Solar thermal System - Marathon water heater?

greenie100August 19, 2012

I am looking to upgrade my home with an active closed loop solar thermal water heating system. I live in Portland, Oregon. Given the information that I can get from my local utility, PGE, and Energy Trust of Oregon this should save roughly 2,800kWh or roughly $280 per year on my utility bill.

The solar thermal system is made by Rheem USA. They have two systems that I am considering:

Rheem RS80-48BP - this is an 80 gallon tank with heat exchanger. Refrigerant is circulated through the pipes and solar panels. The system has 48 square feet of panel surface. The heat is transferred to the water via heat exchanger that comes with the Rheem Solaraide tank.

Rheem RS120-64BP - it is the same system as above, but has a larger 120 gallon tank and 64 square feet of panels.

I am wondering - what size should the system be for a household of 4 people?

The smaller system would force me to have a booster tank installed. I am thinking about using a 50 gallon Marathon water heater, since it would easily match the life of the solar equipment. Marathons have a lifetime guarantee and can be used in standard stand-alone installations. If I went without the a second tank like the Marathon water heater I'd run the risk of running out of hot water.

The larger system can go as a stand-alone, but the guaranteed hot water amount in the Rheem Solaraide tank is roughly 40 gallon....the amount of water above the upper heating element. This risk of running out of water still seems real to me.

What are your thoughts? Go smaller? Go bigger? Would you use a Marathon water heater as booster tank in both systems just to make sure?

The second portion of my consideration revolves around whether to execute the project as a DIY project or to go with a specialized contractor?

While the equipment is somewhat specialized I was able to find either one of the solar thermal systems for purchase online:

I think that I can probably get the system installed on my own for roughly $5-$6k. I have seen contractor quotes for more than $10k. This seems way too high for me.

Last but not least - Rheem Marathon water heaters cost between $800 - $1,000 retail. If I used a regular electric water heater I could save between $400 - $700. With that said I do like the durability and high temperature resistance on the tanks. Marathon water heaters can be heated up to 180 degrees without voiding the manufacturer's warranty. This should be beneficial if the pre-heated water from the Rheem Solaraide tank comes in too hot.

Here is more information on Marathons I was able to find:

I appreciate your input on the system configuration and choices. I don't want to end up spending the money just to set up a system that falls short.

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Hi Greenie100,

Great questions. The answer between the two choices probably involves how much water your family uses(including habits) and how much insolation you get in your OR climate. I would guess the bigger system. I have used the Marathon tanks and love them. The biggest selling point to me is that they have no anode rod to change out which is usually the death of most tank water heaters.

Dont take a DIY installation lightly. If you have skill with copper plumbing then it can be a great way to save some money but be prepared for lots of time and tinkering if you have no experience with solar hot water. Be very careful about how you install your backup tank and monitoring for expected performance is crucial. Dont skimp on controls and thermometers.

The bottom link is troublesome for me to include because I dont necessarily agree with all of the authors points. It is however getting tougher and tougher to spec solar hot water these days due to the incredible efficiency of heat pump water heaters and falling prices of PV. I feel Solar Hot Water still has its place but it is making less financial sense these days.

Here is a link that might be useful: Green Building Advisor article; Solar Thermal is Dead

    Bookmark   August 19, 2012 at 2:21PM
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I have a Marathon water heater, and am very satisfied with it. Not real easy to tell how much electricity the thing uses, I've always had gas water heaters prior. It has had no issues since I bought it close to 3 years ago, at any rate.
It is hooked to the desuperheater of our geothermal system.

I was able to buy mine at a discount thru our rural electric co-op, which made it an easier decision.

Interesting article above on the PV heating. I installed a solar heated shower here. It is a plastic barrel painted black up on a crude 2x4 stand outside. I fill it with a hose. It works well from about May-Sept. I was unable to use it for about 1 week this summer during the 110-114 days because the water was too hot. It was also too hot to be outside trying to drain water and replace it. Unreal.

    Bookmark   August 19, 2012 at 2:49PM
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I definitely think that I will go with a Marathon water heater tank in this situation just to make sure that I have reliable tank that will do its job well.

I like that it is made of polybutane on the inside and fiberglass on the exterior. It should withstand higher temperatures that we may encounter on solar thermal systems.

I still need to toss a coin on whether a smaller or a larger system is appropriate. It is a $500 difference.

Here is a link that might be useful: Marathon water heater diagram + info

    Bookmark   August 20, 2012 at 4:33PM
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Check your utility because some offer substantial rebates on the water heaters and they aren't wait till tax time rebates. Ours you order the water heater through them and its about 200-300 cheaper than home depot or lowes.

    Bookmark   October 10, 2012 at 9:53PM
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A builder we just interviewed uses primarily Marathon water heaters, which he purchases through the local utility board. He says they are extremely efficient and have a lifetime warranty. I don't have any personal experience with them (yet)!

    Bookmark   January 25, 2013 at 9:05AM
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