with the cost of home heating rising steadily, i have noticed many people installing woodstoves, or pellet stoves, even coal stoves, myself included. i have noticed one major inefficiency with this system, the exhaust is carrying away a huge part of the energy produced. my idea is to alter the current system of chimney pipes. anyone with a woodstove is most likely familiar with the insulated stainless steel flu systems, they are a dual layer tube with insulation between, to prevent the metal heating and igniting anything around it inside the house. why can\\'t the insulation be replaced by a layer of copper tubing creating a coil around the exhaust, which could be used to heat water either for your hot-water needs, or to be used for additional home heating. the water would be absorbing the heat, thereby acting as an insulator for safety, and carrying away the heat to actually be used. the only thing that adds any complexity to the setup is that you have to make sure that the water never boils or the system could burst, which is not as simple as a regular waterheater, because you have an inconsistant heat source, but could still be acomplished with a temperature sensor controlling the water valve entering the system, or in the case of a closed home heating system, it would control the rate at which the water pumps through.
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It is my understanding if you cool the flu too much the efficiency suffers. The creosote will build up in a cooler pipe faster. If the creosote ignites you could have bigger problems.
I think recovering as much heat as possible of the heat from the flu is a great idea, but I'm afraid that circulating water is too much trouble.
I don't think the water-heater runs that often during the day--so when there's no demand for hot water and you have all the hot-you need, would you continue to circulate cold and let it out on the ground? Like you've said, if you leave it in the pipe, you could end up with steam and dangerous pressure.
I personally think pellet stoves are a great idea, but I would like to see an easy way to convert normal biomass from my yard--grass, branches, etc--into pellets.
High efficiency gas furnaces are so efficient now that the flus are made of plastic. How do we do that for pellet stoves?
As for Hyunef's comment about creosote, I'm still waiting for a flu that will allow you to just let the creosote burn without any hazard. It's a pain to clean the flu.
Why not just have a reserve tank placed where it will be heated by the stove and distibuted like a regular water heater. but could only be used as supplemental during winter months. All this could be in a basement with ducts.