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Community and Q&A

On Demand Electric Water Heater sensible?

cpk1 | Posted in General Questions on

Hey everyone,
I’m in the midst of planning my ADU that I intend to self build and try to approach pretty good house standards with it. I live on a small city lot so I am pretty much constrained to my existing garages footprint of ~18’x20′ and one story with a loft. Due to having such limited space I’m trying to figure out the best way to provide hot water for this small studio. Originally I was anticipating using an on demand gas fired water heater, however my city has implemented a moratorium on gas fired appliances for new residential projects so that idea is out.

Since space is at a bit of a premium I’m wondering if an electric on demand water heater would make sense? The unit will have a washer, dishwasher and shower (probably no bathtub) and some solar panels (2-3KW I hope), however the neighbor has a large tree just to the west of the structure so the panels will be heavily shaded the second half of the day.

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  1. walta100 | | #1

    If you want to go tankless electric you need a huge electric feed from the utility. To have a real shower (3 gallons per minute) will require 24-36kWh depending on the incoming water temp. That is 3 or 4 40 amp 240 volt circuits. If you plan on using the stove the AC and hot water at the same time a 200 amp service maybe pushing its limits. A 320 amp service is not a welcome last minute upgrade.


    1. cpk1 | | #2

      Thanks Walter. I'm planning on getting separate service for the ADU but anything over 200amp seems ridiculous and I'm not sure if that complicates the permitting process.
      I'm worried with a heat pump heater that is only 40 gallons showers will have to be very short if there are two people living here and getting ready at the same time. I have a 40 gallon in my mine house but with gas the recovery is very fast - I'm not sure with a heat pump heater if it would be. Would it make sense to have some kind of in line booster to help a tank recover faster or to heat water if it's low temp leaving the tank?

      I guess if a heat pump water heater has fast recovery in ~40gallon size I can try to squeeze that into the laundry area, or in the loft on the low head room side.

      1. brad_rh | | #11

        A heat pump WH does recover slower, but you won't have a problem with 2 showers and a 40 or 50 gal HPWH. I have a geospring 50 gal & it can easily handle 3 showers as long as someone doesn't get greedy (low flow heads). Also consider your peak and off peak pricing if you have it. Unless we have company I run mine in off peak hrs, and coast thru the peak time.

  2. onslow | | #3

    Alex D,

    I think you should just go with a large standard electric water heater. The size of the ADU you appear to describe (18x20 with loft) probably only encloses 6000 cu ft or less. Heat pump water heaters take heat from the air and one with a 300cfm fan will move 18,000cfm in an hour. Not an unusual time period when running in economy or energy saver mode, even when accounting for the wide variations in energy management programming in use. If you are in a net cooling climate, this might be okay, but do plan on louvers or ducting to prevent isolating the air volume for the HPWH. Do not plan on the loft for water, weight and vibration reasons. If you are in a net heating area, the energy savings will be at the expense of your heating costs.

    Demand electricity pricing would be another reason not to go with tankless electric if that is enforced in your area. Of course, any thing electric will be subject to demand rate, its just picking something that doesn't ask for 30-40 kwh in one bite is a good idea. You might also end up being put on a commercial rate for electricity in addition when the meter gets hit that hard.

    Walter and I seem to agree on the Marathon water heaters being a good choice. I have an 80 gal. that more than handles two people, one of whom loves their long hot showers. It is a bit of a pig Kwh-wise being resistance heat, but for my climate and house design, it is still better than fussing with a HPWH for the savings derived. It also doesn't make any noise.

    Again, if you can make use of warm outside air and appropriate ducting to feed and vent an HPWH, then it will certainly be gentler on your power bills and circuit breakers. Keep in mind that 80gal HPWH will offer much better buffer room for multiple showers over 50 gal. Particularly if the city water feed comes in very cold which can trigger programming that overrides the HP part of the system until certain set points are hit. Fast recover HPWH's are pretty much sure to be utilizing the resistance heat bars more. There is limited old data to support this, but I have beat that drum elsewhere on GBA.

    1. cpk1 | | #6

      Thanks Roger, I totally forgot to mention this is in San Jose California so heating demand is pretty low, however if the HPWH cools the area down enough the ADU might not need to run the AC which would be great. Our non-demand electric pricing is $0.23522/KWH but I'm hoping with solar panels that could help offset using electricity during high demands if we went that route.

      I'll look into sizes for 80 gallon units, I have plenty of height to work with so maybe a tall model can save enough floor space for me.

  3. Expert Member
    AKOS TOTH | | #4

    If you are looking for shower performance with tight space limits, might be good to install a drain water heat recovery unit. This would let you stretch a small tank further.

    You can also run the tank hotter to get more shower time. You'll need to add a thermostatic mixing valve to bring water temperature down to a safe temperature for house use.

    If you really want to sharpen your space pencil, you can probably get away with a 20gal electric tank in series with a 12kW modulating electric tankless unit. This would let you run a low flow shower head continuously without needing too large of a service but it would also handle a standard shower head and any other larger intermittent water draws. This seems like a pretty complicated way to save space though.

    You can save a lot more space with good layout (minimizing circulation space) and efficient kitchen layout with compact appliances.

  4. MFJerling | | #5

    Alex, consider my case and see if it fits yours as most people will try to talk you out of an electric tankless water heater. I have found that under certain conditions, it can be a very good choice and I am 100% happy with mine. It was the last item that allowed me to go 100% electric in 2016 and get rid of my gas service ($20/month just for administrative fees and taxes before any gas charges). It also allow me to get rid of a 13 year old conventional tank heater (natural gas) that was likely to leak at any time. This freed up a nice 2 x 2' space in my garage for storing mops, buckets, recycle bins, etc. which was a nice little bonus. The heater is a Rheem RTE-13 which is about $300 at Home Depot or online. The input is 13 kW and required me to run 220V 60 amp service from my 200A panel, but I DID NOT have to upgrade the panel and have had NO overload issues in 4 years. I also took the opportunity to add a 220V outlet nearby (low additional cost) for a future electric car.

    Here are pluses relative to a tank heater (electric resistance or heat pump). I didn't compare with gas since you said you cannot have it where you live.

    1) installed cost is about 1/2 electric resistance tank, or 1/4 heat pump tank water heater
    2) no tank to leak in the future
    3) mine has a 10 year warrantee, and you can easily replace it yourself in 20 minutes. The 6 gauge wiring is stiff but workable, and the water inlet/outlet has compression fittings which are easy to work with. It is small and weights just a few pounds. I can hold it in one hand!
    4) no safety valve required (at least in my area-- near Phoenix AZ)
    5) instant, unlimited hot water. It will put out 140F but I have it dialed back to 120F for safety reasons. We have a lot of guests in winter and have never had a problem taking 6 showers back to back, then doing 3 loads of laundry followed by a load of dishes.

    The following are not negatives, but you should consider:

    1) depending on your water, you may need to descale it every few years with white vinegar, similar to a coffee maker. In my case I can go 2 years and its a simple matter of disconnecting the fittings and laying the unit on its side with a cap on the low tap to keep the vinegar from draining, then dripping the vinegar into the high tap with a turkey baster and letting it soak overnight until the bubbles subside. I leave the wiring installed but definitely trip the breaker first!
    2) Since this is Arizona we have low-flow shower heads which put out about 1.6 gpm. This rate is perfectly fine for showers (don't let anyone tell you otherwise) and there is no issue with the 13 kW heater putting out enough hot water. However, you cannot run 2 showers at once, although I have taken showers with the washing machine filling at the same time and it wasn't a problem. Our inlet water into the house is shallow as we don't get frost here, but it does get pretty cold in the morning in January so yes it can overcome this.
    3) as an enhancement, when we had our master bath remodeled I had a thermostatic shower controller installed. These are more expensive (about $500 vs $150 for conventional) but becoming very popular where there is concern about the risk of scalding (nursing homes, etc.). Otherwise, if someone flushes a toilet, etc. in another room, the shower temperature can go momentarily get very hot or very cold no matter how you heat your water. The thermostatic controller controls to within about 1F regardless of the hot/cold pressure balance, and you will not feel any fluctuations. I highly recommend it.
    4) ignoring the higher installation cost and the part of the year it is pulling heat out of a heated space to heat the water, the HPWH will be somewhat cheaper to operate if you use a LOT of hot water, but it has disadvantages too. If you do use a lot of water than it may be a good choice, otherwise I would give the electric tankless heater some consideration.

    Hope this helps!

    1. cpk1 | | #7

      Thanks for all the detail. It sounds like I might be in a fairly similar position as you - we don't get quite as hot in San Jose, but low flow plumbing is required by code and water never comes into the house cold. Sounds like the biggest expense is having 200amp service running the right gauge out to the heater.
      Good point about the water quality - our water here is so miserably hard that I will be putting in a water softener for the ADU, before we got a softener for our house, our dishwasher lasted less than 2 years...

      Do you also have AC? Any problems showering and using the AC? This unit will have a mini split and while I don't think heating loads will get too bad I am wondering about a hot shower on a 100+ degree day while the AC is going. It sounds like I might need to do a little bit of homework to make sure the city will allow a 200amp panel for an ADU and how reasonably I can load it with a dryer, mini split and on demand heater.

      Are you able to comment on your energy bills? Did you notice a difference, if any when you went to on demand electric? I really want the on demand electric to make sense, and you certainly make it sound promising!

  5. Jon_R | | #8

    It may not be here yet, but I expect that the ability to time-shift electrical load will be increasingly important. You can do that with a timer on the tank or less conveniently, by changing when you shower.

    A 1 GPM shower (what I use) needs about 40A.

  6. MFJerling | | #9

    Alex, I purposely left out some detail in my first response as it was already getting too long, but your confirmation of low-flow appliances, warm inlet water, and water softening are encouraging for an electric tankless heater so I'll provide more info.

    I also have a 6.9 kW solar PV system (20 panels) and am grandfathered for net-metering until 2o35, which APS no longer allows for new installations. The system has consistently put out 12,5oo kWh per year since 2015. However, even on this plan I still have Peak and Off-Peak rates if I exceed my net-meter "banks", with Peak from Noon-7p weekdays being 3X off-peak in winter and 4X in summer. The Peak rates are very high here.

    In 2016, I also got rid of my gas furnace and rusty, inefficient 13-year old AC unit and replaced them both with a 3-ton two-stage heat pump (Bryant). The 220V, 60A service was run from my panel to the heat pump indoor air handler, along with the electric tankless heater, plus the future outlet for EV charging. The electrician simply ran the 6 gauge cable inside metal conduit from the panel outside, into the garage along the ceiling. It gets even better though... I also have a pool and used to heat it with a gas heater, but I replaced this with a heat pump as well and it draws 5 kW. Even with the two heat pumps, the tankless water heater, electric oven/range, electric drier, and an electric golf cart that I use as a car, I STILL have a 3000 kWh surplus at the end of year and get a credit for it at the end of year. So, to answer your question I do not see it on my bill because it is effectively zero other than the administrative fees and taxes that cannot be avoided, which are about $25/month.

  7. thegiz | | #10

    I have a tempra 29 plus tankless water heater, it’s a tiny box. Had a 50 gallon heater that broke that needed to be replaced and this is before I had gas. My 2 kids take a bath every night, along with 2 adults showering, etc. I live in nyc suburbs where electric is expensive but is still cheaper than having a tank. Basically when I had the tank it had to remain on the entire day. I would also run out of hot water all the time. Now it just turns on demand, takes a few seconds before it gets warm but so far it’s good. Will eventually switch to gas on demand but for now it’s still cheaper than what I had. I guess if your place is super insulated might be cheaper to have a tank, mine was in a uninsulated basement.

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