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Should I install electric tankless hot water, a heat-pump water heater, or solar hot water?

Irishjake | Posted in General Questions on

So I’m in the process of specing out and finishing design on a Deep Energy Retrofit/Whole House Remodel on an old 1858 farmhouse.

House Demographics
We are in Zone 6a – NH.
PERSIST Technique is being used throughout.
R20 slab/R20 Sandwich Foundation walls/R40 above grade walls/R60 roof
We will have 9.3 kW of PV solar, and our current plans include two Solar Thermal Panels and and 80gallon Steibel-Eltron Water Heater/Storage Tank for our domestic hot water…..

That is until someone mentioned that I should consider a Heat Pump Water Heater. I’ve also considered Point of Source Tankless Electric Hot Water Heater by Steibel-Eltron (Tempra 20 & 24).

I like that my hot water would only be produced when needed.
We also go away a lot in the summer to visit family and I am concerned that the SHW will just be wasted.

With the large PV array won’t the Electric Tankless Steibel-Eltron Tempra 20 & 24 make sense?
The Tempra 24 is 100 amp, 240volts, and 24kW. The Tempra 20 is 80 amps, 240volts, and 24kW.

My utility is paying for my underground to my house whether it needs to be 200 amp or 400 amp service.

What should I do? Articulate, educated thoughts welcomed!!!!!!

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  1. GBA Editor
    Martin Holladay | | #1

    The most cost-effective option for you would be the heat-pump water heater. Compared to an electric resistance water heater, the heat-pump water heater will save enough energy to pay back the added investment cost in a few years.

  2. wjrobinson | | #2

    I'm a residential Electrician and Builder/carpenter. What I first do is find out who you are and work to discover your needs. I now try to anticipate three hair dryers curling irons and five countertop George Forman grills all active at once. 200 amps is plenty for such with a standard Water heater but not with three tankless units sized for 40 degree incoming water and a multi head two person luxury shower.

    So to answer the question, we need to know who are you? What are the needs? House size , number of bathrooms to be in use at one time, number of kitchen counter appliances used at once, natural gas?.

    Look at your kitchen counter and count plugged items, same in baths....

    That's a starting point for me.

  3. jinmtvt | | #3

    Helo Brad,

    i would suggest that you take time to calculate the tankless heater power range as during mid winter to early spring it might not be sufficient if you like hot showers.

    I had plan to install a single tankless ( Tempra 36 ... yes ~180A @ 240v ) when i designed my house ~8 years ago. Little did i know back then about building science. So i learned the "VERY EXPENSIVE HARD WAY " that incoming water temperature up here in Quebec; combined with multiple head showers = impossible to run tankless.

    I had to pay additional 200A capacity for my electrical entry ( probably ~2000$ ) additional wiring and panl inside the house, only to find out that during winter time, the 36KW unit was not able to maintain steady temperature when my wife was using kitchen hot water faucet at the same time i was in my shower ( which has 1gpm body spray and 1gpm hand shower ) .

    We have 2 showers in the house ( one of which is still unfinished ) , just imagine when having guests using both showers at the same time.

    Tankless is only worth it if you are going to be out of the house more often than in,and even then could always turn off the water heater when going out for long period of time.

    My regular 60gal electrical tanks have a sticker for energy rating here in Canada, which says something like ~ 80$ worth of standby loss for whole year, and then you have to remove the heating season portion of that.

    Either get a regular cheap water heater and save on initial costs ( also makes it possible to upgrade cheaply to a new technology in ~ 10 years or so ) or get a reliable heat pump water heater
    and save energy while using it.

    Don't forget to plan for a drain water heat recovery .

  4. Expert Member
    Dana Dorsett | | #4

    Active solar is expensive, and the maintenance has a cost too. Applying that budget to more PV to cover the power use of a heat pump water heater would be a more cost-effective way to spend that money.

    An electric tankless is ridiculous way to heat water in a zone 6A climate due to the low hot water delivery volume performance in winter. It's also a peak-power-pig. If residential rates ever get re-structured to apply "demand charges" to the largest 15 or 30 minute draws at any point in the billing period (as is commonly done with commercial electric customers), a huge intermittent load like that would send you off the charts, even if you don't use much power overall. Demand charges are fair way to assess the grid infrastructure costs, since piggy loads are what drive up the grid capacity requirements, whether they are used constantly, or only for 15-30 minutes at a time. With net-metered solar customers who zero-out their energy use every month (as you are likely to do), utilities looking to recoup their infrastructure costs from those customers as well, either as fixed monthly fees, or demand charges.

    So far only one utility in the US has gone the demand-charges route, but many are looking at it, since it is a fairer way of assessing the cost of the infrastructure required by customers, rather than applying it as a fixed per-kwh adder, as has been traditional for residential rates. While PV owners are the impetus for the utilities wanting those changes, demand charges would fairly address the cross-subsidy of energy sippers who never draw more than a couple kilowatts at a time, to energy gulpers with 8 ton air conditioners and electric tankless hot water heaters. SFAIK no utility in NH is currently assessing increased fixed fees to PV owners or applying demand charges, but that will eventually HAVE to change, as distributed generation becomes the rule rather than the exception. A the rate that PV costs are falling (against a backdrop of rising electricity rates due to gas pipeline constraints) this will likely be coming to a head well within the lifecycle of a tankless hot water heater.

    Bottom line- at any total power use it's safer / better / greener and nicer to the other folks on your side of the substation to be a power-sipper than a power gulper, which also tilts in favor of heat pump water heaters, and strongly AWAY from electric tankless HW heaters.

    A tankless HW heater is an underperforming 20-24kw energy gulper in a zone 6 climate. A standard electric tank will still hit 9-10 kilowatts or more if you fill a tub with it and the bottom element turns on.

    A typical heat pump HW heater is on the order of 1-3kw if you put it in heat-pump-only mode, but can be as high as 10kw or higher for those with multiple resistance-heating elements. eg:

    The all-stainless ATI-66 draws 2.5kw in heat-pump only mode, but has two 4kw elements (much like a standard tank), if you choose to run it in fast-recovery mode, which would put it at 10.5kw if all three were active at the same time:

    The Stiebel-Eltron Accelera 300 draws a paltry 500W in heat-pump only mode (which takes forever to recover), but has a a single 700W resistance element, which raises it's peak draw to about 2200 watts:

    The Accelera 300 is probably the lowest peak draw of any heat pump water heater out there, but one with the slowest temperature recovery too (which is why it needs to be 80 gallons). But that's a full order of magnitude less power draw than any electric tankless that could support more a single 2 gpm shower in a zone 6 winter, and it could support 10-15 gpm draws no problem, for fast tub fills and no screaming person in the shower when somebody tries to run the clothes washer or wash their hands during the shower, which is the scenario you'd be looking at with a tankless. From a demand charges and grid infrastructure point of view, this is the right choice.

    If you're away for days/weeks at a time during the summers the solar hot water is not only wasted, you have to actively heat-dump (by design) to keep the temperatures within the operating maxima of the system components, which adds cost & complexity to the solar hot water system. If you're still going the solar-thermal route, the up-charge for going with evacuated tube collectors (which don't have the same stagnation temperature issues as flat panels) may be cheaper than heat-dumping.

    But with PV and heat pump water heaters there is no summertime down side, whether home or away. Half the heat going into the water with a heat pump water heater is being drawn from the house, taking a small amount off the cooling load. In the winter that adds a modest amount to the heat load, but in YOUR house the heating energy use is likely to be smaller than than the hot water heating use anyway, and the cooling season will be more like 5-6 months compared to the pre-retrofit 3 months, since your heating/cooling outdoor temperature balance point will have moved to something like 50F, and the solar gains from windows will very likely slightly overheat the house on sunny days that average over 30F.

    How are you intending to heat this place?

  5. Irishjake | | #5

    Thanks for all the great answers....
    I imagined that a HPWH was going to be the best choice, but analysis/paralysis has taken over a bit.

    What a about an Electric Marathon Hot Water Heater with add on Heat pump?

    I'm heating with mini-splits.
    One ducted unit in the basement (one zone) and
    One ducted on the first floor (one zone) and
    One 1:3 upstairs (one in each bedroom)
    Electric mats in the floors of the bathrooms and laundry room.

    Condensing heat pump clothes dryer.
    Induction cook top
    LED lights
    DWHR unit
    Nest Thermostats WiFi enabled
    anything I'm missing?

  6. wjrobinson | | #6


  7. STEPHEN SHEEHY | | #7

    Brad: From what I've been able to determine, the newer model GE Geospring HPWH (made in KY) is reliable. That was not the case for the older model made in Asia. You can buy one for less than $1000. According to GE, it only draws 550 watts in HP mode.

    Putting a heat pump on the top of a Marathon would cost more.
    The steibel eltron gets great reviews, but costs about $2500.

  8. jinmtvt | | #8

    stephen: woa thanks for the tip

    quickly found it at ~ 1020$ and is USA made..superb!

    Will be looking for it on my next projects for sure.

  9. Dana1 | | #9

    The Geospring is a glass lined tank which has a ~10-12 year lifecycle. The AirTap ATI xx series features an all stainless steel tank, and should go for decades. (I believe that's the case with the Accelera as well.) The stainless tank is a big part of the price difference.

    Key to getting max efficiency out of these is to get one big enough that you never have to run the resistance elements, running in heat-pump only mode.

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