One of the advantages of heating with wood is that just one stove can be used for every need.In addition to keeping us warm, wood-burning machines can cook meals, dry clothes, and toast cold toes.But wouldn’t it be fancy if that black box could also take a hot shower?
In fact, domestic firewood water heaters are nothing new…over a century ago, many stoves had tank attachments.However, the advent of “closed” wood burners and pressurized water systems has put most of the old batch heating techniques on the back burner and new methods based on closed cycles have been developed.
Most water heating accessories use heat exchangers installed in the firebox or equipment chimney.The best commercial example of this approach works really well.If the furnace is running most of the day, they can provide hot water for the whole home.However, for safety, these devices are often made of stainless steel (an expensive commodity) and must be pressure tested to ensure they can withstand the very high temperatures that may be encountered within the heating system.As such, a good internal heat exchanger comes with a pretty hefty price tag.Homemade internals, on the other hand, are notorious for scalding steam explosions.
Also, extracting heat from a firebox or wood stove’s chimney can have an unfortunate side effect: extracting Btu directly from the fire (using a firebox exchanger) reduces combustion efficiency… if the products of incomplete combustion are cooled below the temperature at which they condense (either through a combustion chamber or a chimney heat exchanger), a large accumulation of creosote can occur.Make no mistake, the combination of a chimney fire and a water-filled heat exchanger can spell disaster.
Recognizing the fact that there is no unpaid midday meal, we took a conservative approach to designing our own wood stove water heating attachment.Instead of putting an exchanger inside the heater or chimney, we attached one to the outside of the firebox.By adopting this strategy, we avoided any major modifications to the heater, which maintains Underwriters Laboratories accreditation.More importantly, several of the safety criteria we’ve already mentioned are met: the temperature encountered outside the heater enclosure will not boil the water (as long as the liquid remains circulating), the heat used to heat the water is radiated by the heater anyway , so no excess heat escapes from the firebox.
Our water heating attachment consists of only about 50 feet of 1/4 inch copper tubing coiled into a Paris-filled drywall.The gypsum-based material helps distribute heat evenly to the coils and allows the exchanger to be in direct contact with the furnace body without overheating.(We would like to thank Ed Walkinstik for the suggestion.) The assembly bolts to one side of the heater and plugs into a recycled 42 gallon water heater (we used a water heater with a burnout element but a soundproof box).Just like a solar preheater.
A 10 gallon per minute pump mounted on the heater drain circulates water through the coil and back to the “T” just below the relief valve at the top of the tank (this valve is reserved as a safety precaution).Cold water enters the vessel through the normal inlet, and wood-heated water enters the conventional electric heater through the standard heat outlet.All wiring is well insulated with 1 inch thick high density foam.
Of course, if the water is constantly circulating, heat may be lost to the stove when there is no fire burning.To prevent this, researcher Dennis Burkholder put automatic on/off controls on a line-voltage air conditioner thermostat connected to the pump’s power cord.(You can also use the more common combination heating/air conditioning control, set to cooling mode.) The thermostat is mounted on a wall three feet away from the heater, about a foot from the top of it.When the air temperature reaches 80°F, the 120-volt controller turns on the pump and the water begins to heat up.When the temperature drops to 76°F, the built-in differential switch turns off the circulator again.
The components of the heat exchanger system are shown in the attached drawings, but of course every installation requires some changes to the basic dimensions.For example, if your furnace is larger than ours, you can expand the panel enough to get a full 60-foot coil of 1/4″ soft copper pipe within the larger exchanger frame.However, those with smaller heaters will have to use a smaller amount of wiring.
In any case, it is easiest to use the tubing as it is coiled for transport.We just put the crimped wire into the frame and gently bend the pipe to fill the rectangle.The flexible material can be bent to a radius of about 1-1/2 inches without kinking, so it’s not difficult to force it into any potential “hot spots.”We work from the outer edge inwards, attaching the coils to the backplane as we go.(Without wires to secure the outer ring of the tube, the whole thing wanted to jump out of the frame.)
After the copper pipes are evenly distributed within the frame, stir a thin layer of plaster of paris and pour the mixture into the frame.Level the surface by running a ruler over the angle iron and allow the material to dry for a few days.The panel can then be attached to the side of the furnace and the 1/4 inch line can be connected to the 1/2 inch pipe of the preheater tank.
We performed extended testing to determine the most efficient configuration of the switch and to give ourselves confidence that the equipment would operate safely.For example, to see what would happen if a power failure shuts down our pump, we sealed the pipe coming out of the preheater tank and installed a pressure gauge on the relief valve.The highest pressure we were able to develop in the system was 3 PSI…that’s after our Atlanta Stove Works Catalytic stalled the flow for 8 hours at the highest possible burn rate!
Additionally, to determine if conduction heat exchange through the furnace walls was being encouraged to unhealthy levels, we examined the interior of the wood burner’s firebox daily for increased creosote buildup.We found no differences in the appearance or depth of the deposits on any of the four walls, suggesting that the exchangers were primarily receiving radiant energy from the outer furnace walls.(The ceramic may have played some insulating role, offsetting the increased conductivity.)
How much hot water will the exchanger produce?Well, on a typical 7 hour cycle, we would load 55 to 60 pounds of wood into the Atlanta catalyst, which would raise the contents of the 42 gallon tank to nearly 140°F.This 8 pounds per hour burn rate is probably a bit higher than what most people use, so you may get slightly less hot water from a similar device.Of course, if you keep burning intensely throughout the day, the 24-hour total should still be enough hot water over 100 gallons per day.Even if you frequently operate your stove “off,” this system will significantly reduce your utility bills.
Depending on the size of your household and everyone’s water consumption, this system can eliminate your winter hot water bills.So if you can get wood for much less than the equivalent amount of electricity or gas, the energy you use to heat the water from your wood stove (minus the space, of course, the heat the appliance will provide) will be well worth it Invest.Plus, you’ll be happy to know you’ve taken another step toward replacing non-renewable energy sources.
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