Solar Hot Water Heater W/internal Heat Exchanger

Q: I'm trying to help a friend with his leaking 120 gallon solar hot water heater. He wants the same design that has served him well for the last 20 years. It was made by "Ford" and had the heat exchanger inside the tank. There was a warning with the tank to use glycol with a brightly colored dye to warn if the heat exchanger developed a leak. I suspect that design died a horrible, litigious, death. He's tried several dealers and I've spent some time searching the WEB. About all we've found is that quite a few people are looking for the same thing. Considering the breadth of expertise here I thought I'd ask. I suspect he is going to have to live with a design that has the glycol tubing wrapped around the outside of the tank (but inside the insulation) which seems to be the currently popular design.

A: Maybe a good copper or bronze shell tube or a stainless steel brazed plate exchanger mounted external to an insulated tank would work as well. Two cheap sealed can hot water circulator pumps could circulate the two fluid paths. Maybe Propylene Glycol/water mix would be a better choice on the collector side due to it's lower toxicity. For a brazed plate unit, Flat Plate Inc. in PA. makes some pretty cost effective brazed Stainless units. Problem with all of those designs is you have constantly changing domestic water bringing fresh oxygen into a supposedly coated and protected steel tank. There are always flaws in the protection system, and eventually the tank fails. If it was one of those expensive designs and you need to throw away your exchanger along with it, that hurts. Makes you think buying the cheapest four-port "glorified electric water heater" tank and an external exchanger would be more economical. I went the other way - a tank with a double internal exchanger, so that the antifreeze from my collectors is isolated from the dead oxygen-free water in the tank, and so is the domestic water in the other exchanger coil. Without the constant supply of oxygen, the tank should last indefinitely. My favorite "plus" is that the domestic hot water is freshly flash heated, not a long-incubated soup of the slime that collects in a normal hot water tank.

Discuss It!

Tom Wills said:

There are several good options for replacing an internal heat exchanger tank. Two we use often are: the integral (wrap-around) heat exchanger version you describe made by Rheem/Ruud/Richmond and the stainless steel internal HX tank sold under the ThermoMiser brand by Lever Edge and AET in Florida. Converting to an external HX is an option but increases the moving parts, electrical consumption, and inherent failure rate. Just because Ford recommended a bright dye does not mean they were using toxic ethylene glycol. Solar-formula propylene also has a bright dye. The main problem with a cross-leak between HX and potable is not contamination of your shower water (the lower pressure in the glycol means the leak goes mostly the other way), but loss of freeze and corrosion protection in the solar loop.

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