I doubt a natural gas furnace even has a PCIx16 slot much less match even a budget Nvidia's hashing specs.
^See what I did there? Another joke.
Secondly, another difference favoring Bitcoin mining on AMD GPUs instead of Nvidia's is that the mining algorithm is based on SHA-256, which makes heavy use of the 32-bit integer right rotate operation. This operation can be implemented as a single hardware instruction on AMD GPUs (BIT_ALIGN_INT), but requires three separate hardware instructions to be emulated on Nvidia GPUs (2 shifts + 1 add). This alone gives AMD another 1.7x performance advantage (~1900 instructions instead of ~3250 to execute the SHA-256 compression function).
For GPU programming I've enjoyed Nvidia's CUDA package greatly over wrangling OpenCL that Radeon relies on.
Kill-A-Watt says I've used approx $68.23 of electricity at 11.5 cents per kWh. Bitcoins currently trade at 1 to $30 and I've got 2.2 bitcoins. The Radeon 6770 was (and still is) ~$110.
Additional factors to consider:
-The bitcoin machine is also my daily workstation so if it were running headless and otherwise unused it would have probably done better in the electricity used category.
-It makes a damn fine space heater and I've enjoyed it immensely this winter.
-My focus in this project was to learn hands-on about scientific computing applications and it's been great for that.
In conclusion: as a business it would have been a flop, partially because I haven't optimized the setup for that application. As a learning opportunity and 200 watt heater it's been phenomenal.