Is an AirTap for water heating worth the retrofit?

Hybrid water heaters have a conventional electric storage heater paired with a heat pump that extracts heat from the air and uses it to help heat the water. This makes sense if you're replacing a conventional heater. But what if you wanted to retrofit a heat pump onto a fairly new storage heater, to save on a hybrid's roughly $2,000 outlay?
 
AirGenerate's AirTap, $700, is basically a heat pump that you add onto an existing storage-tank water heater. Its cost compared to that of a hybrid, along with its compatibility with electric heaters, seem to be a win-win. (Gas heaters are already efficient enough without the heat-pump attachment.) But our tests of the AirTap might indicate otherwise.
 
The AirTap saved 28 percent in energy over a conventional electric heater in our tests, which was roughly half of what we saved with the ready-to-install hybrid systems. (Our full report is available to subscribers.) That testing was conducted in a 65 degree F room consuming hot water to simulate the usage of two to four people.

Recommended maintenance includes replacing the heater's corrosion-fighting anode every two years, which costs about $75 per year with installation. (We initially reported "twice a year," as the manual for the unit we tested incorrectly advised.) This would double the time it takes to break even on the AirTap's initial costs, presuming you pay the national-average price for electricity.

Part of the challenge in replacing the anode is that the AirTap, installed over the tank, can block access to the anode—which can be as long as four feet. In other words, a plumber would likely need to remove the AirTap whenever the anode needs replacing. One potential solution from AirGenerate, now in pilot testing, is a new product called Super Anode Rod that the manufacturer expects you'll be able to use with AirTap or any storage-tank water heater. AirGenerate claims the refillable tube will keep particles from breaking off into the tank as the anodic material disintegrates. The tube is also flexible, which should make for easier installation.
 
In the meantime, one of the hybrid heaters we tested will likely cost less over the long run than the AirTap—and save you more. If your hot-water heater needs replacing, consider an Energy Star-qualified gas heater, which typically provides more savings than a comparable electric model.
 
—Ed Perratore