[Grumbles] [Goedjn]
So, the major question I have is: Does burning fuel in an electrical generator produce less heat than burning that same fuel in a furnace or boiler? (that is, assuming you make an effort to recover the "waste heat" from the generator?) I'm fairly sure that recovering heat from an engine costs you some of the mechanicle energy, but I'm not sure that the converse is also true. If not, then that leads to the followup question: Why have a fuel-oil furnace/boiler in your house, if you can have a water-cooled electrical generator that produces the same amount of heat, and a bit of juice, besides?

Take, for example, a notional house 28'x36'x2 floors, insulated as follows:
Ceiling: R50, 28*36=1,000 sqftΔU=20
Walls R22, 2Fl*(28+28+36+36)*8ft/fl=2048ΔU=93
doors: R2 3doors, 3x7=63 sqft sqftΔU=31
windows: R2, 24windows x 3x4 = 288 sqftΔU=144
floor: R12, 1,000 sqftΔU=83
Total U=372
Temp/Dif=20°F
BTUs/Hour=7,438
BTUs/Gal138,000
Heat recovery efficiency=85%
Temp/Dif=20°F
Required Gallons/hr0.063
Ok, now assume that you burn this fuel in a generator. Lets use this Onan 8.0HDKAQ I found described at http://www.backwoodshome.com/articles/thomsen43.html as an example. The article claims that at 75% load, it generates 6kW, while eating about a gallon of fuel every four hours. If our combination unit runs at about the same efficiency while burning .062 gallons/hour instead of .25 gph, then it should generate about 1/4 as much power or around 1.5 Kw.

As part of a total home-energy management system, I'd hook up battery banks to the thing, and also put in some electrical resistance heaters. That way, whenever you generate heat, you pump up the batteries, and if the batteries are at max charge, you can divert the extra electric to the electric heaters. (giving, apparently, about about 5000 extra BTUs..) for scaling purposes, that means we get around 138*.85=117000 BTUs in direct heat and 5000 btus in elecric heat. This implies that around 4% of the energy available in heating systems is being wasted, currently. That would still leave a need to pump the batteries when you DON'T need sufficient heat to generate power as a byprodUct. This leads me to wonder about the plausibility of building a diesel oven/generator, but I suspect that the thing would take too long to heat up.

I suspect that a diesel engine just can't be made cost-competetive with a straight burner, even if you are getting power out of it too.

[Grumbles] [Goedjn]