so a couple hundred feet of black poly probably isn't new too most people playing with this sort of thing, but i recently bought a neat little circulating pump (i can almost close my hand around it, uses almost no power and works right off 12v (20$ ebay). Did i mention the poly coil rests on foam insulation and has double glazing over a frame?
so i have a hot water heater (just using the tank right now), bottom drain plumbs to ball valve to intake of pump, pump goes to poly tubing on roof (about 8-10 feet higher than pump) in a loop that returns to hot water manifold ( or just T into "outflow" of hot water heater ) cold water plumbs as normal to cold water intake of hot water heater
if the valve to the pump is closed hot water to your system comes from tank, if it is open the hot water system is directly fed through the poly coil--be careful this water can actually be hotter than water in the tank tends to be and can be dangerously hot in fact this is not just a toy and the tank water can easily also become scalding
currently i'm manually turning pump on and off--watch sun and guess when the tubing has hot water, then turn on pump for about 5-10 minutes till water starts running cold, then let it recharge for a half hr or so and repeat.
the other day i turned it on and got distracted (while the sun was shining full blast) and just let it keep running for an hr or more and the whole tank had heated up pretty good, cause the intake to the hot water heater was running fairly hot.
i don't spend a lot of time watching the system, occasionally when i'm by the switch i turn it on and let it run for a bit.
i have a thermo relay and sensor, but haven't quite figured out how to hook it up, guessing the outflow from the coil might be best, and make sure i loop that bit of pipe under the rest, so it is coolest part of the roof pipe and the switch only turns on when everything is hotter than the pipe where the sensor is.
I think it's important to note there is no freeze protection for this, so when sun hours and temps are insufficient, i drain this down and basically do the same thing with a copper coil around the rocket stove--with appropriate precautions,
the solar system can be scalding hot, the rocket stove could be much worse.
being off the grid, the whole thing became possible when i found the miniature 12v circulating pumps
the search was for" hot water circulating pump" on ebay--like most items they come from china, only you have your choice of whether to wait or not
i have used the second one in the links with good results, it has a cover easily removed with 4 screws to take out clogs or reposition the outflow relative to the base -- hair seems to build up in them and eventually balls up and stalls the motor (my system has no filters on it) The first one is more of a flow through design with in and out inline, it also is a bit more expensive and ships from within the us so you don't have to wait two weeks for delivery--i bought it when i thought the first one had expired, but then discovered it was in the electrical supply, so by the time it came in the mail i no longer needed it, although i'm keeping it as a backup, or possibly to set up a separate secondary system for the rocket stove
This is my first attempt at integrating a radiant floor into a system with minimum excess plumbing and such, so i'm still in process of discovering how to incorporate all the variable possible needs with least complicated plumbing--
I looked at a thermostat system to turn mine on. They are common with solar swimming poolheaters. One sensor measures temp of storage tank, another measures the panel temp. If panel temp is higher than storage, it opens solenoids to divert the filter pump water through the solar panels and then through the filter. That system could be used to turn a pump on rather than open a solenoid. It could probably be used to maintain a max temperature.
My plan is gonna be simpler. Simply put, a 20 watt solar panel direct wired to a pump. If sun is shining it will pump. If it's not it wont. This doesn't leave any safety precautions such as too much heat or freezing. It's an off grid cabin and not used daily. System will be shut down when not there.
sounds real simple and easy, mine is a bit more complicated with regulators and batteries, and sometimes barely enough power to light leds and run the computer.
the thermostat system is no sweat to turn on and off to bring the hot water tank to temperature, but once it reaches temperature it needs to shut off, hence the idea of two thermostats, or perhaps some more complicated electronic solution.
Also, i think the pumps are good to 90 C, and while i wouldn't worry about exceeding that temp (unless i lived in the mohave, i try to keep machines in general well below their tolerances to get longer use out of them. Locating the pump at the drain valve and the hot water intake at the top means the pump is always drawing on the coldest water in the tank, and it is only after reaching the high temp all through the tank that it actually has to handle hotter water, so i'm often running with just half a tank of hot water, but because the supply comes off the top, the temps there are much hotter than the water coming out the drain below
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