Things to think about
Can take longer to reheat than a gas or electric resistance water heater unless they have backup resistance heating elements. All 240 Volt, 30 Amp units have the backup heaters. Some 120 Volt units have smaller heaters, some have none. Can offset this by putting in a larger tank so you have more hot water to start with.
Require sufficient air in the room to extract heat from. Without any venting most require around 100 square feet of floor space, so a garage is fine but a closet or utility room may be too small unless they have louvers in the door. Most units can be fitted with ducts to bring in warmer air (or exhaust cooler air) from (to) another room.
Make more noise than a standard water heater (similar to a dishwasher). Check the decibel ratings on the unit before you buy.
The 120 Volt models were developed to make it easier to replace a gas water heater with a heat pump. They typically can plug into a 120 Volt, 15 Amp circuit that is also used by another appliance. This can avoid the need for an electrician to install a new circuit, saving money.
The main disadvantage of these models is they either have no backup resistance heating elements or have small backup heating elements that will take longer than the 240 Volt to reheat the tank if all of the hot water is used. Also, due to the lower UEF these versions are not eligible for some government incentives (e.g. Golden State Rebate $700 to $900).
There are two methods to avoid running out of hot water and having to wait for the longer reheat time.
Install a unit with a larger tank than the gas water heater you are removing. For example, if you are removing a 40 gallon gas water heater you should install a 60 gallon heat pump water heater.
Install with a thermostatic mixing valves on the output pipes. You set the hot water temperature in the tank to 150℉ and the mixing valve adds enough cold water to lower the actual output water temperature to the normal 120℉. The added cold water simulates having a larger tank.
A O Smith 120 Volt model
Rheem 120 Volt model
