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Community and Q&A

Electric tankless hot water

[email protected] | Posted in Mechanicals on

My name is John, and I’m an instructor at Madison Area Tech College in Madison Wi, in the Construction and Remodeling Program. We train or students in a 1 year diploma program how to be wood frame carpenters. We build super high efficient tiny homes with the students, that we sell. We incorporate lots of things talked about on this site. One thing we are trying to figure out: How to efficiently heat water without taking up too much space. Our current couple of houses are going to a site that is going to be installing 50 various tiny homes in a village setting and renting them out in a hotel fashion. They are planning on putting a very large PV system on the main building, which will be a rehabbed sugar beet mill. Very cool project all around. They are looking for these to be Net Zero, and are hoping to produce enough electricity to run the homes. Thus, the electric, tankless HW heaters. We are currently considering the Ecosmart 18, but are open to suggestions for good electric tankless HW heaters. Being tiny homes, we don’t have much room for tank style HW heaters, so settled on electric tankless.


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  1. GBA Editor
    Martin Holladay | | #1

    Tankless electric-resistance water heaters work. Some green builders try to avoid them, however, because they tend to put a big strain on the utility grid. Utilities often worry that if a high proportion of homes use these heaters, the grid won't be able to handle the peak power needs caused by everyone taking showers between 7:00 am and 8:00 am.

    These heaters also have very high power ratings, and therefore require expensive upgrades for electrical service.

  2. charlie_sullivan | | #2

    Your space constraint and your desire to use the PV (with net metering, I assume) could lead to this being a logical choice here even though I don't like them for the reasons Martin mentions. However, you might also be setting up an experiment that will demonstrate the worst-case scenario that those of us who don't like them worry about: If you have 50 tiny homes all on the same electric feeder, perhaps even all on the same transformer, and they all have tankless water heaters, there might be a day when everyone showers at the same time, and the feeder and the transformer(s) get overloaded.

    It might be that the electric service gets sized for 50 McMansions because the design rules don't have a category for tiny houses, and the water heaters' power requirement no worse than a McMansion air conditioner, and it's all fine, but then again, if the electric service is sized for 50 McMansions, you are losing some of the potential cost advantage of tiny houses.

    Bottom line is that this is a conversation worth having with the engineers planning the electric service for the whole development.

    Have you considered district heating solutions? You could put some water-heating technology in the big building that wouldn't be cost effective in the individual houses. CHP, wood-chip boiler, ground-source heat pump, perhaps some of those combined with solar thermal collectors, etc. All of those things are unpopular on GBA because they don't make sense for a single-family dwelling, but you might have a big enough overall community to make them work.

  3. [email protected] | | #3

    It would be a net metered situation. I think they can upgrade their transformer for the whole site. It does require a 150 amp panel for a 400 sq ft house which is interesting. Are there other options that are solid beside the Ecosmart? Have never installed a Ecosmart, or any tankless electric, for that matter. Curious if anyone has thoughts on different brand names?

  4. STEPHEN SHEEHY | | #4

    You might need something bigger than the Ecosmart 18, since it needs 62° water to be able to produce 2.5 gpm. I suspect Madison's water, at least in winter, is colder than that.

  5. user-2310254 | | #5


    So gas and propane are off the table? Or are they planning to install a PV system that is large enough to offset the peak demand of 50 cottages with electric water heaters? I like Charlie's idea of using a community-wide system to serve everyone.

  6. [email protected] | | #6

    Nothing is off the table, but they are trying to keep the mechanicals/utilities that run to each tiny house to a minimum. We should be able to heat and cool with Mitsubishi split systems without a problem. I did base the size of the Ecosmart 18 on their website, which takes into consideration Wis. water heating issues. Natural gas to each unit definitely adds costs to the project overall. It's understood that if they were relying solely on utility supplied electricity, it doesn't make sense, but with the large amount of locally produced electricity from the PV system that should mitigate the long term cost increases inherent in resistance heating. Has anyone any feedback about the various brand names of the units themselves?

  7. iLikeDirt | | #7

    400 square feet? An electric tank water heater shouldn't take up more than about 4 square feet, leading to the loss of only 1% of the usable floor space. That's the direction I would go in if it were me.

  8. Expert Member
    Dana Dorsett | | #8

    I'm not particularly enamored of how some of the utility monopolies tend to operate in WI regarding small scale PV and residential rates, arguing that since the PV homes don't cover the grid costs, they shouldn't net meter. While that's pure BS for the average small scale PV home (where the presence of the PV LOWERS the peak loads), it's absolutely NOT BS for homes with 10 ton air conditioners or tankless hot water heaters.

    There's almost no point to being Net Zero energy then putting 200 amps of peak load on top of a tiny energy sipper house. It's about the worst thing you could do to the grid (short of outright sabotage :-) ), since it oversizes the grid infrastructure requirements by about an order of magnitude.

    If thermal storage in tanks of water isn't an option, 2-4kwh 20kw+ battery and inverter between the grid and the tankless to shield the grid from the abuse would be reasonable, but expensive. Under present rate structures behind-the-meter grid batteries aren't cost effective. But if/when residential rate structures change to include demand charges, so that grid (ab)users are actually paying the upsized infrastructure necessary to support the loads it will be.

    Focusing soley on net energy use, and ignoring increases to the peak draws is a mistake. It's the opposite of "green".

    A low-boy tank HW heater in a conditioned crawl space under the house that also includes a 4" x 48" drainwater heat exchanger feeding the HW heater and is FAR greener. (WI used to offer rebate subsidies for those, not sure if they still do.) A drainwater heat exchanger that size will deliver better than 50% heat recovery on low-flow showers, if the output of the HX is feeding both the cold side of the shower and the cold feed to the tank. The recovered heat from showering more than covers the standby loss of the tank if it's used daily.

  9. user-2310254 | | #9

    Stiebel Eltron mini tank water heaters are well regarded.

  10. jshorey | | #10

    I have installed two of these type of water heaters, one was a stiebel eltron and the other was an ecosmart. The plus side is they are very small and I have built them into wall cavities. Both have been working great for 3-5 years. However I did have to install the special valves to allow flushing the system with vinegar periodically. For the ecosmart I also put in a manifold system for the pex lines so I wouldn't need shut-off valves anywhere and to minimize heat loss.

    You definitely can't put in rainshower heads or anything that will get you over the rated GPM. I would recommend a small tempering tank in a wall or in the floor to get the water up to room temp if it is in a cold climate.

    The downside is you need A LOT of current to run these things. One of them needed 2 40amp breakers. That adds substantial wiring cost and possibly a larger feed to the house.

    Doing it again I might put a small 120V heater under the kitchen sink and then a larger unit for the shower. That would minimize draw. Most dishwashers, washers, etc heat water anyway.

  11. itserich | | #11

    I would start by choosing a low flow showerhead. This will minimize the size of the heater which reduces the amount of wiring, water, and ongoing electricity.

  12. wisjim | | #12

    In few communities near me, in western Wisconsin, tiny homes are built without individual bathrooms but share facilities in a community building, avoiding this question.

  13. srenia | | #13

    I would use a central hot water tank using wild DC. Saves a lot of money on so many fronts. I would also use a separate central boiler system to heat all the units. A simple window ac unit for the summer.

    Of course a normal apartment complex with individual yards would be more efficient than separating the units in the first place. It's like your reinventing the wheel except it doesn't work as well,cost a lot more. and resembles a trailer park.

  14. greenleopard | | #14

    Depending on the layout, I would put centralized heat pump water heaters, maybe 1 per 2 houses? Or put one in a house, then run the hot water of the nearest house of it. If it's more than 50' put a Eco 18 to provide hot water while waiting for the heat pump water to arrive. So many options...

    Heat pump water heaters operate at a cheaper rate than even gas. BTW drain water heat exchangers are very much not worth the money if your water heating is passive solar or heat pump.

  15. GBA Editor
    Martin Holladay | | #15

    What do you mean by "passive solar" water heating?

    -- Martin Holladay

  16. [email protected] | | #16

    It's my understanding that solar hot water systems aren't all that, after all, and you're better off installing PV and using an electric HW heater. But that's a different conversation. The developer of the property is most interested in the solution that is easy maintenance, along with energy efficient. The 50 units will be scattered around the property in pods of 10, and hundreds of feet away from the main building, in most cases. So a centralized bath is not an option, especially as the developer wants the units to be self contained. And being 200 sq ft to 400 sq ft, and rented occasionally, tank HW doesn't make sense from a standby perspective and a space perspective. Thus our thoughts on tankless. And since there will be a large PV system on the main building, a Net Zero tiny home is desired, and they don't want to run gas to all the units. And yes, it would be Net Metered. We did our blower door test last week and passed, but aren't achieving the numbers we'd like, and that's in part due to very small footprint gets harder to achieve the numbers you want. But that's a different discussion as well. Thanks all!

  17. Anon3 | | #17

    You want a tank with PV, that way you can heat the water when the sun is up and use hot water in the middle of the night.

  18. KeithH | | #18

    I'm surprised no one mentioned mating battery technology to your PV and all electric utility system. The cost of a battery system, either for the community, or individual (aka Tesla Powerwall) is probably overwhelming at this time (3 bedroom system is currently $6k+?) but if you end up doing more of these in the future, it's worth looking into. Additionally, if your local utility imposes a peak metering charge in the future, you might find that such a system would be more cost effective. Perhaps there is a whole community option that would cost effectively reduce peak load?

  19. GBA Editor
    Martin Holladay | | #19

    No one mentioned buying batteries because they are so expensive that the investment makes no sense except for isolated off-grid buildings.

    If someone asked you about the various means of transport between Cleveland and Memphis, you could suggest, "Why not hire a helicopter?" Sure, it's possible -- but most people will choose from more affordable options like driving, taking a commercial flight, or taking the bus.

    For more information, see Batteries for Off-Grid Homes.

  20. Expert Member
    Dana Dorsett | | #20

    Grid attached batteries can make economic sense if the residential rate structures evolve to apply demand charges, or if (like grid-aware electric tank water heaters) some control of the battery can be rented to third party, to allow it to be used for ancillary grid services such as frequence & voltage conrol + demand response. Right now people with electric tanks in the PJM grid region can be paid that sort of control by demand response aggregators such as Mosaic Power:

    Since the Supreme Court upheld FERC Order 745 early last year electricity markets are required to compensate grid-response programs for their services at the same rate as generating assets. While few places have those markets fully developed yet, they will be coming.

    A grid battery or grid-aware tank water heater is the opposite of at tankless water heater- it's an asset reducing grid capacity requirements rather than a liability, increasing grid requirements.

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