A Heat pump water heater can reduce energy use by up to 60% compared to electric storage tank water heaters. The limiting factor of heat pump water heaters is their ability to keep up with hot water demands in larger homes with more than 3-4 persons.
Larger heat pump water heaters with capacity of up to 80 gallons are available but how efficient are they if they do not operate in the efficient economy mode provided by the heat pump technology? Many hybrid electric water heaters are forced to use their electric back-up elements when ambient air temperatures around the unit drop.
To open up the market for large household installations for heat pump water heaters, Energy Unlimited borrowed a concept used in solar hot water heating systems:
Step 1: Install efficient water heating technology to preheat the cold water coming from the water main into the house.
In solar thermal applications the heating (single tank system) or the pre-heating of water is accomplished by an open loop or closed loop solar thermal system. The system transfers heat from the sun to the water either directly (open loop) or indirectly via a heat exchanger (closed loop).
Instead of using a solar thermal water heating system, Energy Unlimited chose an efficient AirGenerate AirTap Hybrid heat pump water heater to preheat the cold water to a temperature range of 120-125 degrees Fahrenheit.
Energy Unlimited programmed the AirGenerate electric heat pump water heater to work exclusively in the so called economy mode (heat pump mode). This operating mode disables the electric backup elements in the AirGenerate water heater; therefore, all of the heated water provided by the preheating tank is efficiently generated by transferring free heat from the air to the water that is stored in the tank.
Step 2: Install the most efficient electric water heater storage tank that will store the hot water and give it a boost when necessary to bring it up to the desired water temperature.
When the home owner uses hot water, the preheated water is fed to a second water heater or a so called booster tank. The booster tank receives the preheated water from the AirGenerate AirTap Hybrid water heater, brings it up to the desired temperature if necessary, and stores it with as little stand-by losses as possible.
In the best case scenario, the water routed from the AirGenerate electric water heater to the booster tank, a super-efficient 50 gallon Marathon water heater (MR50245), will already be in the desired temperature range when it enters the booster tank. If the water is at the right temperature, none of the Marathon’s electric heating elements have to do any work. The primary role of the Marathon water heater is to efficiently store the hot water generated by the AirGenerate AirTap heat pump water heater.
The reason Energy Unlimited chose the Rheem Marathon water heater is the low standby losses. In case of a power outage, water stored in a Marathon tank loses less than 5% of its heat per 24 hours due to its superior insulation. The unique Polyurethane Envirofoam™ insulation significantly reduces heat loss compared to standard water heaters. Marathon water heaters are also extremely durable, come with a limited lifetime warranty, and are manufactured by Rheem in USA.
Using a 40 gallon, MR40245, or a 50 gallon, MR50245, Marathon water heater as a booster tank will effectively double the thermal storage capacity of standard water heating systems. The two tank solution provides a total of 116 gallons of efficiently heated hot water, which is an ample supply for larger homes with 3-5 bathrooms or up to 8 adults. The system can supply more than 8 to10 back-to-back 7-minute showers to a family in the morning.
The use of a two tank installation provides energy efficiency benefits to larger households that want to take advantage of energy efficient electric water heaters. This combination of a AirGenerate Hybrid heat pump water heater and a Rheem Marathon water heater is a good alternative to solar thermal water heater installations.