Water Heaters: Electric tankless or Heat Pump?
carpeverde
| Posted in Energy Efficiency and Durability on
It’s time to replace my gas water heater and go all-electric. Should I install an electric tankless water heater or a heat pump water heater? I live in the hot humid climate zone 2A.
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Replies
Electric tankless can be a problem to install due to their VERY high electric demands. I would avoid those. If you want to go all electric, a heat pump water heater is your best option.
Bill
An electric (gas too frankly) tankless has zero benefits besides a small physical footprint. They are inefficient and have poor performance vs tanks.
Tank, whether heat pump or resistance, is a much better choice unless you need an extra few sqft of floor space.
Heat pump. And get the rebate!
In CZ2A, a heat pump water heater makes sense because you can place it outside in a garage or carport if you don't have space for it inside, and if placed indoors, you get year-round air conditioning as a side benefit. In colder climate zones the options are more limited and heat pump water heaters aren't always the best choice.
The only way I would consider an electric tankless unit is if you spend a lot of time traveling with your house sitting empty, and if your power supply is robust enough that the water heater can easily get the 120 amps or so they require to run.
Rheem, at least, has many models under 50A and even under 30A. (https://s3.amazonaws.com/WebPartners/ProductDocuments/7F39BA60-854F-417D-A1BC-05C26B591073.pdf) Depending on the building type — for example, a one-bedroom apartment — this may be perfectly workable as a replacement for an electric tank heater.
Unless space is a concern, low performance tankless heaters make little sense.
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My HPWH has a published COP of 3.7. That means a tankless would use 3.7 times as much electricity.
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Standby losses have nothing to do with the CoP of the unit. Standby losses are just heat loss through the insulation of the tank. The CoP has to do with the amount of energy consumed by the heat pump unit in relation to the amount of heat it puts into that tank.
This is similar to comparing the rated size of your furnace to the amount of heat it takes to heat your home. The efficiency rating of the furnace in this case has nothing to do with the amount of heat lost through the walls of the house.
Bill
Scott, good point, and I have spec'd similar units by Stiebel Eltron for in-law apartments or in one case for a kitchen sink that was far from the heat pump water heater and bathrooms. But they aren't big enough for typical whole-house use.
The tradeoff with low electric draw heaters is that they either can't raise the water temperature much, or they can only handle very low flow rates. Physics dictates how much energy is required to raise a given amount of water a set number of degrees, and there is no way around that. In theory, electric resistance water heaters are 100% efficient, so there is NO way to get more hot water out of one with less input energy -- you ALWAYS have to trade off one or both of either reduced flow rate or lower temperature output water.
Bill
Thanks, Bill. We're a senior couple with very low hot water demand. Hot water use is mostly for showers only because clothes washing uses cold water and dish washing is warm. We're planning on electrifying our house to remove the gas range/stove and vintage 1956 gas wall heaters, so some upgrading of electrical service is in the cards anyway, but we don't need to bump it up so much to take on a watt-heavy tankless water heater on top of everything else. Appreciate your insight.
The issue with tankless hot water heaters is not the total hot water used, it’s the instantaneous amount used. Even if you only took 1 shower per month, you could easily exceed the capacity of an electric tankless. Even a 2 gpm shower at 100 degrees with 60 degree incoming water would need almost 50 amps. With a tank, you could use less than 15.
Thanks, Michael. We can sure use the little bit of cool air venting indoors for most of the year, and no, we don't spend time traveling. I appreciate your point of view and help answering this question.
If you have an indoor garage then a heat pump water heater is perfect, free dehu and cooling :). If it isn't going into a garage then keep in mind that heat pump water heaters can be a bit noisy ... but they also require air-flow so you can't shut them up inside a closet.
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A heat pump would be 3-4x more efficient than a resistance heater, whether tank or tankless.
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It depends on the CoP of the heat pump, and that, in turn, depends a bit on the operating conditions of the heat pump (primarily the ambient temperature of the room it's in). On average though, the 3x-4x improvement Paul mentioned is probably pretty close when comparing energy use of the heat pump to energy use of the tankless. The standby losses (which are zero for the tankless) of the heat pump are not going to be enough to offset the savings of the heat pump in terms of operation, so I don't see a scenario where the tankless comes out ahead, except possibly in a seldom occupied cabin.
Bill
A HP water heater comes out ahead in the cabin scenario too, as long as you turn it off during periods of no occupancy. Pretty standard practice.
Yep, mine has a "vacation" button for just that purpose.
I wish we could bury the “tankless means efficient” narrative!
pnw_guy, here's a rough calculation of standby losses for a tank:
A 50 gallon tank has a surface area of about 30 sqft and has an r-value of about 16. Assuming the water inside the tank is 120 degrees and the ambient room air temp is 70, you have a 50 degree delta T. 365 x 24 x 50 x 30 / (16*3412 (conversion to kwh)) = 240 kwh. Divide this by the COP of the heater, so call it 69 kwh per year lost for a heat pump. That's $10.35 per year using my rates, assuming the tankless heater has 0 standby losses.
Tankless heaters, on the gas side, are mostly more efficient that a typical Home Depot gas tank heater because they condense, not because they are tankless. The marketing focuses on the wrong feature.
Which of the heat pump units are the quietest and most reliable? With the HERS rebates I will essentially get one for free so it is a no brainer.
I installed a tankless in my previous house and absolutely loved it. In the six plus years we had it the only issues I ever had was the fuse it came with died within a month but a new one for a few pennies at Radioshack fixed that, and the little water movement sensor got clogged with minerals which was a super easy fix too. They are very simple to own and operate, and of course you never run out of hot water! Yes they consume massive power while in use, but zero power when not in use. My wife and I are pretty eco conscious so we only shower twice a week each, and since I work from home I don't go through a lot of clothes so we really only do laundry once every two-ish weeks. Add in one load of dishes per week and that's our entire hot water usage. We saw a pretty big savings switching away from a 30gal non-heat pump water heater. People say heat pumps are more efficient but I find it hard to believe unless you are using tons of hot water every single day (eg family of four who all shower every single day). Tankless units are also way cheaper typically. I think ours costs me $300 and in your climate, the ground water will be so warm you won't need some big 36kW unit; we were in Zone 3 (South Carolina) and got the 27kW which was way more than we even needed, it just happened to be on sale for cheaper than the 18kW one. I can't speak for all brands but EcoSmart uses multiple 9kW heaters and only use which ones you need, so if you got the 27kW one for example, but only used a trickle of water for something it would only turn on one 9kW coil. The biggest annoyance (and cost) is having an electrician come out to wire it all up. The EcoSmart one I had took very thick wire and needed three 40amp fuses which meant I had to get an entire new electrical box installed. The electrician bill was twice as much as the water heater itself.