Rheem Marathon water heaters used as booster tanks for AirGenerate AirTap™ Hybrid water heaters

by rnaleway on August 28, 2012

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.AirGenerate AirTap heat pump water heater with Marathon water heater

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 Marathon water heater provides boosting capacity, thermal storage, and buffer to the AirGenerate water heater allowing it to operate in efficient heat pump mode 24/7.

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.

{ 8 comments… read them below or add one }

Loren August 30, 2012 at 4:24 pm

The Airgenerate hybrid heat pump water heater is currently the only NEEA (Northwest Energy Efficiency Alliance) Tier 2 Northern Climate qualified product on the market. This means that it will still effectively operate in temperatures as low as 20F in heat pump mode, without needing the backup electric element to kick in, unlike the other brands that require electric element backup at 35F or so. Not to mention it’s the only water heater with a stainless steel 304 tank with a lifetime warranty, so basically the sacrificial anode rod is only there to make you feel good!


Roch September 2, 2012 at 4:30 am

The lower operating temperature helps the water heater to produce savings throughout the year. While other heat pump water heaters need to use electric resistant elements during cold spells (when the products installed in garage spaces or basements) the AirGenerate AirTap Hybrid water heater is still able to continue to deliver savings. I think that consumers can expect to save an additional $50 – $150 per year on their electric bill per year when this is installed in a garage space in Oregon, Washington, Montana, Idaho, or Wyoming. It all depends on the way you use water, the installation site, your climate, and your utility rates.


Kim January 31, 2013 at 7:59 am

Great post.. Thanks for sharing such a nice post of heat pumps hot water with us..


Michael May 1, 2013 at 10:19 pm

I just received my delivery of the ATI66 and plan on using this method of the preheating the water with it. This should eliminate any fear of the heat pump being unable to keep up with 5 people as well as provide redundancy should one of the water heaters fail.


Http://www.ana3araby.net June 12, 2013 at 3:45 pm

It’s hard to come by knowledgeable people about this topic, but you sound like you know what you’re talking about!


Matt Risinger December 30, 2013 at 8:50 pm

Great Post! It sounds like you’ve really thought through this install. What do you think about reversing the order of the install? Marathon first, then ATI66? I’m about to build a Lakehouse that will be occupied by 2 people most of the time, but occasionally has high weekend guest occupancy. I was thinking of using the Marathon first (with a wall switch to turn it off/on), then using the AirGenerate second. Most of the time with two people in the house the 66 gallon ATI66 could handle the load, but when company is coming they could flip on the Marathon heater to pre-heat for the HPWH? I’d love your thoughts on that setup. Best, Matt Risinger


rnaleway January 25, 2014 at 12:57 am

Matt, I saw this set-up on your blog with a Rheem heat pump water heater. It’s an interesting concept to use an electric water heater as a pre-heater. In all solar thermal installation (the concept of a booster tank was borrowed from solar thermal installations) the water is pre-heated efficiently via the sun. If the water is preheated efficiently to the desired operating temperature…let’s say 125 degrees Fahrenheit…the less efficient booster tank will not use very much electricity at all besides maintaining the water temperature at the desired setting and mitigating stand-by losses. If you cannot preheat it all the way to the desired temperature the booster tank will handle the balance.

Reversing the order of the tanks: Your concept would eliminate the penalty of standby losses when the home has a lower occupancy. That’s good. However, you’d always use the maximum energy to power the resistant elements when heating up water all the way from the inlet temperature to the 125 degree setting when the home is occupied by more more folks while the regular electric heater is in operation. You would probably see very little savings from the system when it is in high occupancy mode.

An optimized solution would be to use the HPWH as a pre-heater and the Marathon water heater as a booster tank. Put both water heaters on wall switches. Then go ahead and install additional water lines and valves that would allow you to shut down and bypass the booster tank. This solution would eliminate any standby losses when the home has low occupancy and provide booster capacity when it is experiencing high occupancy rates.


Ankara web yazilim April 2, 2014 at 7:45 am

It’s nearly impossible to find educated people about
this topic, but you sound like you know what you’re talking
about! Thanks


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