Solar heat to boiler conversion.

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Posted by Charles Campbell on March 31, 2009, 01:04:59 PM

I currently have an Amtrol 25 gal. hot water maker with an external propane boiler which I use to heat water for a zoned radiant floor system and regular hot water use. I would like to convert this to a closed loop solar hot water/heating system, which would be my primary hot water source and the propane boiler would secondary or backup. The Amtrol storage tank has an internal heat exchanger and is thermostatically controlled so that when the temp. drops below a certain set point it triggers the boiler to heat water. My question is this: Can I reroute the cold source to the 120 gal solar storage tank and then plumb the hot water output on the solar storage tank into the cold supply on the 25 gal. hot water maker? As a preferred alternative, could I do away with the 25 gal. hot water maker altogether and plumb the back up boiler directly to the solar storage tank? By the way, I am not an installer or contractor.

Posted by Tom Mayrand on April 01, 2009, 12:26:21 PM

Re: Solar heat to boiler conversion. (Reply #1)

charles, sounds like you got the right idea. You can pipe the solar tank as you describe, cold water in, then hot water out to the cold water supply on the Amtrol, then hot water from the amtrol out to the house. Basically the solar is preheating the water coming into the amtrol keeping the thermostat satisfied and the boiler off. You can also add valves so you can use the solar tank only. Best to keep the solar tank separate from the boiler, otherwise the boiler may heat the water confusing the solar controller and keeping it off.

Posted by Charles Campbell on April 01, 2009, 10:10:11 PM

Re: Solar heat to boiler conversion. (Reply #2)

Thanks for the timely and very useful info Tom.

Posted by on April 02, 2009, 08:05:59 AM

Re: Solar heat to boiler conversion. (Reply #3)

I would recomend that you pipe the system so that you can run either directly off the solar hot water tank or as a makeup to the Amtrol. I have the same setup and find that somewhere around this time of year in northern NH, that I no longer need the Amtrol as the water in the solar hot water tank is hot enough. Once you get to this point, the boiler will use natural gas to keep the Amtrol tank hot when it really isnt needed. If you turn off the zone to the Amtrol tank to save gas, you are essentially taking hot water from the solar and running it through a cold tank and it will take longer to get hot water to the tap. There is a way around this by installing a circulating pump to recirculate water from the solartank, through the Amtrol and back into the solar tank but thats getting complex even though it increases the amount of hot water storage.

A few precations are neccesary if you adopt this approach, The first one is that you need to have a tempering valve with a high limit between both of the tanks and the house. This keeps someone from getting scalded as the solar hot water can exceed 140F. There should be one off the Amtrol tank, but some plumbers skip it and trust the temp controller. The second precaution is to drain the Amtrol tank during the summer when you are running direct from the solar system. Its okay to leave the Amtrol on line for a few weeks with no flow, but having a stagnant tank full of luekwarm water for an entire summer has some potential for legionella and other bacteria especially if you are on well water and dont chlorinate.

I also have additional valves and piping so that I can isolate the solar tank, while keeping the hot water system on line, but thats getting into the overkill! (but it is handy when you are working the bugs out of the system).

Posted by Charles Campbell on April 04, 2009, 09:55:36 AM

Re: Solar heat to boiler conversion. (Reply #4)

I agree, I too have had to do numerous repairs on my deep well/pressure tank system and have found that you really can't have too many valves. Makes working on the system so much easier.

As for the circulating pump between the solar storage and the Amtrol. If the Amtrol tank sits on a shelf above the solar storage tank will I not need a circulating pump anyway? Don't think I want to drain the Amtrol, we live in the high desert of New Mexico and get a lot of year round sun. I am trying to avoid complex plumbing, but am willing to accept a certain amt. of complexity if it will increase efficiency and will help avoid routine tasks such as draining and chlorinating tanks, such as getting hot water to the faucets sooner.

At this point, I am not sure whether the Amtrol is plumbed as one of 5 separate zones, or whether it the priority diversion zone. Don't know if I worded that correctly. I will attempt to diagram my system and get back to you on this. If the Amtrol is plumbed as the priority zone, are there downsides to circulating the hot water from the solar storage thru the Amtrol and just letting the thermostat on the Amtrol be the controller for the boiler? Ideally, I would like the solar heated hot water to be primary, and the Amtrol/boiler system to be backup.

Another option, I thought of was getting rid of the Amtrol altogether and just plumbing the boiler into the solar storage tank. Do you know if the Heliodyne tank comes with a thermostatic controller similar to the one on the Amtrol, or if one can be purchased that will do the job?

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