originally Posted by peeler View Post
    Thanks for the replies. As far as the individual models, I'm afraid that may get too pricey. I'm almost done with this project and I'd have to run 240 circuits to
    these locations. This is an historic building and I don't have cabinetry to hide things in (pedestal sinks) so equipment in the bath isn't feasible. I do have easy access in the basement under the first floor baths, but its not heated down there. I will point out that all the lavs, regardless of which floor they are on are fairly centrally located and all at the middle to rear of the building.

    I'm starting to wonder if using a traditional tank (30 or 40 gal?) and putting it on a timer to turn down at night, and even setting the max. temp to hand washing levels as opposed to shower levels makes good sense?
    The Bosch Ariston series mini tanks all run on 15A 120V service (either plug outlet or hard-wired). But if it's all truly centrally located and a single tank can be located directly beneath the vertical runs to minimize distribution lengths a single tank might be the right solution.

    If it's actually cold down there (under 60F), insulating all of the basement plumbing to at least R6 (including the nearest 6' of the cold feed, and the pressure/temperature valve outlet) puts a big dent in the standby losses, as does going with at least an R20 tank (or bringing it up to over R20 with an extra blanket.) Setting the outlet temp to the minimum-legal value is also a standby saver. (If, as in some locations, you're required to maintain 140F+, maybe a small tankless IS the right way to go.) If it's ~65F or higher in the basement on average, R4 on all of the plumbing would be still be cost effective. (You can get suitable closed cell pipe insulation at Grainger, if you don't have another source. Box stores tend to only carry R2.)

    Also, smaller/fatter tanks are (all else being equal) less lossy than taller skinnier tanks- the surface-area/R-value issue. Depending on your actual peak hour HW usage (which may be difficult to determine), you might get away with an 8-10 gallon mini-tank and insulate the hell out of it, but if it's not enough, there are options. In general, smaller tends to be slightly more efficient than larger- go 30 gallon, not 40 or 50. Since your volume use is low (nobody taking showers), you can probably go with a 120V version if you go as big as 20-30 gallons, since recovery times can be long without consequence. A 30-gallon shorty/compact would have minimal surface area losses and plenty of capacity, and under 50W standby if insulated to R20 with near-tank plumbing at R4. (Figure ~350-500kwh/year in standby loss.) Look for something small, easy to insulate, with an EF of .93+ (eg Rheem 82VP30-1, but there are many others.) Most are available in 120V 1500-2000W versions.

    Turning off electric tanks at night with timers yields vanishingly small savings, since standby losses are low, and well over 90% of the electricity you didn't use overnight you end up using to bring it up to temp in the morning. The standby losses don't drop significantly until the tank is 10s of degrees F cooler, and in 12 hours a 130F tank might cool off to maybe 122-125F (depending on how cold the room is), not 105F (unless it's outdoors, in January, in Maine). You'd save at most $8-10/year with a timer even in high electricity rate areas, probably less where you are.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default heater

    ALL of the small heaters, even up to 12 gallons are 120v, unless they were special ordered for 240v. I would not consider the Ariston, unless they have stopped using the fiberglass/plastic tanks. Before I went that way, I would install a standard tank model, with a Grundfos "Comfort" system. The pump would be at the heater and the control valves at each, (maybe only one, depending on how your system is piped), farthest lavatory. This will give you "rapid", not necessarily "instant" hot water at the sinks.
    Last edited by hj; 12-18-2009 at 09:12 AM.

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