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In my understanding, heat pump water heaters pump energy from the room around the water tank into the water.

Clearly, this must be energy efficient in the summer time, especially if I am using AC anyway. The heat pump water heater may actually cool the house a bit.

However, in the winter time the savings are not so obvious. If I am using a heater to heat the home, including the water tank room, then I don't see how there can be any savings. Instead of generating heat using a coil in the water, I generate heat in another area of the house which gets pumped into the water. A joule is a joule and I need to get it from somewhere. It seems to me that if I am actively heating the house, and lets assume everything is electric, then a heat pump water heater cannot be more efficient than a traditional one.

Am I missing something in the winter case?

  • best installed outdoors. – Jasen Jan 30 at 0:17
  • Are you only considering unitary heat pump water heaters, or are split system HPWHs an option here as well? – ThreePhaseEel Jan 30 at 0:42
  • installed indoors it would steal house heat to warm the water. could be useful in the summer. but not so much in the winter. – Jasen Jan 30 at 0:44
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    Where in your house is your water heater placed? The recommendations for a heat pump water heater are that it cannot be in a small enclosed space or it will chill the space to such an extent that it becomes inefficient or makes the space too cold. – Jim Stewart Jan 30 at 0:44
  • @Jasen yes you are right, I had it backwards. Inside is best for summer, outside for winter. – SDiv Jan 31 at 1:25
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You are right that a unitary HPWH is a parasite heating load in the winter...

When most people think of a heat pump water heater, they think of the unitary heat pump water heaters that were mandated as a replacement for large electric tanks (bigger than a 55gal drum) by the NAECA 3 standards, released in 2015. These have all the heat pump machinery atop the tank, and as a result, draw heat from their surroundings to heat the water in the tank. This means that they are as Harper said: nice for the utility since they don't have to ship as much electricity to you, but not so nice for you in that you are still paying for 100% of the heat that goes into your water.

However, unitary systems aren't the only game in HPWHville

While unitary heat pump water heaters are the most common type out there in the US, they are not the only type of system on the market. The Japanese, in particular, developed a very different approach to using a heat pump for water heating, generically called a split system heat pump water heater, and often called an "EcoCute" as a trade-nickname of sorts.

These systems consist of two parts: an interior water tank and an exterior heat pump unit, the latter similar in form-factor to the outdoor units used in mini-split heat pump systems. Water in these systems comes into the tank, then is circulated from the tank outside to the heat pump unit, which heats the water and sends it back indoors to the storage tank. Hot water can then be drawn from the tank as if it were any ordinary storage water heater, although some split systems use a mixing valve at the heater to maximize efficiency by way of storing water at higher temperatures than it is actually used at.

While more difficult to install than a unitary HPWH (due to the need for outdoor plumbing and accompanying freeze-prevention measures), these do not have the "parasite load" problem in the winter months, and can provide plenty of output in just about all weather conditions short of Arctic winters (the Sanden units are rated for operation down to an ambient temperature of -20°F) without the need for an electric backup heating element. They also draw less current than the current generation heat pump water heaters under all operating conditions (15A@240VAC for the Sanden units I have researched vs the 30A@240VAC circuit required by most current hybrid/unitary heat pump water heaters), making them more amenable to running off of a backup generator or even off-grid without losing significant capacity, or putting an unreasonable load on the genset or inverter.

Note that while the Sanden units aren't the only split-system heat-pump water heaters out there, most of the other units (the Daikin Altherma comes to mind, although there are others as well) are designed for space heating or combination domestic hot water & space heating loads, and have poor support for being used purely for domestic hot water.

If that's not possible...

In situations where a true split-system HPWH (Sanden EcoCute, Daikin Altherma, etc) isn't feasible, some unitary HPWHs (most notably the Rheem/Ruud units) support duct kits that allow outside air to be pulled in to "feed" the hot water heater. This can improve efficiency in moderate climates, but is subject to limits as the heat pumps in unitary HPWHs are not built for low ambient operation (with ambient low limits in the 30°F to 40°F range). It's also possible to put the unitary HPWH in an unconditioned space, but those spaces are becoming scarcer due to trends in modern construction and it also can increase the standby losses the tank is up against in colder climates, so one must evaluate the tradeoffs of this approach carefully.

  • Actually all the 'split' systems I've seen don't circulate water but coolant between outside unit (motor) and inner unit (tank). – DDS Jan 30 at 14:41
  • Is this equivalent to having the heat pump outside during the winter and inside during the summer? I think that's probably the best solution. – SDiv Jan 31 at 1:21
  • @DDS -- there are units that use a more traditional split-refrigerant architecture yes (such as the Daikin Altherma), but finding documentation on them is harder, and they aren't quite as efficient as the Sanden units are – ThreePhaseEel Jan 31 at 2:17
  • @SDiv it's basically equivalent to having the heat pump outside all the time -- it does mean you lose "free" cooling during the summer, but that's not that big a deal in the grand scheme of things – ThreePhaseEel Jan 31 at 2:19
  • A lot of houses have their existing water heater in a garage or other unheated space. If that's the case, a unitary heat pump water heater can still be worth it. Basically your garage is going to be colder in the winter. – Vectorjohn Mar 13 at 23:28
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A joule is not a joule

The joules used by a resistor water heater must be made at an electric power plant. Electricity is an inefficient way to transfer heat, since thermal generating plants are 33-40% efficient, 50% at the absolute outside for turbine+boiler plants which have a gas turbine turning one generator, then its exhaust (waste) heat is used to boil water to fire a traditional steam turbine/generator. Very Rube Goldberg and it relies on expensive, maintenance-heavy gas turbines, which in turn require gas or petroleum fuel. The long distance power transmission is reasonably efficient but still a source for 5-20% loss.

It is nowhere near as efficient as using gas or petroleum to make heat locally right in your house.

The most efficient way to heat both house and water is with local flame using locally supplied fuel.

So a heat pump water heater "third-party"s the core issue of heat generation off to your house's main furnace. That, however, is not in their bailiwick, so they don't have to account for it. The power company gets to declare victory, "look at all the energy we're saving, who says you can't do over-unity! It's magic!"

The general concept is that you use a little electricity plus the normal anount of oil or gas, which is cheaper and more efficient because you're making heat locally at 80-90%. Except really, you're using almost as much fuel as you would've running both furnace and water heater, because you must reheat the house against the ice cold exhaust of the water heater. You can "toss the old air outside", but then, you must suck an equal amount of air in the house from outside, so ejection is only a win if this replacement air is significantly warmer than that ejected. But yes, in the winter it is a net lose.

On the other hand, it's a big net win in the summer. And summer is when most regional power grids suffer their worst peaking conditions. So from a grid engineering POV, it's a major win.

  • Thanks for a nice answer. In the DC area most people have only electric heating (crazy and inefficient I know, hence $250 electric bills). In this case with a unitary system, I think the conclusion must be that there is no gain in the winter (but probably no loss either) but potentially a nice gain in the summer, as you suggest. – SDiv Jan 31 at 1:24
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    This is partly right. The thing to consider is that heat pumps can do more (much more) than 100% efficiency in energy use. If that difference is enough, than its worth the losses of electricity to use a heat pump. A typical, decent heat pump water heater can do 300% efficiency, which makes them use less total energy. It isn't magic, it's thermodynamics. Heat pumps don't generate heat, they move it around. So they're not limited to 100% efficiency. – Vectorjohn Mar 13 at 23:25
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This is more complex to understand then it sounds. There are at least a couple of non-obvious elements to take into account -

  1. A heatpump does not rely on the difference between inside and outside to create heat - heat is vibration of molecules, and it is theoretically possible to cool something down to almost ABSOLUTE 0 (ie approx -460f). It is true, however, that heatpumps are generally more efficient where the "outside" temperature is closer to the desired temperature.

  2. Water is about 1000 times as dense as air, so the amount of energy needed to heat the water is a lot higher then the same quantity of air, but this will impact on the calculations of heating.

  3. Burning raw fuel is almost always cheaper then using electricity - depending on how cold it is "outside", there is often nothing in it "pricewise" between using a heatpump and burning LPG or equivalent. (But burning waste oil, while bad for the environment and a pain to maintain is a lot cheaper then even a heatpump)

I have a water heater which is driven by a heatpump. I live in a temperate climate and the heatpump is outside. This means it has virtually no impact on temperature of my home.

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    A water heater based on electric resistance heating is reliable and the usual failure modes simple to repair. I have heard horror stories of problems with HPWHs. What has your experience been? However, I do recognize that we (the human species) simply must reduce our energy consumption. And personally I would rather spend my money hiring repair techs than paying fossil fuel producers to mine and transport the fuels needed for inefficient, polluting power generation. – Jim Stewart Jan 30 at 13:25
  • What brand of heat pump water heater do you have? – Jim Stewart Jan 30 at 13:32
  • @JimStewart Its an Australian Brand - Quantum. I confess that I no longer own it (i sold that property), but for the years between the time I installed it and I sold the property it "just worked". I would have no hesitation putting one in in any property which did not require gas. (in my experience gas and older rentals here dont mix because of added damp and mould) – davidgo Jan 30 at 18:16
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I recently installed a heat pump water heater because of its efficiency my utility basicly paid the cost of the heater (got it for free just had to install). Yes they do blow cold air and are best in an outside or vented area. I asked the utility if they were really saving that much power because of the times standard heat pumps need emergency heating, the administrator of the program said that these need to be vented because of the cold they produce if in the house but outside the house they would be fine. He said they were more efficient than a standard electric water heater and that's why the have the large incentive to switch over. So they may be slightly better than a electric as long as some fresh air can be exchanged out side even in the winter, but are way more efficient in the spring, summer and fall. I haven't had mine long enough to really compare but our bill did not go up and it was colder than normal last billing cycle and I still had plenty of hot water. Not that we had problems in the past but that is the info provided by the utility not someone in sales.

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