We have a wood stove and it's one of the ways I rationalize living in this silly rural/suburban town I live in, because we can just use trees on our property. It doesn't heat any water, so the oil-fired furnace (which my parents have written off for modification) still has to heat the water. I was thinking of adding an air-to-water heat exchanger to the exhaust right after the stove and and plumb it into the existing hot water loop. Has anyone done anything like this?

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition

A friend tried it years ago. It was a good way to cool a wood stove and a poor way to heat water. Putting the heat exchanger in the flue is a very bad idea as it cools the chimney and makes for a lot of creasote and a poor draft.

search this site. http://nepacrossroads.com/forum-67.html . Lots of good discussions about your topic. Here might be a start .http://nepacrossroads.com/about14511.html EDIT: a better discussion > http://nepacrossroads.com/about1990.html

What happens when it gets too hot and flashes to steam from the hot flue gasses?

I looked into an outdoor wood furnace since I have an abundance of firewood. Shipping was the deal killer.

If you poke around that site I believe it is discussed.

In my honest opinion IF you are close to NE PA and by close I mean with in 800 miles or so I would burn anthracite coal. If you have wood to cut then sell it to buy coal.

I have a woodlot, and I have in the past used hard coal in a stove. Wood is dirty heat and coal is much worse. Wood is becoming a drudge to cut and split and haul so I am looking at corn burning stoves.

Was it Anthracite? I believe it is much cleaner than wood and much better than corn. Corn has to be kept dry and can attract rodents. Anthracite burns much hotter (BTUs) has little if no effect if it wet and no smoke to speak of.

Coal has it's own set of problems. When kerosene became available about 100 years ago there was a huge move to convert the coal furnaces to kerosene. Coal makes dangerous gases when it burns and makes a messy chimney. I have an oil fired furnace that makes heat and hot water, but I like the feel of a wood stove in winter. The local farmer has a market heated by corn and it's as nice as wood. Corn can attract rodents and that's the main concern I have with it.

I do not doubt that corn works well. Anthracite coal just has a much higher BTU output cost about the same if not less if your near NE PA . Also corn stoves top out about 55000 BTUs as where coal stoves can go as high as 180K. Not sure what problems it can cause on a good chimney. Here is a link to a company selling Automatic stoves. Worth at least a look. http://www.leisurelinestoves.com/index.html & a nice boiler http://www.harmanstoves.com/products/details.asp?cat=central-heating&prd=boilers&f=BLRVF3000

Thanks for the links. I was offering the OP some observations in answer to his questions. I'm really all set for my heating needs.

Anthracite coal burns with a blue flame like a gas stove. There is no smoke to speak of. The big problem is getting rid of the ash.

I would at least have a metal tank near the stove/heater, in order to let the water warm up a bit before it enters the hot water heater - this way it shouldn't have to work so hard to heat the water. I have heard of people who put a 4" PVC pipe, 8' long in their basement, and plumb them so the water gets a chance to warm up a little.

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09 Jetta TDI
1985 300D

I suppose a tank-style heater wrapped around the sides of the stove would work well, but then you get into licensing and insurance issues since it will be a heated pressurized vessel.
I built a waste-oil heater inspired by the Mother Earth one, but I used a 25gallon propane tank as the casing, with an Army surplus 40kbtu tent heater instead of their half-assed attempt at a burner, with a heat exchanger from an old steam pressure washer mounted near the top of the main chamber. A circular baffle plate spaced down one inch from the top edge slowed the escaping exhaust and allowed more heat to remain, then a 4" stack out from the top center. I plumbed the coil to a car radiator with an electric fan, and a 12v circulator pump completed it. Roughly 20% anti-freeze mix, since western WA didn't get that cold, and I had a heated garage.
You could possibly adapt this to pre-heat water, as long as it's circulated, but again you're into the heated pressurized vessel thing again. Not a big deal in a detached garage, but in a house it generally is a big deal.

Therma-coil website with drawings: http://www.therma-coil.com/plumbing.htm



PLUMBING

Thermosiphon (passive) Method

The easiest, most economical and desirable method of Thermo-coil water heating is the thermosiphon, using no pumps, controls or electricity. Heated water from the Therma-coil is less dense, lighter, and naturally rises to the top of the water-heater or storage tank. The cooler water in the bottom of the water-heater/storage tank is heavier and "falls" down the plumbing line to the Therma-coil, completing the cycle. The flow is slow and is reduced by long runs. This is only possible if the water heater or storage tank is higher than the woodstove and close to it. An ideal thermosiphon arrangement would be to place the water-heater on the next floor above the woodstove, in an attic, closet, or elevated in the garage opposite the woodstove wall (a stand can be built under the water-heater).

Sloping the piping at least 1 foot rise for every 2 feet horizontal run would be a good rule of thumb using 3/4" (well insulated) copper pipe. Soft copper reduces sharp turns and is easier to install in most situations. Local plumbing codes and methods should always be adhered to.



Pumped (active) Method

When the water heater isn't located close to and higher than the stove, a small circulator pump and controller is used. The differential controller compares a probe sensor inside the bottom of the water heater / storage tank, and a probe sensor inside the therma-coil exit line. Whenever the stove water is hotter than the storage tank, the controller turns on the circulating pump. The pump circulates the cooler water from the bottom of the water heater, through the Therma-coil where it is heated, and back to the top of the water heater (costing only pennies a day in electricity.)