Originally Posted by Shanaly
Hi in theory the water just circulates thermally, with the HW coming off the top of the heating
coil (be it solar - say a black hose curled on deck) or heatex (copper coil round exhaust). Feed would be cooler water from the bottom of the closed circuit, or from CW tank.
But how efficient is this circulation? e.g. how much lift
is there per degree of heat? How effective is heat transference?
If I need a circulation pump, which is likely I feel, does anyone know of one, or of a thermostatically controlled one? Or could I just depend on the pressure difference of a pressure - actuated pump?
Thanks in advance!.
For gravity circulation, its required that the tank be installed above the heat exchanger
, by say at least a few feet. You need to install piping from the heat source to the tank without any dips, that is a smooth rise. It works but not so much on a boat. Hot water being lighter then cold water (it expands when hot) so it will rise. How much lift depends but figure you need oh 30 degrees F (16 degrees C) differential for lift to happen
For a circulator system, something like 2.2 GPM / 8.5 LPM - 12V Solar Hot Water Pump - Can be run by a 5W solar panel - With Brass NPT Threads - Amazon.com
You can use a 10 watt solar panel to control the pump as that is simplest or you a differential controller
Amazon.com: Universal Solar Differential Temperature Controller: Patio, Lawn & Garden
For a solar collector its best to use black pained copper plate/ flashing with 1/4" tubes on 6" centers. Installed in a frame with insulation
on the bottom and tempered glass on the top. For a 6 gallon water heater 5 SF of panel will be enough to give a tank of hot water per day.
install a pressure relief on the panel in case of no flow heating. If the panel is in sun and the circulator is not running the water in the panel can get VERY hot. An air vent will be needed too.
For ball park sizing I assume 175 BTU's per SF of copper panel/hour. One btu will heat one pound of water one degree. So 5 SF of panel will heat 6 gallons about 100 degrees F per day. More toward the equator, less as you head
north. Most of the heating will be from 10 am till 3 pm ish depending on season and latitude
Hose and other materials has a lower heat conductance and so you would need more area for the same amount of heating. If your engine exhaust
is water cooled then the heat is of poor quality and gravity circulation with water probably will not work