Last week I wrote about “hybrid” water heaters, a relatively new type of water heater that includes features of both storage and tankless models. This week I’ll cover another type of water heater that is also (confusingly) referred to as “hybrid”: heat-pump water heaters. These produce over twice as much hot water for each unit of electricity consumed as any other type of electric water heater (storage or tankless).
You’re going to be hearing a lot about heat-pump water heaters over the next few years, because new federal regulations that take effect in 2015 will require heat pump functionality for larger electric water heaters — more on that below.
Why it’s worth considering water heating carefully
Before diving into heat-pump water heaters and what makes them tick, it’s worth spending a minute to say why I’ve focused so much attention on water heating in this blog recently. As a fraction of residential energy consumption, water heating has become more and more significant over the past several decades.
In 1978, water heating accounted for approximately 14% of a home’s average energy consumption, according to the U.S. Department of Energy, compared to 66% for space heating. By 2005, those percentages had shifted to 20% and 41%, respectively. I assume that this isn’t because our water heaters are using a lot more energy, but rather that our houses are better insulated and our heating systems more efficient.
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In an ultra-efficient Passivhaus building (built to the German standard for low-energy homes that is gaining popularity in the U.S.), it’s not unusual for water heating to be the largest energy user in the house, and it can be as much as twice that of space heating.
Electric-resistance vs. heat-pump water heating
Up until recently, almost all electric water heaters relied on electric-resistance heat. Electric current flows through a special element with high electrical resistance, and the electricity is converted directly to heat. The conversion of electricity into heat is virtually 100% efficient — though heat loss from an electric storage-type water heater always results in an overall efficiency lower than 100%. (Note that if we’re looking at primary or source energy that the power plants use to produce the electricity, the efficiency is far lower.)
Heat-pump water heaters are very different. Electricity isn’t converted directly into heat; rather it is used to move heat from one place to another. This is counterintuitive, because the heat is moved from a colder place (the room air where the water heater is located) to a warmer place (the water in the storage tank).
This seemingly magic process happens because a specialized refrigerant fluid is alternately condensed and evaporated in a closed loop. This process relies on phase changes of the refrigerant that capture and release significant amounts of heat.
A detailed explanation of the refrigerant cycle is beyond the scope of this blog. Trust me that it works. (It’s the same basic principle used in your refrigerator, which extracts heat from inside that insulated box and dumps it into your kitchen.)
The net result is that for every one kilowatt-hour (kWh) of electricity consumed, two or more kWh of hot water are produced. The energy factor, which is often thought of as a measure of efficiency, is 2.0 to 2.5 for most heat-pump water heaters on the market, while a 100% efficient electric-resistance water heater would have an energy factor of just 1.0.
Growing interest in these water heaters
There are a few heat-pump water heaters that have been on the market for decades, but these never really reached the mainstream. All that has changed in the past few years, however, as the largest water heater manufacturers, including A.O. Smith, Rheem, and GE have all introduced heat-pump water heaters.
While standard electric water heaters have no moving parts, heat-pump water heaters have compressors (to compress the refrigerant vapor causing it to condense into liquid) and fans (to circulate room air across the heat exchanger so that heat can be extracted from it).
Noisier than other water heaters
Be aware that these mechanical components produce noise — often significantly louder than a refrigerator. Heat-pump water heaters I’ve examined have noise ratings from 55 to 65 decibels (dB), which is a large range of variability (65 dB is ten times as loud as 55 dB). Most refrigerators are 40-50 dB.
If you are particularly sensitive to noise and don’t have an acoustically isolated place to install it, the energy savings from a heat-pump water heater might not be worth it.
New water heater regs to require heat-pump water heaters
New federal regulations that are due to kick in on April 16, 2015, will require that electric water heaters larger than 55 gallons have energy factors close to 2.0. The exact energy factor required is based on a formula that factors in the storage volume, but for all sizes in this category the required EF is close to 2 — a performance level that can only be achieved with heat pump technology.
The energy factor requirements for smaller water heaters — up to 55 gallons in size — are also rising in April 2015, but will remain below 1.0 and will be achievable with a very well insulated electric-resistance water heater.
Heat-pump water heaters rob heat from the house
Because heat-pump water heaters extract heat from the air where they’re located, with most installations they increase heating loads somewhat. If you have an expensive fuel, such as baseboard-electric and are in a cold climate with a significant heating season, a heat-pump water heater may not make sense.
These water heaters can make a lot of sense when there is a lot of waste heat, such as in a basement where an oil or gas furnace or boiler is located.
Heat-pump water heaters come in various sizes: from 40 to 80 gallons for products I know about. For most families, the larger sizes make sense, primarily because heat-pump water heaters heat the water quite slowly — often just eight gallons per hour.
Most heat-pump water heaters have different settings that regulate how readily the back-up electric-resistance elements will come on. With larger models, users can operate them on the heat-pump-only mode (the most economic) more of the time. The first-hour rating will give you a sense of recovery time, but which setting the water heater is on makes a big difference.
My next water heater will likely be a heat-pump model
I’m pretty sure we’ll install a heat-pump water heater in the house we’re currently renovating. Given what’s on the market today, I will probably select the GE GeoSpring water heater, a 50-gallon model that’s 10 dB quieter and half the cost of the German-made efficiency leader, Stiebel Eltron. I’ll also look at the Rheem Hybrid Electric and the A.O. Smith Voltex, which have the same energy factor (2.4) as the GeoSpring — though noise will be the biggest determinant. The GeoSpring is the only heat-pump water heater that’s made in America.
At an electricity cost of 15¢/kWh, a heat-pump water heater will be significantly cheaper to operate than the highest-efficiency, condensing propane water heater (we don’t have natural gas in southern Vermont) — even if propane were to drop to $2/gallon (far below its current price). Where natural gas is available — and assuming the price of natural gas remains so low — heat-pump water heaters will have trouble competing on economic grounds.
A big attraction to me of heat-pump water heaters is that they can be powered using a photovoltaic (solar electric) system. Our new place will be net-zero-energy and we hope to entirely avoid fossil fuels in the house.
Be aware that heat-pump water heaters aren’t cheap. That GE GeoSpring I mentioned above lists for about $1,200, plus installation, and the Stiebel Eltron model costs about $2,500. Unlike electric-resistance water heaters, heat-pump models require condensate drains, which can add cost. By comparison, a standard electric or gas storage water heater can cost as little as a few hundred dollars.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also recently created the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.