Back to the subject of derating furnaces... :D

If modulating furnaces only change the inducer blower speed and the gas manifold pressure, I don't see why I couldn't wire a dimmer switch inline with the inducer motor and adjust the manifold pressure on the regulator. The inducer motor seems to be a shaded-pole AC motor, so a dimmer switch should be sufficient to control the motor speed. I would want to use a more sturdy dimmer, not just an off-the-shelf cheapo. I have used those before for this purpose and they don't maintain motor speed very well.

The fuel-air mixture could be adjusted with reference to the exhaust temperature and temperature rise over the heat exchanger.

I'm thinking something like this for the inducer speed control...

KB Electronics Solid State Variable Speed AC Electric Motor Control 6.0 Max amps 115V # K177-1006

http://ecx.images-amazon.com/images/...500_AA300_.gif

The only thing that I am concerned about is the dial moving from the set position by means of vibration, etc., so it would have to have some resistance in the dial and would have to run the motor at the same speed every time, regardless of line voltage variations. Inducer speed is critical to proper flame adjustment.

I have also been thinking about installing these on the furnace and water heater exhaust pipes to monitor exhaust temps...

F&C 2" Barbecues BBQ SMOKER PIT GRILL THERMOMETER TEMP GAUGE

http://ecx.images-amazon.com/images/...SL._SS500_.jpg

They are actually grill thermometers, but they do have the proper temperature range for this application. I'm not to excited about the big wingnut on the back though.