I've got a curiousity question regarding the Honeywell VisionPro thermostats. When you set them up, you have to select the type of system. And you also have to set the number of cycles per hour. They use that method instead of setting a temp differential that turns the system on and off. That way of doing it raises it's own questions, eg, what happens if it's only 55 outside and it's set to 5 cycles an hour? Does it still fire up for 5 cycles of just a minute or even less?
But here's the main question that has me stumped. They recommend setting the cycles per hour to 5 for furnaces of less than 90% efficiency and to 3 per hour for furnaces that are 90+% efficiency. That seems backwards to me. Older, 80% eff furnaces have fully open paths from combustion air in right through to the chimney. Each time it turns off, whatever heat remains in the furnace is lost mostly to the chimney and I would think there would be more of it left in the old furnace too. Think heat exchanger mass.
So, why would you want the 80% furnace to cycle 5 times an hour, but the 95% one to cycle only 3 times?