AirTap: A Heat Pump For Your Hot Water Heater: Hot or Not?
There are few appliances dumber than an electric hot water heater; it uses energy made from boiling water to run turbines to turn generators to be distributed over wires...to boil water again. The AirTap is a heat pump that improves the process somewhat, by taking energy from the air and using it to heat the water. Like most heat pumps, it is advertised as having a COP (coefficient of performance) of up to 3.5 times that of a straight electric element. It seems like a good idea, but is it?
Let's look at their claims. First of all, the issue of the COP. A heat pump is efficient because it moves heat, rather than making it. But the heat has to come from somewhere, so if you are paying to heat your house, then you are heating the air and the heat pump is removing the heat and sticking it into the hot water. It isn't free; you are essentially running an air conditioner inside your house and exhausting the heat into your hot water tank. So the unit makes no sense in any part of a country where the heating degree days outnumber the cooling degree days, and certainly makes little sense in Canada, where I saw it at a home show.
Where homes need cooling more than heating, it makes a lot of sense; they even sell an optional duct to supply cool air to other parts of the home.
Then there is their marketing, suggesting that it is more cost-effective than a solar water heater. saying:
Solar paneled water heaters are also a pain on the pocketbook: the initial cost of a solar water heater system is 8 times as expensive as the AirTap™, while requiring the same amount of electricity to provide hot water on cloudy days. What's more is that the environmental benefits of the AirTap™ are the same.
But I can't figure out their numbers; they don't add up for me.
The AirTap unit costs $ 700; that would put the cost of a solar hot water system at $5,600 (I have been quoted as little as $3,000 after rebates and subsidies). Their chart puts solar with a five year cost of $ 7063; since hot water systems have tanks and the evacuated tube ones work on cloudy days, I wonder what the difference is.
They also show the 5 year cost of the AirTap at $1850; with a US average electricity price of 11.6 cents per kilowatt-hour, I calculate the electrical cost to run the unit at $3658 and the unit costs $ 700, giving a five year cost of $4358. They only can show an ownership cost of $ 1850 by subtracting the purported energy savings; given the fact that a conventional electric hot water heater costs $ 485 per year to run, or $ 2428 for five years, using the same math would put the solar system's five year cost at $ 3171, or considerably less. I don't think they are comparing apples to apples.
Then there is the way they explain how much power the unit actually uses:
To put into perspective, AirTap™ uses less power than an 8-cup coffee machine to run the compressor, and its energy consumption level is equivalent to keeping two coffee machines on for a day.
Which sounds so much better than saying that the unit burns the same amount of electricity as six one-hundred watt incandescent bulbs burning all the time.
In the end, this is a relatively cheap way to reduce hot water heating costs if you live in a warm part of the continent. It is probably a terrific product under those circumstances. It was originally designed for a laundromat, where there is a lot of waste heat; it makes perfect sense for that kind of commercial operation.
But I do think they are overselling its benefits, are unfairly harsh about solar systems (which I am biased towards, and think should be on every house and perhaps even be mandatory in the building codes ) and I think their math is fuzzy.