Your water heater is cooler than yours: a heat pump water heater only drops the room 4-6 degrees in winter
A heat pump water heater moves 1600W of heat using 400W of electricity, so a summer shower is basically free air conditioning; in winter it only drops its room 4-6 degrees, because the heat needed to heat a whole tank is only enough to run the furnace for 40 minutes.
The argument · tap a timestamp to hear it
A heat pump water heater is a refrigerator running backwards
An ordinary water heater uses electric resistance elements or gas to "create" heat from outside; a heat pump water heater works like a refrigerator in reverse: it pulls heat out of the surrounding air and dumps it into the tank. Because it is only moving heat that already exists, its electricity use can be as low as a quarter of a resistance element's. This one plugs into an ordinary 120V outlet, draws only a few hundred watts in operation, averaging about 400W in the author's measurements, yet releases about 1600W of heat into the water.
— AlecPlugging into an ordinary outlet is the real breakthrough
Electric water heaters used to require a hardwired 240V dedicated circuit, so switching from gas to electric was almost never worth it: you had to run a new circuit, which was expensive and not always feasible, and the running cost was still higher than gas. A 120V plug-in heat pump water heater changes that equation, with running costs even slightly below the gas water heater it replaced. But the author cautions: the unit itself costs much more, and at current gas and electricity prices, payback takes about ten years, depending on local electricity rates and incentives.
— AlecThe claim that it's unsuited to cold climates is nonsense
The author lives in the Chicago area, has no basement, and has his water heater crammed into a utility closet inside the living space, which a plumber called "crazy." But after doing the energy math, he thinks people who say heat pump water heaters are unsuited to cold climates are "full of it." Using temperature loggers, he measured that in winter the water heater only drops its room by 4 to 6 degrees Fahrenheit, and only while running; upstairs and in the living room he could barely measure any effect.
— AlecSlowness is the biggest hard flaw
A 120V plug-in heat pump water heater is absurdly slow: the heat pump can only raise a whole tank by about 7 degrees Fahrenheit per hour. The author measured that in winter, heating cold water up to 130°F takes 11 hours, and even in summer with warmer inlet water it takes 7 hours. An ordinary water heater recovers in an hour or two after you use it up; this one might make you wait 11 hours in winter. If your household runs out of hot water often, this plug-in model isn't for you.
— AlecThe free summer cooling is overrated
The author originally expected to get free air conditioning and dehumidification while showering in summer, but was disappointed by the measurements: for a family of four showering 20 minutes a day plus the dishwasher, the water heater only runs about 5 to 6 hours a day in summer; and his two-ton central air conditioner draws about five times the power of the water heater. At best, the water heater can only offset about an hour of air conditioning. Still, by his math, every dollar you spend heating water in summer saves you a dollar of cooling.
— AlecEnergy is energy: once you do the math, there's nothing to fear
The author used the intuition that "energy is energy" to work out in advance that the winter impact would be small: raising 66 gallons of water from 55°F to 120°F takes about 36000 BTU; his furnace puts out about 55000 BTU per hour, so heating a whole tank is equivalent to running the furnace for 40 minutes. The cooling side works the same way: a two-ton air conditioner removes 24000 BTU per hour, so running it for 90 minutes is equivalent to the cooling produced by the water heater heating a whole tank.
— AlecStratification makes the slowness tolerable
A storage water heater uses a dip tube to deliver cold water to the bottom of the tank, while hot water floats on top, with a sharp boundary between them and no mixing. So when two people use half the tank in the morning, the remaining 33 gallons are still scalding hot, and the water heater only has to heat the 33 gallons that were used, halving the recovery time to about 5.5 hours. The author measured it: after filling a bathtub (about 30 gallons), the remaining 36 gallons of hot water let him shower for over 45 minutes before running out.
— AlecIt deliberately ignores small draws of water
The author worried that the water heater's dehumidification would fight with his humidifier, but found the designers had already thought of it: the tank is so well insulated that when no hot water is being used it only kicks on about every 13 to 14 hours; and unless roughly 15 to 20 gallons are drawn at once, it basically doesn't respond. The author guesses this is the result of moving the bottom temperature sensor upward, specifically to be compatible with humidifiers. All winter, his humidifier was never disturbed by the water heater.
— AlecIn their own words · checked verbatim
But water heaters are among the most energy intensive appliances in our lives.
Alec0:00
This water heater, on the other hand, uses the same technology as your refrigerator - but in reverse.
Alec1:02
I do not regret this one bit and can now state with pretty high confidence that anyone suggesting these are inappropriate for cold climates is full of it.
Alec5:07
On average, when this water heater runs, it only draws about 400W from the wall. But since those 400W are running a heat pump which can use about 1200W of heat out of the air, 1600W of total heating power ends up being released into the water.
Alec17:21
The reason I'm constantly saying “energy is energy” is that once you develop an intuition for it, you'll be able to make predictions.
Alec26:26
Water heating is energy intensive, but space heating is much more so.
Alec28:34
The heat pump can only raise the temperature of the entire volume by about seven degrees Fahrenheit per hour.
Alec36:41
Figures
| Ordinary water heater monthly cost | over $50 | 0:00 |
| Heat pump water heater monthly cost | under $20 | 0:00 |
| Heat pump water heater average electrical power | about 400W | 17:21 |
| Heat pump water heater total heating power | about 1600W | 17:21 |
| Winter temperature drop in the water heater's room | 4 to 6 degrees Fahrenheit | 26:26 |
| Heat needed to heat a whole tank | about 36000 BTU | 26:26 |
| Furnace output | about 55000 BTU/hour | 26:26 |
| Winter time to heat cold water to 130°F | 11 hours | 31:38 |
| Summer time to heat cold water to 130°F | 7 hours | 31:38 |
| Heat pump temperature rise per hour | about 7 degrees Fahrenheit | 36:41 |
Glossary
- heat pump water heater
- A water heater that pulls heat from the air to heat water, like a refrigerator running in reverse.
- dip tube
- The tube in a storage water heater that delivers cold water to the bottom of the tank so hot water floats on top without mixing.
- anode rod
- A metal rod in the tank that sacrifices itself to prevent corrosion of the tank lining, usually not needing replacement for years.
- demand response
- A grid program that lets devices automatically cut power use or delay operation during peak hours.
How to listen
Homeowners and engineers considering whole-home electrification, replacing a water heater, or just curious about heat pump technology, especially those in cold climates without a basement.
The last section on smart plugs and demand response can be skipped; the author himself says it won't save much money.