Water Heaters: Electric tankless or Heat Pump?

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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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!
I have an hpwh but sometimes I wish I had a tankless electric. I have a 200 amp panel and can handle it. I know they're not nearly as efficient but not only is it a space saver but there's other issues. I know hpwh's are much more efficient but the two times I've taken the lid off of my Rheem hybrid the insulation had a lot of black mold. I've sprayed it with lysol, where the insulation was thin I removed it. So to me there's more maintenance involved. That and I run my condensate line outside and here in the suburbs of Philly I can't do that in the winter because it'll freeze. So for now in the winter I just have it dump into a 5 gallon bucket and empty it every couple of weeks. I have it in a separate unheated storage room that in the winter will get down to the mid-40s but it's ducted to the house and I'm not real thrilled about the black mold situation. Also tankless electric is not nearly as efficient but I would imagine it's more efficient than it electric tank water heater because of standby losses.
Thanks, cldlhd. I hope you are able to find a solution to eliminate the mold completely. I imagine your mold growth is seasonal and you'll be able to tackle it just as it appears and not have to deal with spores the rest of the year.
I have a state 80 gal HPWH and have the temp set to 145 to kill most anything that may be in the water. Mine uses foam insulation so there doesn’t seem to be a mold risk.
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.
What if you heat the water up to 140° and have a mixing valve on the way out to cut it down to 120° because you don't want legionella and other forms of bacteria swimming around in your tank? Also what if you have the tank in a basement or room that's cooler than 70°? I have mine in a separate storage room that gets about 25° colder than that in the winter.
Then it will almost double to about $20 per year, except you only need the 140 once a week and you only have the cold surroundings in the winter so it's maybe $14/year.
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.
Several months back I installed a Rheem Platinum hybrid HPWH. Extremely happy with it. 80 gal unit set to 135, with downstream mixing valve set at 120 = never going to run out of HW. COP stated as 4.0. Quiet: I measured 56 Db at 4', 60 Db on the side with the cool air discharge. Sitting in the basement in center of house by furnace, I can barely hear it run when it's otherwise completely silent in the house, much quieter than the furnace. $700 rebate on the unit brought it to $1500 delivered, not so much more than a quality NG unit. Install was a bit spendy, $1700 included an expansion tank, overflow tray, and getting rid of gas line and two 40 gal NG tanks. But income tax credits will bring TIC to just over $2k. Bonus: was able to seal the natural draft flue, a 3x10 oval from basement to attic. Heat pump only setting, size of tank and mixing valve handles long showers and guests; Resistance element in my view just there as a backup in case HP goes out. Worried about reliability so bought the 5 yr warranty HD offered ... time will tell I suppose. This purchase was almost a no-brainer, I spent a lot more time thinking about the LG large capacity heat pump clothes dryer but went ahead, So far so good.
In addition to the higher efficiency of HPWHs that others have mentioned, they can also cost significantly less to operate than tankless electric if you have time-of use electric rates or solar on your roof. This is because you can make them run when electricity is cheap (or the solar is producing), effectively operating them as heat batteries.
In some areas, the utility or an affiliated program will even pay you to heat your water during off peak demand periods, or not heat your water during peak demand periods.
Using such strategies with a HPWH, I pay about $30/month for hot water heating (family of 4) even in California, where electricity prices are ridiculously high.
Danan you are leaving money on the table by setting the tank temp at 135°.
Try setting it a 120 and find out is 80 gallons is enough for your home.
The greater the temperature differential between the air around the HPWH and the water in the tank the lower the efficiency of the water heater will be.
Is more than 32 minutes of shower run time really required?
Walta
A good rule of thumb when it comes to heat pumps is if you're mixing, you're doing it wrong.
Some newer HPWHs have built in thermostatic mixing valves. As an occasional way to increase hot water capacity it seems OK.
Walt, I think you meant to address @dadams, not me 😁
You got me curious so I took a closer look at the different types available. I can't load the excel file or else I would.
Household Size (ppl) Average Hot Water Usage (gal/day) Annual Hot Water Usage (gal/yr) Electric Rate ($/kwh) Gas Rate ($/therm) Tank Standby Loss(%) Regional Groundwater Temperature (F) Water Heater Temperature (F) Tankless Gas Efficiency (%) Tankless Electric Efficiency (%) Tank Gas Efficiency (%) Tank Electric Efficiency (%) Heat Pump Efficiency (%)
1 29 10585 $0.17 $1.45 40% 55 120 89% 100% 75% 90% 370%
Annual Energy Consumption (BTU) Annual Energy Consumption (kwh) Annual Energy Consumption (therm) Annual Cost ($)
Tankless Gas 6,447,335 1,890 64.49 $93.51
Tankless Electric 5,738,129 1,682 57.40 $284.88
Tank Type Gas 10,711,173 3,139 107.14 $155.35
Tank Type Electric 8,925,978 2,616 89.28 $443.14
Heat Pump 2,171,184 636 21.72 $107.79
I don't buy the tank standby loss of 40%. That seems way high.
All the green cells are intended to be user input. What's a good value for standby losses? Are these published with the specifications from the manufacturer? It looks like it is, what is yours?
This version allows for different standby losses for different types. Wish I could attach the excel file....
A document I had from Lawrence Livermore National Lab gives a total standby loss of around 18-22% or so. Most of that are "Standby stack losses", which is basically heat from the water going up the stack vent when the burner isn't running. Using a sealed combustion type water heater would probably significantly reduce that.
Bill
Or electric.
And why would we expect a tankless gas heater to have less standby stack losses?
Put me in the camp that there's no reason to believe a tankless water heater has significantly less standby loss than a tanked one.
A tank type[gas] water heater is a chimney surrounded by water. That water is cooled by the airflow through the space designed to heat it, so a pretty efficient cooling process.
A tankless has no water to cool off, so while not zero standby losses, they are pretty small in comparison. Most of them seem to be direct vent, so little inside air migrating out.
I don't consider myself a fan, but there is some unsubstantiated rhetoric bouncing around here.
https://www.energy.gov/eere/buildings/articles/tankless-gas-water-heater-performance-building-america-top-innovation
https://www.energy.gov/energysaver/tankless-or-demand-type-water-heaters
Not sure I would buy one for several reasons, and these comparisons may compare tankless to the 'worst' tank type.
heat pump water heaters are 'newish' and have had their share of teething issues....
I think very little of tankless heaters. One thing that is always missed in this discussion is the variety of gas tanks and the variety of gas tankless options. You can, today, purchase a gas tank that is more powerful AND more efficient than a gas tankless heater. But the comparison made by the tankless proponents always seems to compare the crappiest Home Depot gas tank to the condensing Rheem. I think once people understood that efficiency is the same or better and performance can be better, the tankless product would wither, save for the space constrained.
Since the industry uses UEF to measure the efficiency of water heaters. I just eliminated the standby losses altogether and used the UEF instead since it incorporates the standby losses into the factor. Since I was looking up UEFs I went ahead and added some product data as well for comparison. Looks like the standby losses were indeed high but I guess that depends on which heater you buy too.
What I'm wondering though, looking at this data is, how can a tank type electric water heater only be 1% less efficient than a tankless water heater?
Want to hear something funny? A few years back I thought, well, if there's tankless water heaters and heat pump water heaters, then why not a tankless heat pump water heater? It'd be the best of both worlds, right? Lol, cause it's a tank that's why not. And by tank I mean a heat pump that is like a 7.5 ton/90mbh unit.
"What I'm wondering though, looking at this data is, how can a tank type electric water heater only be 1% less efficient than a tankless water heater?"
Because tankless isn't really any more efficient?
Modern tanks have low standby losses, they're well-insulated. I've had the experience of turning a tank off and coming back a few days later and the water is still hot. At the same time tankless don't have zero standby losses, they always are going to heat more water than you need.
It really depends how frequently you use the hot water.
This is also not accurate as you are not factoring in whole house energy consumption. With tankless you are getting no heat gain from standby loss. From heat pump, you are actually shifting your energy consumption to your heating system as it is taking energy from your indoor space during heating conditions With tank electric you are recovering all standby loss in indoor heating if you are in a cold climate. With tank gas you have, as bill mentioned stack loss.
The house climate zone will significantly effect these numbers, as well as the houses primary heating and cooling mechanicals
@#42
just wondering, what are some examples of gas tank heaters that are decently priced and as efficient as condensing tankless gas?
i was shopping for water heaters this past month and spent $900 on a condensing tankless 160k btu rinnai unit. the few condensing tank units i could find were 3x that.
They’re the high performance option! They should be more expensive than tankless.
Typical tank is around 0.7 UEF or less, condensing tank around 0.9 but about 3x the cost.
Condensing tankless are typically 0.95 and about the same cost as a power vented tank. Don't see how a tank is the more efficient option.
I'm inclined to think tankless water heaters are probably better than all but heat pump tank-type water heaters too. I expect the tankless has essentially zero standby losses, which is where all the savings come from.
The difference with a heat pump water heater is that you essentially get more heating BTUs put into the tank than the energy you consume to run the unit, because the heat pump scavenges BTUs from the space around it. THAT makes things more complex, and would mean the tankless would now only be more efficient in situations where there is very little hot water use, such as a cabin that is only occupied part time. The "very little" unit of measurement though would need to be worked out if you really want to know where the exact breakover point is.
Personally, my main issue with the tankless units is they seem to have more issues over time, especially on well water, compared to tank type units. Since I'm on a well (a brand new one as of a few weeks ago, but that's another, expensive, story), the longevity of the water heater is more of a concern to me than overall efficiency is.
Bill
Thanks, Bill. That's a very important point - the durability and usefulness over time of a water heater being more of a factor than overall efficiency.
It’s close enough and you get performance that tankless cannot offer
but what does that mean? what is the impact of that in reality if the uef numbers for a condensing tankless is the same, if not better than a condensing tank?