I've ordered a 3.5" thick 12"x12" water to air heat exchanger (boiler rad coil) because this year I'm going to try to use my efficient LP gas hot water tank to also heat my tiny dwelling. my living space is only 20x20, and less than 400 sq ft inside. the heat exchanger is supposed to provide 57,500 btu (with a certain water and air flow rate).
MY QUESTION: can I use a thermosyphon to circulate the hot water to the exchanger? I can mount the coil rad exchanger immediately beside the tank if need be. I have ordered a small hot water circulation pump, but I'm all for the thermosyphon if it will work, being much less maintenance. also my LP tank does not rely on electricity, I'm sure the blower fan will require electricity, which I have, but if I can circulate without electricity or pumps, I will. my tank has pex lines on top, and it has fittings on the side also, near the top and bottom both, which I figure I could use for a thermosyphon...?
yes, a thermosiphon will probably work, but with the small size of the coil, the circulating pump may be necessary to keep up with the cooling effect of the fan on the water in the coil. perhaps if the tubing had a larger diameter a thermosiphon could move enough water to heat the place,
My bet would be you will end up using the pump when it gets really cold but a lot depends on how well insulated the room is, how cold it gets, etc, all 400 sq ft of floor space are not heated equally.
The only question I have is about the continuous cycling of the hot water heater as heat is being continuously removed to heat the room, and whether that might shorten the lifespan . especially if it's not a heavy duty heater.
If you try and use the thermosiphon, mount the coil so the intake is high and level with the output of the heater and let the water fall to the bottom fitting on the heater. type of pipe isn't as important as constrictions in the pipe. Pex fittings often reduce the effective diameter considerably, which may not be a problem depending on the size of the coil, but it's good to keep in mind--insulate the top horizontal run very well.
You will also likely need a thermostat on the coil to regulate the pump or fan or both--some thermostats will turn one appliance on and another off at the same time/temperature.
remember, if you're not having fun, you've got the design wrong
the LP gas hot water tank is barely 2 yrs old, 40 gallon, it has a built in thermostat and I keep it dialed in about half way, temperature wise, which i prefer for my showers. the tank is super efficient, 65$ canadian in propane (100lbs) gives me at least 3 months of hot water in the summer! and turns itself off an on with the temp setting. in the winter i was previously burning 65$ in only 3 weeks using the tank plus a gas fireplace. however when I'm not using the fireplace, the LP gas lasts for several months, and the excess heat wouldn't be going out the chimney if i recirculate hot water. it would be going back into the tank. it seems to me the hot water to air heat exchanger would be far more efficient and self regulating as far as water temps, whereas the fireplace only had 3 settings, low med hi constantly burning fuel, with zero thermostat setting. i would have the air blower fan and pump only on when needed, if i need any pump at all. otherwise perhaps only a fan on a thermostat, and if a pumpless thermosyphon would work i might just add a shutoff valve for summer time. my 400 sq ft is poorly insulated , basement level, half underground, like a micro earthship, my concrete floor is about 3-4ft underground, which you might consider geothermal but in Canada the frost can go 3ft down give-or-take, just not likely when its being heated. the listed 57,000 btu is about twice what i should require, even in a Canadian poorly insulated dwelling. my geothermal floor only keeps me cool in the hot summer, it doesn't help at all in the winter, infact i use interlocking hi density floor tiles year round because of the otherwise cold floor. i would rather be overkill an be able to shut it off, or have it regulated via thermostat, rather than not have enough heat in January. last year i burned my own free dead wood but I'm not always around to keep a fire stoked. 2 yrs ago i had the LP fireplace but it was too inefficient and fuel hungry.
i love the efficiency of the LP tank and if i could use the one appliance for both the hot water and residential heat, i'd be a happy camper. i've already decided this is what i want to do this winter, and that's why i've spent money ordering parts online. i just need to figure the most efficient way to put it together with as minimal cost and maintenance as possible, for a humble frugal DIY Canadian guy like myself. its already cold here, today it tried to snow in the middle of the afternoon, and were getting frost on the pumpkins at night. i should have had this done by now, but I'm utilizing small electric plu- in heaters temporarily until the coil exchanger and pump arrive from the USA/Ebay later this week, or early next week. by then I'm hoping to know wheather or not pumpless thermosyphon is a good low maintenance possibility.
It sounds like you will have a good adventure this winter, and may be able to report back how well your experiment worked
just remember, this isn't recycling hot water in an endless loop. you are burning more gas in the water heater to reheat cold water coming back from the radiator, so your propane bill will be going up when you are heating,
You are likely correct that the water heater will give the same heat to the room as the lp fireplace for less gas.
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