Water to Air Heat Exchanger

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Mountain
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Post Wed. Aug. 13, 2008 9:54 pm

Can anyone tell me if it is possible to add a water to air exchanger to a coal stove which get placed into a forced hot air system? I know they do this with outside furnaces to forced hot air systems with great success. What I was wondering was if a water coil placed either inside or outside a coal stove could be looped into a water air exchanger and placed into the plenum of a forced air system effetively. I'm looking at this as the same a radiator works for an automobile where the outside air or fan keeps the coild cool enough from over heating. Can the same principle be used if the fan from the forced hot air system is continuosly blowing? Thanks for your time

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LsFarm
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Post Thu. Aug. 14, 2008 8:16 am

Yes, it can, this is the way that a boiler is used to heat a house with forced hot air heat. But I doubt that you will get any amount of effective heat from a water coil in a stove,, the coil is mean to slowly heat domestic hot water. I don't think it will absorb enough heat to do much for a forced hot air system.

Greg L.

Mountain
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Post Thu. Aug. 14, 2008 4:05 pm

Lsfarm, I saw where a heat to air changer capable of producing 60,000 BTU's could be purchased. I have an alaska 140 coal stove. The only hot water coil they sell to fit this stove is a single loop. Based on where the coil is placed in the stove I was wondering if placing the exhanger into the return portion of my plenum would produce enough worth while heat. I currently have a plenum on top of the coal stove with a 1550 cfm blower and two eight inch ducts going into the direct plenum which works fine however, it seem like if properly installed more heat could be captured. I was thinking maybe a closed system running from the hot water coil to the heat exchanger could be a possible solution. COuld this produce enough heat to meet the 60,000 BTU capability of the heat exchanger?

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Yanche
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Post Thu. Aug. 14, 2008 5:48 pm

NO


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LsFarm
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Post Thu. Aug. 14, 2008 6:41 pm

Forum member Steve N posted the approximate BTU's extracted by a hot water coil, I don't remember the number but it wasn't very much, about 10,000BTU I think.. The limitation is the heated area of the loop in the firebox.. With a 60,000btu water to air coil, you would never get the water up to an effective temperature..

If you are trying to extract more effeciency from your 140 furnace,, I'd use the coil for it's designed purpose: heat domestic hot water.. the furnace makes heat all day and night, the most common hot water use times are the morning and evening.. If you use the coil to heat a tempering tank [old hot water tank] and the hot water heater, you can have 80 gallons of domestic hot water for esentially free.. Typically domestic hot water is 25%-30% of your heat bill yearly.. so at least during the heating season you can save another ~25% or so.

Hope this helps..

Greg L
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Mountain
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Post Thu. Aug. 14, 2008 7:42 pm

Thanks lsfarm and yanche for your advice. Its great to know that people understand what is being asked and can give good advice. I really appreciate your input. Since we got this cleared up and the coil would be better to heat domistic water, have you seen or heard of something called an add on boiler? I saw this for sale on e-bay. It was designed by someone in Vermont. There claim is that it will heat an 80 gallon tank of water. The device looks like a radiator from a car only smaller. It gets connected to either the top or side of the coal funace and extracts the heat this way from the outside. The water runs through the capillaries of the device and heats water. Again, I would guess it depends on how hot you burn the stove as to how much heat is extracted from the outside to heat the water. Is it possible that such a device could efectively heat water by sitting on the outside of the stove versus a coil on the inside of the stove? I hate to cut hole into my stove only to find out later that the system I wanted to have doesn't work as planned and now have to deal with patching holes in the stove. I do have a well and and was thinking that a tempering tank next to the coal stove may be the way to go. THis system would work in the summer and winter however in the winter I could circulate the water through the stove to warm it prior to going into the hot water heater. I would appreciate any advice on this. thanks again.

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LsFarm
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Post Thu. Aug. 14, 2008 7:57 pm

I think your furnace may have holes already marked or cut and plugged or a coil..

The ability to absorb heat by a coil or exchanger is directly in relation to the difference of temperature of the heating media and the heated media.. In a coil in a firebox, the internal temps in the firebox are 800-1500*, the water in the coil is ~ 150*. This is a big temp differential so a lot of heat passes into the water...

If you put a fined tube or radiator in the exhaust of your furnace, the air temp probably doesn't exceed 250*, [I'm not sure about this]. But putting an exchanger in the oulet of your furnace will serve to lower the output temps, and not do too much to warm the water in the exchanger.. I'd stick with the DHW coil in the firebox.. it's a tried and proven system.

Greg L.

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Mountain
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Post Thu. Aug. 14, 2008 8:07 pm

Lsfarm your knowledge is impecable. I was thinking about placing a tempering tank less than 4 feet from the coal stove and then runing this into my 40 gallon water heater. I have a family of three and of course hot water consumption varies day to day expecially on laundry day. My water heater would be approximatley 25 feet from the tempering tank. What are some of the anticipated expenses if would have to undergo? Would I need a circulating pump? Also, I know Alaska has a coil specific for this stove but saw some nice hilkoil water coils that includes a pressue relief and saw blade to cut for around 200.00. Any thoughts on the size of a tempering tank or devices to set this system up properly?


CapeCoaler
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Post Thu. Aug. 14, 2008 8:08 pm

Do the coil inside the stove. The add on would be a waste of your hard earned cash. Air is a poor conductor/transfer agent of heat.
The ability to absorb heat by a coil or exchanger is directly in relation to the difference of temperature of the heating media and the heated media.

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LsFarm
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Post Thu. Aug. 14, 2008 8:12 pm

Do a search for DHW, Coil, tempering tank etc.. there is also some info in the Knowledge base about using a coil and circulators..

A quick answer is I'd recommend a circulator, but a 'gravity flow' system may work for you.. it is just slower in recovery,,

Sit back with your favorite beverage and read,, this is a popular subject..

Take care, Greg ..

The storms are almost past NYC, maybe I can still fly my flight to the west coast.,, even if it is 3 hours late.. :mad: :shock:

Greg L

Mountain
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Post Thu. Aug. 14, 2008 8:15 pm

thanks for the info lsfarm and capecoder

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BigBarney
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Post Fri. Aug. 15, 2008 12:01 pm

The most important item in a coil is ,as LSFarm said,the temperature differential but

also is the area of the coil exposed to the hot fire.In a 3/4" tube you only have ~14 sq"

of area per foot which severely limits the heat which can be absorbed by the water.A boiler has

a large area of heat exchanger exposed and can do a much more efficient job of capturing

the heat from the fuel.Coils have to be extremely long to extract much heat and are constrained

by the available space in the firebox.

BigBarney

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