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Community and Q&A

Electric tankless water heater

David Voros | Posted in Mechanicals on

Do any of the pros out there have a recommendation on a tankless electric water heater? The water comiing in is very cold. Any suggestions would be great.

I’m thinking electric tankless for several reasons:
-Vacation home- long periods of no use, but when the home is being used it’s being used by a lot of people.
-It’s got PV Panels
-Limited space for tank heater
-Natural gas is not stubbed, propane is available but needs to be trucked in
-I would like to avoid another exhaust pipe through the building envelope

Any suggestions would be helpful, thanks!

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #1

    David,
    Stiebel Eltron is a quality brand. There are others.

    The major disadvantage is that you need oversized electrical service, which can be expensive to install. (Operating your water heater won't be any more expensive than operating a tank-style electric resistance heater.) There are also disadvantages for the local electric utility, since it has to provide high-current electricity in short bursts, but you may not care about that.

    For more information, see Point-of-Use Electric Tankless Water Heaters.

  2. User avater
    Dana Dorsett | | #2

    The grid infrastructure required to run the very high power intermittent load of a whole-house tankless (or even point of use) are substantial. You may even need to upgrade the transformer serving the house to be able to support a tankless electric. A typical whole-house tankless draws in excess of 25,000 watts and needs a 150A/240 VAC breaker. (Many houses don't even have 150A service for the whole house.) A typical tank draws less than 5000 watts.

    The standby loss of an electric tank type water heater is pretty low. When you're away for more than a few days it can be simply turned off.

    In many locations utilities are struggling to recover their infrastructure costs, particularly on low-use or net zero electricity houses with solar. The concept of applying "demand charges" to the billing has been common for commercial and industrial ratepayers to cover infrastructure costs has become commonplace. Demand charges take the heaviest use half hour or 15 minute interval during a billing period and apply a $/kw (not kwh) multiplier to that period, and it's often the largest single line item for those customers. This type of billing is being proposed for special classes of residential ratepayers all over the US, the most common special class being low total-use solar ratepayers.

    If demand charges for solar customers gets implemented in your area, an electric tankless can easily DOUBLE the demand charges assessed- it's a liability! A single 8 minute shower can add 10kw+ to a 15 minute interval, whereas a typical electric tank adds less than half that. It might use slightly more energy overall (due to standby losses), but it's drawn out over time.

  3. Jon R | | #3

    Keep in mind that a low flow (perhaps 1 GPM) shower head helps with sizing/amp draw (< 10kW).

  4. User avater
    Dana Dorsett | | #4

    And, at 1 gpm a 10 gallon under-cabinet tank storing water at 140F can still serve up an 8 minute shower with margin to spare, but it won't fill a tub.

    The utility Eversource recently had their rate case in MA approved that included demand charges for solar ratepayers:

  5. Calum Wilde | | #5

    Dana,

    You're touching on something that I've thought about a few times. Hot water systems at this point have two distinct uses; everyday low volume uses like dishwashers (3-5gal) and showers (5-20gal), and high volume uses such as filling a bathtub(35-80gal) . There's such a huge divide between those uses I'm surprised we aren't seeing more ideas for a way to split those uses to give better efficiency for both.

    I don't know what the answer is though. I feel like we have a fairly typical family. We have two kids that use the bathtub once in a blue moon (they typically shower) and we use it maybe once a month. It seems wasteful to size the hot water heater for that. Any thoughts on separate systems that would be more efficient than a single use system that is setup for a broad range of uses?

  6. David Voros | | #6

    Dana,
    You've pretty much convinced me to avoid the tankless electric water heater. Now I'm considering a short tank water heater in the sealed crawl space. Is there any wisdom to 2 tanks? Thanks for all your great insight!

  7. User avater GBA Editor
    Martin Holladay | | #7

    David,
    If you need to supply hot water to two rooms (for example, a kitchen and a bathroom) that are far apart (say, 20 feet or more), you might want to consider installing two water heaters (one near each room that needs hot water). Otherwise, a single tank makes more sense.

  8. Trevor Lambert | | #8

    I have two tanks. It's nice to have the extra capacity, but obviously it comes at the cost of the second tank plus the standby loss of that tank. If you can get a single tank big enough to handle your needs, that is preferable. The problem with any electric tank is that the recovery rate is painfully slow when compared to natural gas. If you get two tanks, I would say having them in series makes more sense than having them independent, serving different areas. There will be a lot of times where the entire demand is from one area, and having them in series allows both to serve that demand.

  9. Jon R | | #9

    At $250 (plus 60A wiring) for a 11kW tankless, you could add it in series, after a tank and it would only kick in if the tank ran out.

    It depends on how you prioritize cold showers vs unlikely demand charges.

  10. User avater
    Dana Dorsett | | #10

    There are several manufacturers with 50 gallon lowboy electric water heaters, which would be cheaper than a pair of 20 gallon lowboys. Marathon makes a short 50 gallon version too.

    Measure up the available space for both height and the size of the access hatch/door when shopping for a lowboy water heater.

    The notion that demand charges are "unlikely" isn't well founded, particularly for solar ratepayers. As more distributed solar goes on the grid this has become a utility favorite proposal in rate cases. The recent MA approval only made news because it's one of the largest utilities to be granted demand charges for solar customers so far, but they are far from the first. As utility business models evolve it's conceivable that residential demand charges will be approved for ALL customers in some US utility somewhere before the warranty period of a water heater is up. (No, wait, that has already happened in some local utilities in Alaska.)

  11. User avater
    Dana Dorsett | | #11

    The standby losses of an electric water heater are already pretty small. Upsizing the volume of the tank doesn't change the raw kwh/annum of standby loss by very much. Abandoned heat and distribution losses in the distribution plumbing usually adds up to quite a bit more than the standby loss of a 50 gallon electric tank, and doesn't go away when going tankless.

    The IRC has changed up the particulars over the last three iterations, but in general terms it's worth installing at least R3 on all distribution plumbing on 3/4" pipe, and on any half-inch pipe at some length. With tanks it's also worth insulating the temperature & pressure relief valve, and the first 5-10' of any outflow piping from the t& p valve.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Dana,
    You wrote, "Abandoned heat and distribution losses in the distribution plumbing usually adds up to quite a bit more than the standby loss of a 50 gallon electric tank, and doesn't go away when going tankless."

    Actually, the right way to install a tankless electric water heater is as close as possible to the shower or sink where the hot water will be used. Since most homes with tank-style water heaters have a significant length of pipe between the water heater and the fixtures that require hot water, I would argue that switching to a tankless electric water heater almost always reduces "abandoned heat and distribution losses" significantly.

  13. David Voros | | #13

    Martin,
    Are you suggesting putting a tankless heater near every bathroom and kitchen to avoid the distribution heat loss? I guess that would be possible with the crawl space and it would also take less power because I could use smaller units.....There's only 2 bathrooms and a kitchen so 3 units wouldn't be too bad.

  14. User avater GBA Editor
    Martin Holladay | | #14

    David,
    I'm not suggesting that you install tankless electric water heaters. But if you do, it's important to install each heater close to the fixture or room it is serving.

    My house has one bathroom and one kitchen, and the rooms are side by side, so it wouldn't be hard to install a tankless electric water heater -- if I ever wanted one -- between the two rooms.

  15. User avater
    Dana Dorsett | | #15

    Even kitchen faucets designed to new efficiency standards have 2.2gpm aerators. At a 70F rise (say, 40F incoming cold water, 110F out at the faucet) that needs 22.5kw and a dedicated 100A/240V breaker. Just two of those in a house would add 200A/240V to the service capacity requirements.

    A small volume point-of-use tank water heater for the kitchen might make sense, maybe another for laundry if it's a long way from the main water heater, but never multiple point-of-use tankless units. The only large volume draws tend to be for bathing/showering, and the only location where it makes sense to install a tankless (and only if there is no other option that fits.

  16. Calum Wilde | | #16

    Dana, you wrote "Even kitchen faucets designed to new efficiency standards have 2.2gpm aerators."

    This is just the first one I found, but there's plenty of kitchen faucets with 1.5gpm aerators.

  17. User avater
    Dana Dorsett | | #17

    Calum: Yes there are lower gpm aerators than the standard maximum, but it doesn't come close to solving the problem:

    At 1.5 gpm with a 70F rise is still over 15kw, and the tankless would "only" need a 75A/240V breaker. A pair of those would add more than 150A to the service drop capacity requirements. That's already 100% of the capacity of the panel in my house, and could be more than the existing panel capacity at David's vacation house. The transformer serving my house (and 7 others) does not have sufficient capacity to bump all 8 houses on it to 200A, let alone the 300-350A or so per house it would need to run 8 multi-tankless houses.

    It's also not clear whether the 2kV distribution feeder supplying my neighborhood could manage a whole neighborhood at 300A per house. Even if it has that much raw capacity, the power quality in the neighborhood could spin out of control quickly if tankless water heaters became ubiquitous. The infrastructure needed to manage loads like that costs real money, and YOU (and your neighbors) are ultimately paying for it.

    By contrast...

    Even the largest capacity Level 2 car charger draws less than 100A/240V. Most Level 2 car chargers are under 50A/240V. Unlike water heaters, it's fine to interrupt car charging when the local grid load is at or over it's capacity limits, for a few minutes, and "smart" car chargers that allow the use of that intemittent load to stabilize and smooth out the distribution grid load can even extend grid component lifespans while improving grid stability, and moving more kwh. In Denmark electric car owners with smart chargers are already being compensated with cash for allowing the utility to use it for ancillary grid service, whether it's just a one-way power flow (as people with electric tank water heaters can be paid for that in some parts of the US), or 2-way flow, allowing intermittent draws from the car battery (but still net-charging), where they get paid even more, on the order of €1300/year. A similar (but much smaller) demonstration program exists for the EV fleet at the Los Angeles Air Force Base with energy flows both to and from the car batteries. There the energy is always free, and they are paid between $1000-2000/car-year for the ancillary services provided.

    Over the next decade as EVs and plug-in hybrid numbers grow this sort of thing will become common. It will likely become the go-to method of managing otherwise potentially grid-disruptive loads of EV charging if done in an uncontrolled manner. And in the process it improves both grid stability and grid throughput, and becomes a readily available controllable load to suck up the belly of the PV "duck curve" on sunny cool days with minimal mid-day air conditioning load, and lots of excess PV output.

    But you can't do ANY of that with a tankless water heater- it draws what it needs, when it needs it, and would not provide satisfactory service if it could be interrupted intermittently for grid management. It's just a very heavy, intermittent load that forces the grid capacity infrastructure requirements higher. With no possibility of using it for improving the power quality on the distribution grid, it's just a problem that other infrastructure needs to be put in place to solve, the opposite of "green", despite the very low standby losses.

  18. Calum Wilde | | #18

    Oh wow. Sorry I wasn't trying to argue against your point. I agree with your view on electric tankless, and grid loading in general. We take the grid for granted too much and while some of us are busying ourselves with reducing consumption, I'd love to see more options to smooth out our grid loading. I have several ideas that I'd love to discuss them with you and other members of GBA, but someone else's question thread isn't the right place for that.

    But yeah, I fully agree that tankless electric is a horrible idea for all involved. Has anyone compared the reduction in efficiency of heating the water so quickly and thus losing some heat to the surrounding area, to the standby heat lost in a tank style heater? I've often wondered if the losses of a tankless would even offset that, let alone the increased maintenance costs, and increased purchase price.

  19. User avater
    Dana Dorsett | | #19

    The efficiency of an electric tankless is still pretty good, typically scoring 0.99 or better under EF test protocol (= less than 1% thermal losses to the surroundings). That's compared to the EF 0.95 minimum for the (US) National Appliance Energy Conservation Act since 2013, for tanks 55 gallons or smaller, or a 5% net standby loss.

    The EF test assumes something like 63 gallons of water per day, a storage temp of 140F, a room temp of 65-68F etc (it changes a bit every few years- I'm not keeping close track.) If you're only using 30-32 gallons/day the standby losses don't change, but will be more like 10% of the total water heating energy, but that's 10% of a much smaller number.

    There's really no "payback" in financial terms on an electric tankless, even if the thermal losses were ZERO. A typical 50 gallon EF 0.95 might use ~4500kwh/year at "typical" 63 gallons/day use, 225kwh of which is standby loss. At 20 cents/kwh that standby loss is less than $50/year. If you factor in anything for the higher maintenance cost and the additional cost of the heavier breaker & wiring it won't break-even over the lifecycle of the unit. Most people who end up going that route are all about the space-savings, not energy or cost savings.

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