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Open Loop Solar Collector


New Member
Jun 22, 2021
Does anyone have any open loop solar water heating. Using either commercial or DIY solar collectors. This could be plastic or copper tubing as well. I live in Hawaii so there's no need for a closed loop system or glycerol.

Do you recirculate the water from the cold side of the water heater back into the water heater. How do you prevent the water in the collector tubing from overheating? Assuming it's a really hot day and the water isn't being used. Has anyone ever used a recirculator pump to pump the water through the tubing? Maybe use its thermostat somehow? Automatic dump system or shading system? A pressure release valve?

I know there are kits that used the drain valve to pump the hot water into the tank. A furnace valve is usually added to that pipe lower than the drain outlet on the hot water heater for a drain valve. These are usually full-size water heaters. Has anyone ever seen this done on a 20 or 30 gallon tank. I realize some of the smaller tanks don't have drain valves but some do.
From what I've seen in non freezing climates many people use a "batch water heater". Google that term and you'll find several DIY plans.
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There are all kinds of problems to overcome with this approach.

Boiling and freezing can damage or destroy your collectors and pipework, either quickly or gradually over time.
No way to control the water temperature created.
You only get the volume of hot water stored within the actual collector itself. If you want to run a bath or do several loads of washing, the collector must be huge to contain all of that that large volume of water.
Very sudden changes in water discharge temperature, as cold water chases hot down the pipework, maybe almost boiling water suddenly turning cold in seconds. Not very nice in a shower.
No hot water at all at night and in the early morning. Not very useful.

Nobody does this, the problems are significant and not easily fixed without adding a circulating pump and storage and maybe other things as well.
Ive done the batch type using hot water heater tanks stripped of their cover and insulation, sprayed black and put in an insulated box with a glass top and front. Its just a pre heat unit and water is fed from there to the intake of the actual hot water heater. On non cloudy days i could heat the water to 135f by the late afternoon. i want as far as making an insulated cover for night. I was using a propane water heater and this simple pre heater and cut the propane usage to a third.
Well done Russ !

A pre heater is an excellent and very practical way to lower water heating cost.
Its efficient, because the pre heater does not need to run at a very high temperature, so a lot less thermal insulation will be required, and it will keep warm for days if the storage tank is large enough.
Several ways to heat it too.
Pumped water through solar thermal collectors, or heating can be done electrically with excess solar electrical power, either dc or ac.
So you are using surplus energy available that would otherwise go wasted once the battery is fully charged.

In winter the water coming out of the underground water supply main can be quite cold, and even just warming it up a bit will make a significant difference to the amount of energy required to heat it up to full final hot water temperature.
In summer, the pre heater output should be hot enough that the main hot water heater requires almost zero extra power, and your water heating cost should fall to almost nothing.
Highly recommended.
I use flexible copper pipe. 150ft. Painted black in a metal enclosure with metal black back sheet and insulated, put on the roof. I use a temp sensor/relay to detect temp on the roof at the collector. When the temp reaches 120f on the backsheet of the collector the circulator pump turns on and circulates the water in the 80gal water heater thru the copper pipes. I only use this spring summer and fall. In the winter I have a lot of extra solar production to heat the water heater because I have a wood boiler that’s used to heat the house which uses near zero power. However in the summer I use a lot of airconditioning so the roof mounted water heat helps due to not having much excess solar production to heat water.
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Installed this in 2019 in Montross, Virginia. Collector and controls were from a Californis company. Engineered the rest myself with the solar collector into a propane water heater and then into a heat pump water heater in my basement with many controls installed in the copper tubing and pipe. I also installed heat trace on the copper tubing in the attic and outside controlled by a thermocouple. I would not recommend just anyone to invest in this system especially in environments that are seasonally hot and cold.


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