# Woodstove water heating attachment



## woodsy

Looking for ideas to help increase the water heating potential of our recently installed copper coil on the wood stove top. 
This coil loop is fed by circulator pump from the unpressurized solar drainback/heat exchange tank.
Found this article in Mother Earth News which is similar to what i may want to do. 
The coil laying open on the top of woodstove with 1 gpm flowing through it does not retain heat well enough to be very effective.
Just curious what others may have used or found to be safe and effective.
The coil sets in a recessed area on top of the stove. If i were to cover this area with sheet metal making it a chamber would this increase water heating effectiveness ? Experimenting 1, 2, 3.


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## Harry Chickpea

TMEN projects are often more glitter than action. You could add a baffle in the stove to direct heat up against the top of the stove before exiting, but in any event, a scraper to remove the creosote build-up is needed. Even a thin layer of it would cut heat transfer significantly. You also have transfer inefficiencies because of minimal contact between pipe and stove. CPU fans have flat heat exchangers next to the chips, but even they have to use a transfer "grease" to get good results. I suppose you could build a dike around the edges and flood the top with water, but you are working against good logic. Round pipe on a flat surface with marginal heat isn't a good combination.


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## TnAndy

You don't say what size copper line you used, or how many feet are in the coil. The smaller the line, and more feet you can get in the coil, the better your heat will exchange from the stove to the coil.

But honestly, you would be far better off to put the heat exchanger INSIDE the stove....as per this drawing:










Here is a company that makes a stainless steel coil for inside stoves:

http://www.hilkoil.com/

You can also plumb in inline near the outlet a thermostat ( called an aquastat ) like this to turn your pump on automatically.....I have a "fireplace" boiler I built for my fireplace out of black iron pipe welded to square tubing manifolds, and circulate water from a 500 gallon tank in the basement thru the fireplace loop, and back....then draw out hot water to heat the house via basebd radiators. Mine is a Honeywell model that the sensor goes in a "T" in the line, and also has a dial you can set the "ON" point anywhere from 90 degrees on up to about 160. WW Grainger carries them, as well as about any good boiler supply place.

Looks like this:


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## woodsy

Harry Chickpea said:


> TMEN projects are often more glitter than action. You could add a baffle in the stove to direct heat up against the top of the stove before exiting, but in any event, a scraper to remove the creosote build-up is needed. Even a thin layer of it would cut heat transfer significantly. You also have transfer inefficiencies because of minimal contact between pipe and stove. CPU fans have flat heat exchangers next to the chips, but even they have to use a transfer "grease" to get good results. I suppose you could build a dike around the edges and flood the top with water, but you are working against good logic. Round pipe on a flat surface with marginal heat isn't a good combination.


Yes, this is what I am finding out, round tubing against flat surfacing isn't so great, even if the surface is 350-400 degrees.
Thanks for the great input.



TnAndy said:


> You don't say what size copper line you used, or how many feet are in the coil. The smaller the line, and more feet you can get in the coil, the better your heat will exchange from the stove to the coil.
> 
> But honestly, you would be far better off to put the heat exchanger INSIDE the stove....as per this drawing:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Here is a company that makes a stainless steel coil for inside stoves:
> 
> http://www.hilkoil.com/
> 
> You can also plumb in inline near the outlet a thermostat ( called an aquastat ) like this to turn your pump on automatically.....I have a "fireplace" boiler I built for my fireplace out of black iron pipe welded to square tubing manifolds, and circulate water from a 500 gallon tank in the basement thru the fireplace loop, and back....then draw out hot water to heat the house via basebd radiators. Mine is a Honeywell model that the sensor goes in a "T" in the line, and also has a dial you can set the "ON" point anywhere from 90 degrees on up to about 160. WW Grainger carries them, as well as about any good boiler supply place.
> 
> Looks like this:


The copper coil is 20' X 3/8 I.D., may have enough room for another 20' coil, if that would make much of a difference. I was hoping to not have to penetrate the firebox.
Sounds like you have a great system !


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## TnAndy

Well, you've used the right size tubing....and have plenty. Adding another coil, would of course, add some more transfer.....but as you're already finding, transfer is not that efficient on the top.

I'd do the inside coil. Penetration of the firebox is no big deal....use a hole saw just slightly larger than the OD of the stainless tubing, then caulk around it with RTV silicone...it's good to 400 degrees.....there won't be much gap anyway.....and the temperature of the water in the tubing will always keep the temp right there at the silicone 200 or so.


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## Jim-mi

A cheapo way would be to put a layer of fire brick on top of the copper coil . . .trying to trap what heat that does come from the top.

TnAndy's method is far better...........


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## woodsy

Well seeing its all set up on top we will play with that some more before going inside.
If we can get the 150 gal storage tank up 3-4 degrees per hour would be good enough for me. Its backup for the solar....and electric which I'd like to keep off as much as possible.
Bartered a couple lbs. of ground beef for a 1/6" thick sheet of metal to cover the top of stove where the coil sets making a fairly tight compartment, and have enough firebrick to cover that and give this a test run next time the stove gets fired up.
Appreciate the advice....
I may be able to add on another temperature sensor to the solar controller for automatic operation. 

Will a copper coil hold up inside the firebox ? I see stainless mentioned a lot.


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## WisJim

A coil around the stove pipe has worked well for us in the past, and friends currently do that for almost all their hot water. Gravity circulation to the storage tank works even without power. Today we have a coil in the wood furnace to preheat the water before it goes into the gas water heater.


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## artificer

Yet another heat transfer option: pot of water. Get a pot/pan that covers the same amount of space as your current coil. Put the coil in the pot/pan, and cover with water. You have to keep replacing the water, but its quick and easy.

Next idea is to get a steel or aluminum plate. Use silicone caulk, and caulk the coil to the plate. Place on heater. The plate increases heat captured from the heater top, and the caulk transfers it to the coil.

Better idea is to put the coil inside the heater. Does your heater have the top holes? If so, you could make up a replacement plate and pass the coils through it. No permanent modifications to the heater, and fairly easy to do.

Michael


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## TnAndy

woodsy said:


> Will a copper coil hold up inside the firebox ? I see stainless mentioned a lot.


Copper will hold up to any heat you'll generate out of a woodstove, especially since it will be filled with water, and the copper won't normally get above the temperature OF the water. But the copper you'd need to make a "coil" is fairly soft type, meaning you might beat it up chunking wood in....or if you use a harder copper, you'd have to use elbows to make your turns, in which case I'd silver solder it in case you ever did run the system dry ( like pump failure or power outage ) and boil off your cooling water....regular 50/50 solder might fail.


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## WisJim

To make a coil out of some half or three quarters inch soft copper, fill the copper tubing with fine dry sand, such as the stuff sold at a home center in a paper bag, and crimp the ends beyond the length that you will need. If it is packed tight with sand, you can bend it around a form without damaging it. I used a coffee can (old metal style, 3 pound size, filled with concrete) as a form to make a coil to fit over a 6 inch stove pipe. Then I used a piece of 8 inch stove pipe to make a cover over the coil. When done with the forming of the coil, cut the crimped ends off and solder on your fittings.


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## woodsy

Sand makes a good heat sink too, so I found out.
Third time is a charm.
The solution in this case of copper coil on top of Ashley box type wood heater seems to be sand worked in around the coil. The new compartment housing the coil is about 1 1/2 inches deep, 1' 3" wide X 2' 4" long. On top of coil and sand is a sheet of steel covered by firebrick.
Initial results, although not with a real hot fire is water returning back into the storage at 4-5 degrees warmer than tank temp. slightly warmer than the solar collector water returning. 
Difference between the two being flow volume, woodstove coil flowing at 2gpm, solar 
collector flowing at 3.5 gpm.
Both running together: woodstove coil 1.5 gpm
solar collector 3 gpm
I think we are good to go !
Thanks for all the input.


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## Harry Chickpea

Interesting! I wouldn't have considered sand because of the air spaces between grains, but if you are even doubling or trebling the contact area, you are making progress. Nearly as important, the sand on the top is probably partly acting as a safe insulator. Thanks for the additional info. FWIW, you need to write TMEN and let them know of your experience.


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## greg273

WisJim said:


> To make a coil out of some half or three quarters inch soft copper, fill the copper tubing with fine dry sand, such as the stuff sold at a home center in a paper bag, and crimp the ends beyond the length that you will need. If it is packed tight with sand, you can bend it around a form without damaging it. I used a coffee can (old metal style, 3 pound size, filled with concrete) as a form to make a coil to fit over a 6 inch stove pipe. Then I used a piece of 8 inch stove pipe to make a cover over the coil. When done with the forming of the coil, cut the crimped ends off and solder on your fittings.


 Yeah, I used 1/2" copper coil wrapped around 8" flue pipe...all told, there is 50feet of tubing, works very well. 
My previous set-up had the collector ( several 3/4" pipes paralleled) merely touching the backside of the furnace, this didnt work at all. But the coil-around-flue thing works great. A great use of what would otherwise be wasted energy.
I found it unnecessary to use sand or anything else when wrapping the coil, just go slow, use a stovepipe as your form,(its easier if its done on the floor) and work your way slowly around and around. Just get it nice and tight, so there is maximum contact between copper and flue-pipe.


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## treed

I heated a former home with a 1970's era furnace called a Kickapoo stove. Basically a firebox surrounded by sheet metal for the plenum.

I ran a 3/4" galvanized pipe into the top front of the firebox, made a c shaped loop inside and out the other side of the front end of the furnace. This loop was plumbed into a recycled hot water tank placed on a platform such that its base was level with the loop. Thermo-siphon (non pumped) circulation in the system produced 140 degree water at the top of the tank and "cool" water in the lower third of the tank at all times. There was of course a pressure release valve installed at the top of this tank.

This entire system was then plumbed into the cold source side of my electric hot water system. During the winter months I turned off the electric water heater element because the system provided all the hot water we could use. I also installed a temp gauge at the top of the tank to monitor temp. Never a problem.


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## haypoint

I like the drawing TNAndy posted. I had a similar setup for my wood cookstove. There was no need for any pump. Hot water rises and the cool water is drawn into the coil. The tank inlet pipe went into the base of the tank and the outlet, heated water, comes off the top. I could tell how much hot water I had by simply running my hand down the side of the tank. The "coil" was a casting in the side of the firebox. 

50 feet of copper pipe wrapped around a stove pipe contains less than a quart of heated water. You need a way to slowly circulate heated water into a storage tank.


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## greg273

haypoint said:


> I like the drawing TNAndy posted. I had a similar setup for my wood cookstove. There was no need for any pump. Hot water rises and the cool water is drawn into the coil. The tank inlet pipe went into the base of the tank and the outlet, heated water, comes off the top. I could tell how much hot water I had by simply running my hand down the side of the tank. The "coil" was a casting in the side of the firebox.
> 
> 50 feet of copper pipe wrapped around a stove pipe contains less than a quart of heated water. You need a way to slowly circulate heated water into a storage tank.


 Yes,I have the tank set above the furnace to create a thermosyphon. Anyway, with a good fire going, I get about 15-20 gallons of heated water per hour.


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## ChristieAcres

TnAndy's design is more along the lines of what DH will be doing with our wood stove (the one he built with this in mind). He will be building a heat exchange for inside the firebox. His design will be a bit different, but very safe (multiple shut-off valves, etc...). This set up will produce all the hot water we need. After concluding what he wanted to do, DH called his father (who he runs past his ideas/designs). FIL is short on words, but every one counts (retired Engineer).

Power outage? Back-up generator with relay-switch (under 10 seconds). That would be a concern for me, otherwise!!!

Greg273- sounds like a good setup!


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## survivalpro

A copper coil wrapped around the stove pipe near the stove might give you more heat.

The problem is the surface area of the copper pipe actually in contact with the stove is very small.

Wrap the coil tightly against the pipe and have the coil touch itself as it wraps around and you will have more heat transfer.

Older wood stoves with water heater attachments had a tank of water on the side or back of the stove with a pressure relief valve. That gave a lot of surface contact with the water for faster heating.


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