Efficiency of a heat pump water heater during the winter

Question

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?

Answer 1

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 out 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.

Answer 2

Edit: Things are changing. We're starting to see heat pump water heaters with the ultimate heat source outside the house, negating the "adding load to your furnace" issue. Either this is a split system where the water heater is one of the "Heads" ... OR, the latest thing is the R290 Monobloc. Monobloc means the freon loop is entirely within the outdoor unit (good thing too, because R290=propane!) which is a game-changer for DIY. No more freon plumbing/license. Heat is transferred into the house via a hydronic loop, so you simply need a water heater with a water/water heat exchanger (you want antifreeze in the loop that goes outside). Anyway, back to 2019.

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.

Answer 3

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.

Answer 4

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.

Answer 5

People that put a heat pump hot water heater outside, I don’t get your logic. In my region it gets to -10 degrees F. So outside is a NO. If you live where climate is mild in winter, why would you place it outside unless you vented it inside to gain the A/C it provides. Mine is inside the house (has to be -10) 100 degrees F in summer. In the summer it runs on heat pump for A/C. Winter I switch it to electric only because we need to keep the humidity in our air (not remove it). Also got rid of 2 Dehumidifier that ran in basement during the summer. Indoor Hot water heat pump that’s switchable to electric heat elements saves in 4 ways. 1) the heat pump is more efficient. 2) summer the heat pump pumps cool A/C into house as a waste discharge. 3) summer the heat pump took the place of two dehumidifiers (each dehumidifier puts out heat)that needed removed again. 4) winter we go to electric element so the humidity doesn’t get removed (whole house humidifier is tapped into hot water line). The savings over last 4 years was over $4000.00 / $80+ a month savings (not including water and sewer savings). It may not be a fair comparison because our hot water heater was natural gas. We are now 100% electric. Hot water was our last gas appliances to go and good riddens. Unplugged carbon monoxide detectors and stopped buying batteries for them. Also got a credit of $800.00 from the electric company for getting high efficiency water heater. You do the math!! Thank you.