The photo below is of a passive solar hot water system on the roof of a house about a block away from my house. That one is a double system, I wouldn't need that capacity.

It is used as an auxiliary to pre-heat the water before it goes into the propane hot water heater. The vendor says that in the warm months my hot water propane expense will drop to zero, and that I can expect water temperatures up to 80 degrees C. Even in the "cold" months our weather rarely drops down into the 60's (F) and I have an absolutely unobstructed Southern exposure so it should be effective even then.

At the price quoted for the heater and installation, and the promised savings (65% reduction of my total annual propane bill) it would amortize in four years, and after that would be pure savings. It has an 80 year warranty, meaning I will be about 150 years old when it finally wears out.

Does anyone have experience/knowledge of this style of passive solar hot water heater, and/or recommendations where to look for unbiased information?

tanstaafl.

I've used something similar. It worked quite well, but you have to schedule around it. The water won't be warm until the afternoon and cools off as soon as the sun is off it.

If you're ever in the Los Angeles area, look me up. I'd like to take you to a free lunch.

Is the tank not insulated then?

Remember, too, that this will be used as a pre-heater for the gas-fired heater, so we won't ever be without hot water, we'll [hopefully] be using a lot less propane to heat it.

Currently our propane bill is about 54% of our total utilities expense (gas-water-sewer-electric-telephone-taxes), and while that total is under $100/month, the gas bill is the logical place to start.

tanstaafl.

My first thought was that the tank couldn't be insulated. If it was, the sun wouldn't be able to heat it up to 80. That was the only concern I would have had with the system. If you have it feeding into a propane heater, I can't see any drawback (as long as you don't mind it being on your roof I guess).

The sun does not heat the water tank directly, in that system. From the photos, it looks like an array of glass solar collectors, which are vacuum insulated, with the upper ends of the collectors transferring the heat into the water inside the tank.

Inside the glass tubes are small heat transfer tubes, which do the actual solar absorption and transfer the heat upwards into the water medium. There is no water inside the actual solar tubes.

The water tank at the top could very well be insulated.

A variation of this system has the collector tank located elsewhere, perhaps adjacent to the 'normal' water heater inside the building. A small circulation pump continually moves the water through the solar array and back into the storage tank, which would be insulated. This allows the heat storage tank to be as large as you need it to be to accumulate hot water during the day.

Some systems circulate propylene glycol 'coolant' through the solar manifold, then use a compact heat exchanger to transfer the heat from the coolant into the domestic hot water system, and then into the solar storage tank. More pumps and equipment involved, but the domestic water is then not exposed to the solar collector and all the piping running up to the roof and back down.

There is no upper limit to how hot the water can get. My neighbour has such a system, and when the water circulation pump failed, the water inside the collector manifold boiled, venting through a pressure release valve which is installed for just that situation.

Awesome explanation.

That does sound like an awesome system.

The one I used was clear plastic pipes painted black on the back side, and a small pump. No real attempt at insulating was done, and convection would keep the water moving when night fell. It cooled off right away without sunlight.

However, it did get the water very hot, and we didn't need any additional heat.

I made one for my cabana in Belize.

Looked like this and cost me about $100 or so (perhaps less).

Worked fabulously. Unfortunately, I didn't anchor it well enough and Hurricane Richard destroyed it.