covering copper tubing with Plaster of paris

[steelman]

have been pre-heating my water for the past 2 years using 60ft of 3/8 copper tubing. works great but i seem to only collect about 50 to 55Deg of heat difference. temps are from the well water going in the coil at 56deg and coming out the end of the coil at 100 to 110.
after a good long fire like when the temps outside are around zero for a few days and the furnace running often the Solar storage tank becomes heated, or at least the water coming from the bottom of the tank is coming out at 70 to 75 deg and the water coming from the end of the coil is about 125 to 130 deg. some im collecting about the same amount of heat from by water coil.

im using the Thermo siphon method. the storage tank is next to, and above the Pellet furnace with the 60ft coil just laying on the top of the burn chamber. does anyone thingk i could collect more of the heat if i covered the coil with Plaster Of Paris? i would make a 1 1/2 inch deep "picture frame" on a flat surface and cover the coil with plaster.
my thinking is the blower that is moving the heated air through out the house is taking some of the heat from the coil. if i cover it it may collect more of the heat in the water.

how do you prepare the Plaster? how do you cure it? is it needed?
would it be worth the effort?

i seen were someone did this to his wood burner so it got me thinking i could gather more heat.
ill post a pic soon.

jim

 

[Fred61]

I would worry about trapping heat on the top of your heat exchanger causing some warping if the top is unable to dump the heat as fast as the sides. Also what will happen if the coil heats up too much. Will it continue to thermo syphon or will it turn to steam?

 

[Boil&Toil]

I'm not certain, but there could be an issue of corrosion. Plaster is not concrete, but I know that a classic problem with copper pipes in concrete is that the concrete corrodes the copper pipe. So I'd be cautious at best. I'd also like to see pictures of what you are doing now as I don't have a good feel for it - you have the copper coil on top of the furnace, but not in the hot air path? If it's just laying there in the open, I'd think some insulation over would make more of a difference, but then there's the question of what your pellet furnace would think of extra insulation there (don't want to overheat it....)

 

[steelman]

Here are the pics

 

[Boil&Toil]

If (and I may be getting the wrong idea from the pictures) there is room, I'd say your best bet would be to simply add a second coil of pipe in parallel. You MIGHT also see some benefit from being a little bit more fussy with this coil's coiling - it's a bit of a mess, which may impede ideal thermosyphon flow a little (but that would probably be quite small relative to adding a second coil.) You can use copper wire to tie the coils in place if they don't want to cooperate.

I would expect far more benefit from a second coil than from plaster-encapsulation.

Your milage and perceptions may vary, but I also ended up insulating pretty much all the pipes on my hotwater heater (even the cold supply & relief pipe) when I touched them and felt that they were hot, and therefore conducting heat out of the tank, even without water flowing. Well, the cold pipe only gets hot when water isn't flowing, of course. Can't say I actually calculated any sort of payback on the price of pipe insulation and the probably small amount of heat those were losing, but it felt right to my gut.

 

[steelman]

If (and I may be getting the wrong idea from the pictures) there is room, I'd say your best bet would be to simply add a second coil of pipe in parallel. You MIGHT also see some benefit from being a little bit more fussy with this coil's coiling - it's a bit of a mess, which may impede ideal thermosyphon flow a little (but that would probably be quite small relative to adding a second coil.) You can use copper wire to tie the coils in place if they don't want to cooperate.

I would expect far more benefit from a second coil than from plaster-encapsulation.

Your milage and perceptions may vary, but I also ended up insulating pretty much all the pipes on my hotwater heater (even the cold supply & relief pipe) when I touched them and felt that they were hot, and therefore conducting heat out of the tank, even without water flowing. Well, the cold pipe only gets hot when water isn't flowing, of course. Can't say I actually calculated any sort of payback on the price of pipe insulation and the probably small amount of heat those were losing, but it felt right to my gut.

good idea about wiring the coil together. that little coil took me about 8 hrs of fiddlin. it looks good and tight when you hold it together but when you let it go and the heat dies its thing it looks "not so good"
when making it i stuffed about a pound of table salt in it and hand formed it to a coil. the salt was to prevent the kinks and it worked but getting the salt out i lost my patience after about an hour of rotating it to get the salt out. anyway the wire was a good idea even if it just makes it look better.
i have even thought about a copper flat plate. instead of a coil just 2 flat plates soldered together with about a 1/4 area between them for the water to heat up. in reality i dont think the extra length will heat the water any more than im doing now. i think it gets to temp within a few feet of coil. so the 60ft is a wast. i bet the water would get to the same max temp in about 10ft of coil. but a plate would get to temp just as fast. all we are doing is allowing the water to heat up , get "less dense" and the colder. more dense water drops and pushes the less dense water out and up.

thanks for the post

 

[Boil&Toil]

If the water is getting to heat in ~10 feet, then 3-4 times as much water would get to heat if you turned your 60 feet "In series" into 3 parallel runs of 20 feet each, or 4 of 15 feet each. It's tricky sometimes to remember that "heat" is a game of mass/volume as much or more that of temperature - if you can get 3-4 times the waterflow and half the temperature rise per pass, you'll get more heat...

 

[steelman]

If the water is getting to heat in ~10 feet, then 3-4 times as much water would get to heat if you turned your 60 feet "In series" into 3 parallel runs of 20 feet each, or 4 of 15 feet each. It's tricky sometimes to remember that "heat" is a game of mass/volume as much or more that of temperature - if you can get 3-4 times the waterflow and half the temperature rise per pass, you'll get more heat...

understood.
i have reduced the tubing to 1/2 top and Bott. from the 3/4 NPT bungs on the storage tank.
so maybe i need to keep the 3/4 out the bottom of the tank (the tank has 3/4 NPT threads top and bott.) then to a small manifold that provides 3 loops at 3/8 or 1/4 copper tube with a 20ft loop each (total being 3x 20ft = 60 total ft) would heat more water to the same temp. correct?
the pics i posted is miss-leading i was cleaning the burn pot when i took the pics. so normally i get more heat or at least higher temp. than what is shown. when fired i normally see temp's 70 in and 125 out.

i do have 3 females that love to take 20 min showers, and a female lab that seems to think she gets a hot shower once a week as well.

thanks for posting.

jim