I was walking home and i walked past a house with one of those vents which excess some sort of gas (never really queried what it was to be honest) when i thought couldn't you have some sort of generator and wheel attached to it so the gas turns the wheel and generates power. if you attached a tank and a nozzle to the end it could store it up and blast it out of the nozzle turning the wheel even more.
The amount of energy there is very low and generating power from it is very inefficient and seldom practical, even many small companies find it impractical to fully utilize all the resources their processes produce. It's a little easy with much larger scales like giant corporations or plants where the cost/benefit and system cost/complexity tends to balance out for recouping some of the otherwise lost energy.
Uhh. What houses are seen to burn off excess methane gas? I thought that was only done on oil rigs and then only because the cost of transportation was high. Might be used on landfills as well, but a common house?!
Ducting the exaust gas through a heat exchanger might be useful to pre-heat the furnace air, but heat exchangers tend to be rather large and the increase in back pressure may be too high. What gets the exhaust gasses out of the house is their increased temperature, and I doubt it would increase efficiency by a particularly large amount.