I have a natural gas forced air central heating system in my house with a natural gas water heater. They're both almost 15 years old, and, rated only at 80% efficiency. The rating on my furnace shows 92,000 BTU input and 74,000 BTU output, so 18,000 BTU is going up the chimney. I'm not sure how much the water heater wastes but it's probably alot.
Anyway, both appliances are in my garage, the furnace has a 4" double wall exhaust pipe, the water heater has a 3" double wall exhaust pipe, and they both feed into a 6" pipe that goes out my roof. I'm wondering if anybody has any thoughts on recovering some of this 20-some thousand BTUs I'm pumping outside?
There are several different strategies I could use, mainly I'm considering:
1. A heat exchanger located on top of my furnace in the garage that would be used to heat the air in my garage. (small 2 car, well insulated, would love to keep it warm in the winter)
2. Capture the heat with water and send it back into the water heater.
3. Tap into the 6" exhaust pipe located upstairs in my house, and attach some sort of air heat exchanger there to heat the house.
Mainly I'm just thinking outloud here. Does anybody have any thoughts on any of those ideas? Does anyone know how hot the exhaust gasses should be leaving my furnace? I know they sell "heat recovery units" for wood stoves with a 6" pipe, I wonder if I could use one of those with my gas appiances? Maybe something as simple as coiled copper tubing inside the exhaust vents to heat water? Also, if I try to steal the exhaust heat, am I going to have to come up with some sort of induced draft fan?
Of course, CO detectors everywhere in the house before I even think about touching it...
Please, poke holes in my ideas.