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Community and Q&A

How should I heat my water?

user-6131469 | Posted in Mechanicals on

Hi there. I’m just starting a major renovation of a two story studio space attached to our detached garage to be used as a studio and sometimes guest house. We’re adding water and septic to the house, and I’m looking for ideas on the best way to get hot water. We’ll only have a single bathroom with shower, and a small wet bar sink on the same wall as the bathroom, so there are no long pipe runs. Usage will be infrequent but once in a while fairly aggressive if we have guests there who need to take multiple back-to-back showers. Keeping a large tank hot all the time for the infrequent times we’ll use it seems wrong. I’ve read some advice on here about point of use electric water heaters, but my electrician said they don’t work well enough and take too much power. Especially since I’m taking power from our main house – so we might only have 100amps available. I don’t have municipal gas, and am thinking of supplementing my power use with solar down the line. Do you have any thoughts on what type of heater I should look a? Thanks in advance. -Curtis

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Curtis,
    If you are "taking power from the main house," your electrician is right -- you probably won't be able to install an on-demand electric water heater. You could, of course, upgrade your electrical service -- but that's expensive.

    Your two most likely choices are:

    1. A small tank-style electric resistance water heater.

    2. A propane-fired tankless water heater.

    For more information, see Domestic Hot Water: No Perfect Solution.

  2. Reid Baldwin | | #2

    Would a standard electric tank water heater with an on/off switch work? Obviously, you would need to remember to turn it on before guest arrive and turn it back off when they leave.

  3. charlie_sullivan | | #3

    In general, I'm adamantly opposed to tankess electric, but I actually think this might be an OK application for one. Malcolm, as usual, has a simpler more elegant solution, but if that doesn't sound good for some reason, here's how the tankless electric could work:

    You can put up to a 60 A heater on your 100 A sub-panel. A 50 A heater at 240 V will give you 50 F temperature rise at 2 gallons-per-minute flow. So if you are in a region with 60 F ground temperature or higher, you could get 110 F water coming out at 2 GPM, which is just adequate for a single shower. (It wouldn't matter whether that was achieved by mixing some cold flow with 130 F hot flow, or just the hot flowing at 2 GPM with the heater maxed out producing 110 F. You'd get the same resulting maximum temperature out of the showerhead at 2 gpm)

    If your ground temperature is lower, you could use a lower flow showerhead--at 1.25 GPM, you get 80 F temperature rise which would give you 120 F out even at 40 F in.

    But you can also put in a drain-water heat recover unit, which uses the heat going down the drain to pre-heat the incoming water. Showers are kind of the ideal scenario for them, so you could get higher flow and/or higher output temperature. You could then get a 2 GPM shower even with

    So you can make it work--is it a good match for the scenario? It might be. Simultaneous draws are a problem for tankless, but back-to-back showers are not, and although a good electric tank has low standby loss, it does have some and it is a little more for a bigger tank (although it is less than proportional--doesn't not double when you double the size of the tank). And even if you do Malcolm's suggestion of turning it off between times you have visitors, you would have some losses involved in letting the heat in a large tank dissipate when you turn it off, and then have to reheat it some weeks later.

    Note that if the water temperature is barely warm enough, a non-aerating low flow showerhead is more comfortable than an aerating one, because it doesn't have as much evaporative cooling. Also, a fully enclosed shower stall, with a lid, will get warm and steamy inside making it comfortable with slightly lower temperature water. Neither of these is a huge effect but if your water temperature is marginal it could help.

    Drainwater heat exchanger:
    http://www.renewability.com/

    Curves on temperature rise vs. flow rate for a 60 A tankess electric:
    http://www.stiebel-eltron-usa.com/sites/default/files/pdf/sizing-guide-tempra.pdf

  4. Expert Member
    Dana Dorsett | | #4

    Many vacation homes have electric HW tanks that get turned off at the breaker when unoccupied, which would be the "right" thing to do for serving the guest room shower since you don't have the large showering loads very often. The size of the tank depends on just how many back to back showers you anticipate. (A 75-80 gallon HW heater and a low-flow shower head can serve up at least 3 reasonable-duration showers, but not more than 5 unless they're pretty short. A standard 50 gallon unit can sustain at least a pair of back to back low-flow showers.)

    A pretty good commercial 80 gallon all-stainless electric with a lifetime warranty can be had for about a $1-1.2 at box-store or internet pricing, a glass lined 50 gallon residential units with a 9-12 year warranty run about half that. Installation costs would be about the same, independent of size.

    A smaller 5-15 gallon unit can serve the wet bar & lavatory for the normal daily use.

    I'm a big fan of drainwater heat exchangers when the daily showering load is high, but it's doubtful that it would be worthwhile unless the average annual duty cycle on the guest quarters shower is substantial.

  5. user-6131469 | | #5

    Thank you all for these great points.

    In response: Martin: I actually already have power service to the separate building, and could keep it. I was only thinking of tying it to the main house to save on delivery costs (in NY where I am, it costs about $35 month before I flip on a switch). But if that cost makes sense for opening up the option for a point-of-use or whole house electric tankless a better reality, then I can keep the service.

    Charlie: would love to know why you are usually against electric tankless. How do you feel they compare to, say, a gas tankless? And thank you for the advice on heat recovery options and on using non-aerating shower heads.

    And Dana- thank you for the info on costs and comparison of models.

    One other question: if I go with a tank water heater, should I consider using a hybrid heatpump electric water heater? It would be in the lower lever which is going to be a heated, conditioned space.

    Thanks again for your thoughts.

  6. GBA Editor
    Martin Holladay | | #6

    Curtis,
    A heat-pump water heater costs significantly more than an electric-resistance water heater. I don't think the cost of a heat-pump water heater can be justified for a building where, as you put it, "usage will be infrequent."

  7. charlie_sullivan | | #7

    Why I am usually against tankless electric:

    1) The efficiency is pretty much identical to tank electric in normal use, but they are usually sold based on false claims that state or imply otherwise.
    2) Once you factor in the need for high-current electric service and wiring, they typically cost a lot more, so you are spending a lot of money for them.
    3) They are not good citizens on the electric grid. High peak power draw is something you can get away with now, but it incrementally makes things more difficult for the utilities. Your choice won't make a noticeable difference to the utilty, but widespread adoption might. On the other hand, tank electric heaters are the low hanging fruit for smart grid opportunities.

    To some extent, objections 1 and 3 are more philosophical than practical, but 3) could turn into a practical issue if the utilities start imposing peak demand charges on residential customers like they do for industrial and commercial customers.

    But your case is a little special, because the equal efficiency argument gets undermined by the cumulative standby losses if you leave it on standby for a month, or the re-heat cost if you cycle it on and off for each visit--you are paying to heat 80 gallons of water for each visit even if the guests only use 30 gallons, which is only 37.5% efficiency.

    Dana notes that the drainwater heat recovery system won't pay for itself in energy savings for occasional use. But my recommendation to use it here was not based primarily on energy savings--rather it was based on avoiding the cost of a large tank, or the cost of >100 A electric service.

  8. morganparis | | #8

    If you like the concept of a high install cost solution that is tied into fossil fuel for its lifetime go propane tankless. If you like idea of a high install cost solution that will slow the adoption of renewables by the utilities go electric tankless. If you like the idea of a high install cost solution that will rob the interior of space heating and recover slowly/noisily for the next user go hybrid electric heat pump. If you like the idea of a low install cost solution whose operating cost will be barely more than any of the above and ca function as an energy dump for solar electric go standard tank and switch off between visits. Consider if you really need hot water for hand washing etc. between visitors, if you do a small inexpensive low draw over the sink electric tankless will suffice. As always, invest the money saved on fancy mechanicals in enclosure improvements or solar electric.

  9. user-6131469 | | #9

    Thank you for your input! This all makes sense.

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