Does anyone have any experience with,or knows someone who does,water heating coils in a stove?If so,how well does it work? Exactly how does it work?do I have to buy a lot of extras like pumps or whatever? I have a Harman coal stoker stove and they list water heating coils as an option. Sounds like a good idea but would like to hear from someone with experience first.Thank you in advance forany and all help.

-- larry spearman (lspearma@exotrope.net), January 08, 2001

Answers

My first experience with this was when I put a single u shaped loop through the top of the firebox in my wood cookstove, over twenty years ago. It worked, but took almost all day to heat enough water for a decent bath.

About twenty years ago, I built a wood heater; I made one entire side of the heater double layered, and welded one inch pipe fittings near the top and near the bottom of this "water jacket". These I connected to one inch steel pipes which ran an old dead fifty gallon electric water heater (free). The upper pipe went to the fitting which normally is the location of the upper heating element (which was a one inch thread fittiing). The other, lower fitting went to a fitting on the bottom of the tank. You could also use the fitting where the drain faucet usually is located. Make sure there are no "high spots' which will trap air and make the system operate very poorly.

This system heated so much water that, once the kids grew up and sprouted wings, we had to fill the bathtub a couple of times a day with scalding water, in order to prevent boiling the water. Make sure you know what you're doing, and have the proper heat/pressure relief valves, or you could conceivably cause a disastrous steam explosion if the water boils!!!!!

A coil in the firebox functions in the same way. So does a little collector which is sometimes sold, which also goes in a firebox. This latter is made of a couple of pieces of flat steel, with a one inch or so space between them. It's hard to explain, but it's sort of like a box--maybe 12"x12"x1". This has two pipe fittings on it, also, just like my water jacket.

The way it works is, when the water in the heater/stove, whatever gets hot, it expands. In other words, it has a lower density than the cold water. This causes it to rise out of the collector, through the upper pipe, and into the tank. Simultaneously, the water in the bottom of the tank subsides down the lower pipe and into the bottom of the collector. This is called a "convection loop". There are no pumps, or any other kind of control, other than the aforementioned temperature/pressure relief valves, which are really not needed unless you don't want to be killed by the steam explosion!

The old (or new, if you prefer) water heater tank is otherwise plumbed in exactly the same way as any other water heater. Thus, the hot water which you get from the fire is available at any hot water faucet in your house, just like a regular gas or electric water heating system.

The biggest drawback to this system is that the water in either the water jacket or the coils causes a cooler burning temperature in the firebox, which means that the stove is not operating as efficiently as it otherwise would. This means it does not heat the house nearly as well as a "regular" wood heater, or ESPECIALLY not as well as a newer, "certified" wood heater. But if it's the only way to get hot water you have, I highly recommend it. By the way, if you do a water jacket, like the one I did, be careful that it is strong enough to handle your normal water pressure. I was on a low pressure system, and once, when I had to hook up to a my irrigation water (when the pipes from my spring froze), I noticed that the side of the stove was bowing out by about a half inch. This concerned me so much I left the water on in a sink until such time as I got the spring water thawed out, and my low pressure system functioning again.

For a higher pressure system, talk to a knowledgeable welder, or to the dreaded engineer in your area, as it's too complicated for me to explain it here, what with so many different types of water systems, etc.

Incidentally, a simple, easy to build solar water heating system operates in the same way as the wood water heater: through a convection loop. This is hooked up in just about the same way, only the solar panel is substituted for the wood heater. I have had solar water heat (in warm months only) for almost twenty-five years, and it's awesome!

Good luck,

JOJ

-- jumpoffjoe (jumpoff@echoweb.net), January 09, 2001.

Very thorough, good answer Joe, someday I'd like to sit down and have a cup of coffee with you....

At any rate...

I've welded together a wood stove that has a coil inside of it, I've had to re-engineer the coil and controls a couple of times now, the first time was when the water pipe fittings started to leak (version 1), in version 2 - 1 hour after the fire got going the blasted thing started producing steam instead of hot water and made the pressure relief valve pop (which is a stinky pain in the butt when you are using an antifreeze solution). Im on version 3 now, Joes heater is a "passive" system that relys on convection, mine is called an "active" system that requires a pump and controls. I did an active system mainly because of the orientation of the coil and the hot water tank (and a thermal storage tank) is below the wood stove (in the basement), I still dont like the results yet and I might end up going to a version 4.

Of the two I recommend a passive system since there arent any pumps or switches to break.

The way my coil is put together now is that its a classic squared-off coil, if you are facing the front of the stove you are looking down the "tunnel" of the coil, a mercury switch thermostat(standard wall thermostat with all plastic removed, wires are teflon insulated) on the brick wall behind the stove turns on a small 17 watt DC pump when it reaches 90 degrees back there. A automotive radiator thermometer is attached to the outflow pipe. It has contact points attached to the needle that turns on a 25 watt DC pump when the temperature reaches 200 degrees F (water boils at 212)(fixed my steam problem).

Circulating through the coil is a 50/50 glycol/water solution that goes to a storage tank, inside the storage tank near the top is a copper coil heat exchanger that the potable water flows through when I need hot water. The water reaches scalding temps all the time and I had to install a (blasted expensive!) water temp regulator valve on the outflow from the storage tank to keep from frying me and the wife.

If you want I can give you more info with pictures, just email me.

Hope it helps a bit.

Dave

-- Dave (Ak) (daveh@ecosse.net), January 09, 2001.

Jumpoffjoe:

Question on your design. Would it have been possible to put shutoff valves between the stove and 50-gallon water tank and then a drain valve under the stove tank area? When you didn't need hot water, the tank could then be shut off and drained. But then, might the heat warp an empty tank?

-- Ken S. in WC TN (scharabo@aol.com), January 11, 2001.

I received a private email asking me to clarify so I thought I should post a part of my response:

Im sorry, Im trying to keep my posts clear not write a novel but I guess Im leaving too much out.

It sounds like you're going to need an active system, the reason being is that since your hot water tank is in the basement you will need a pump to force the hot water to move DOWN a pipe to it. In a convection system the collection tank has to be ABOVE the heater coil in order to work since hot water rises and cold water sinks. Im using a antifreeze solution in mine but for simplicities sake its probably easier to just heat your water directly from the stove.

However, if you live in an area with hard water then you might have to rethink that and move to an indirect system. The reason being since it is going to be heated by flames and occasionally run dry (because you dont need to heat up any more water) you end up depositing whatever is in the water onto your pipes and pretty quickly clogging them up.

An indirect system is one in which the heated fluid (whatever that might be; I've heard that silicon oil works well but a food-grade glycol/water solution is probably the cheapest) is pumped (or convected) to a copper coil heat exchanger inside your hot water tank and indirectly heats the water, the hot fluid goes into the coil in an intake near the top of the tank and the cooling fluid goes down the coil and out the tank near the bottom of the tank. This is done to stratify the hot water in your hot water tank and give you decent amount of hot water near the top for use fairly quickly.

You will need to have a sensor of some sort in your tank at around one quarter up of the tanks height. Probably it'll be something like an automotive radiator thermometer with a couple of contact points epoxied to the needle, (used that setup a lot, rugged, reliable and cheap!) they look like a really really big turkey thermometers, the contact point will be wired to a relay that will turn off the pump, vapor pressure in the coil in the stove then empties out the coil, just make sure you have a big expansion tank to allow for the increased pressure. This is to keep from over pressuring your pipes causing a explosion. If you are in an off-grid system make sure your relays are the type that only use power while they are switching, there are a couple of cheapie types that use power continuously and can drain your batteries.

Keep in mind that a volume of water when turned into steam takes up 1600 (thats sixteen hundred!) times the original volume of the water, if you dont ensure that you have good relief valves and expansion tanks you can literally blow up your entire house, there's a stick of dynamite in a pound of water. You wont have to worry about the entire volume of all the water in the pipe turning into steam though, as it begins to boil it will push out the rest of the water and any extra steam that leaves the stove will condense back into water, I'd recommend a 30 gallon expansion tank connected to your heating loop and even that is probably too large.

This is why I like tube heaters more than plate heaters, even if for some reason the water gets stuck in the heater (like it gets frozen or something) they just tend to just spring a leak and spurt steam instead of blowing up, explosions are a bad thing and being dead is kinda permanent (grin).

I tend to like indirect systems mainly because I never have to worry about the pipes or anything else freezing up, if I have to leave the house during winter for more than a couple of days its pretty easy to just empty out my pressure tank (or in your case your hot water tank) I also dont have to worry about the pipes getting clogged up with scale.

In version 4 Im going to replace the squared off coil with four serpentine pipes (two sides, a back and a top), this does two things for me: the first is that I get more heating pipe inside the firebox and thus more hot water, the second is that if I ever have to turn off the pump because my hot water tank and the thermal storage tank is completely charged to 200 degrees F (which is not likely to ever happen, the thermal storage tank is about 5000 gallons which is way too big) the vapor pressure will empty it out more efficiently and produce less steam that has to be re-condensed.

Hope this helps

Dave

-- Dave (Ak) (daveh@ecosse.net), January 14, 2001.

I purchased a solid fuel furnace shortly after we bought our country home and also purchased a stainless steel heat coil at the same time. This "U" shaped devise is suspended in the firebox from the back of the furnace(Jensen). After a call to the Jensen stove works I was asked how big my family was. When I told him it was just me and my wife at the time he advised me against it because it was quite possible to send my waterheater into orbit if enough hot water was not used while the stove was in use. He also siad it required a hot water pump and an additional storage tank or water heater to handle the amount of hot water it would produce. Impressive but scarry! I think it would be a great accessary but would require a certain amount of caution. Good Luck!

-- kevin p. doyle (doywood@wworld.com), January 20, 2001.

Just as a quick thought here, I think it might be a reasonable idea to run a small glycol loop outdoors. Run the pump continually, and use a diverting valve (three connetion zone valve) controlled by a thermostat to divert excess heated water out to the outdoor (could be buried) coil to prevent the production of steam.

Hate to waste energy, but what else to safely do with excess heat? Could always use it to heat your driveway I guess.

stephen

-- Stephen Scott (cje@snip.net), March 29, 2001.

http://www.geocities.com/vysocany/cz_heizkessel/

Solid fuel boiler for coal, coke, wood etc. with 24kW 28kW 45kW max performance

-- autogas-systems (autogas-systems@post.cz), December 01, 2001.