Update: I followed the instructions in the furnace manual for calculating the BTU/h input rating. This involves watching the gas meter and determining the amount of time that it takes to use 1 cubic foot of gas, then comparing that time to a chart in the manual. 100k BTU should take 36 seconds according to the chart. The time was taken with the water heater in pilot only.
The first reading after my little adjustment yesterday was 42 seconds. Extrapolated from the data in the chart, this works out to 87,500 BTU. So I adjusted the gas regulator on the furnace until I got exactly 36 seconds for 1 cubic foot. I can tell that it was overfired before I adjusted it yesterday because the regulator was maxed out, which gave me a time of 30 seconds today on the same setting.
So now, at least by my home DIY methods, the burner should be set to the 100k that it's rated for. I measured a consistent 68 on the return and 135 on the supply, giving me a rise of 67 degrees. That's 10 degrees lower than it was before I messed with it. I also tried setting the thermostat to the recommended warm air setting. I didn't record how many times it cycled in a period, but it was more than at the hot water setting. I changed it back to the hot water setting and it does seem to go a little longer between cycles, still not quite long enough for my liking though.
I'm wondering if I might get a few efficiency points by adding a combustion air supply pipe. The manual says that it can be done, but there is no cutout in cabinet for one. I'm thinking that I could cut a hole and run some PVC pipe straight in to the cabinet. Of course I would want to cover the existing vent slits in the cabinet and access panel so that it only draws fresh air from outside (I can already see the dirty looks from the HVAC guys out there
).
... Which raises the question: Would preheating the combustion air with heat from the exhaust vent (HVAC guys really glaring now) increase the overall efficiency of the system? My idea is to revamp one section of the galvanized exhaust vent with a double-walled heat exchanger can, similar to the heaters used on the mufflers of small aircraft (and old Volkswagens). Basically adding metal strips or spikes to the outside of the galvanized duct for heat transfer, then covering it with a larger diameter duct and sealing it with a cool fresh air inlet on one side, and a heated fresh air outlet on the other. I have worked with industrial furnaces that had combustion air recuperators on the exhaust vents and they seem to work quite well. I doubt that it would cool the exhaust enough to condense before exiting the house, and I don't really see it adding enough heat to the heat exchanger or cabinet controls to damage them. Just a thought...