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Solar Hot Water Preheater Idea? Answered

Hi everyone, I'm new, from the UK, I will shortly be moving into a static caravan/mobile home on family land for a few years to save up, so with money saving being the aim, I'd like to run this little project by you all. 

I'm thinking about making a solar collector and placing it on the roof of the home, as large as I can afford (caravan is 12 wide) and feeding that into a heat exchanger inside an old 47kg LPG gas cylinder (I have access to several old cylinders and believe it can hold around 110 litres, so could even use 2?) and having the mains inlet at the bottom and outlet to the boiler at the top. 

For the heat exchanger, I'm thinking cut both ends off and drop a coil through from top to bottom and weld back up but i'm open to suggestions.

I want to a heat exchanger instead of having the mains water go round the solar collector, as it can get cold in the UK at night, even in the summer and I dont want it freezing in the winter (i'll have enough worry of water pipes freezing anyway), so will use some form of copper friendly anti-freeze.

I'll need a pump for this, as the collector will be on the roof, above the cylinder, so thinking to run it on a light sensor, but how complicated would a thermal switch be, so when the temp in the collector ir warmer then the cylinder be to make? If anyone knows of a  place for the circuits (or of the shelf product), I can see if it's something i'm competent enough to take on. 

One last thing, Would having the cylinder painter black and left in sunlight be a good idea. Would it get hot enough to cause problems with the water mains pressure? 

Look forward to your feedback and advise. 




6 years ago

Apologies, i didnt mention that its a preheater for on demand gas water heating, which is run off bottled gas, which is the reason for wanting a preheater, to save on the amount of gas used. Regards tuns

That is a great idea.
I have warmed up a swimming pool with pumped pool water through
a single black ¾" plastic tube looped on a smaller roof then your caravan...

Have you taken the added weight on the vehicle into consideration.


6 years ago

Out here in the Nevada USA desert a friend related the Point-of-delivery
propane heater economy with instant heat as his preferred system to
the waste of maintaining an insulated tank of hot water.


Modern hot water heaters are built for a quick-recovery-time for the water to heat back up after some is used. This takes a LOT of heat energy. Therefore, if you can live with a longer recovery-time... say 15 hours instead of 1-hour, then you can start thinking of a very small heat source such as a candle flame or a 10watt light bulb under the tank. My 30 gallon hot water heater in my house is heated with just the PILOT LIGHT !!! The water temperature gets up to about 120 degrees but it takes 20 hours to attain that temperature after 2 showers every morning. The water heater is heavily insulated and has no vent. The no-vent is not a danger in my house, but in a small van or trailer, it might be deadly due to carbon monoxide gases... so you might want to heat with a small electric bulb. Just an idea, maybe you can use it or adapt the idea to your situation. If a "military-shower" method was used, the tank would always stay hot due to very low water usage. a military shower is... wet yourself down, turn off the water, soap up , then turn water back on just long enough to rinse off.

If you go with the solar idea, try to position the collector lower than the tank. This way the hot water will rise into the tank without any pump. Another hose at the bottom of the tank takes the cold water at bottom of the tank back to the bottom of the collector. At night, the water will reverse flow and cool off unless you put in a small check-valve that only allows water to flow one way. You might want to look at the site builditsolar.com for more ideas

You can get temp sensors for arduino but I think what you really need is this: https://www.instructables.com/id/Sous-vide-cooker-for-less-than-40/ . Not the whole thing, just the temp relay.