Co-gen Gas Boiler Kit

I've noticed a recent rash of domestic co-generating devices that churn out both heat (hot water, I guess) and electricity. The problem with these is that they require you to throw away your old gas boiler and have everything set up especially for them.

What if you could get electricity out of your old gas boiler without buying a new one?

Gas boilers always end up blowing some heat up the flue in the exhuast gases, even modern condensing boilers will still lose some this way even though they can operate with 95% efficiency. The idea is that a thermopile system is retrofitted to the boiler flue and generates electricity from the temperature difference between the hot flue gases and either the ambient air or some other cold thing, like the cold water going into the boiler. This is intended to be 100% efficient as removing all the heat from the flue gases would, knowing my luck, destroy the convection that feeds fresh air into the boiler.

It doesn't matter that this system doesn't extract all the energy available to it; it's like getting solar PV panels put on your roof, you're capturing energy that's normally just thrown away so it's still a gain regardless of the level of efficiency. The biggest hurdle is, as with solar power, the initial equipment and installation costs. But, that aside, wouldn't it be nice to know that all the time your boiler is running, your electricity meter is ticking over that little bit slower.