Honeywell VisionPro Thermostat Question

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?
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Each time it turns off,

I don't have the answers to your other questions but in my older (oil fired hot air) furnace with a massive cast iron heat exchanger, when the burner shuts off, the air circulation blower remains on until the heat exchanger is cooled down.
So most of the stored heat is recovered and NOT lost to the chimney.
Mark
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Same with gas. I think heat pumps do it as well. One could argue that there is some loss in heating the inside of the duct work back up but not a lot.
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Mark wrote:

Hi, That is why blower turn off is delayed when set temperature is reached. The amount of delay is adjustable on the furnace control board. Mine is set at 90 seconds.
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It took 5 minutes for my old oil furnace fan would kick on, and vent temperature would be as high as 140 degrees, that's after I made the fan go faster. Before I moved in an uncovered the two main return vents, I think the furnace would just cycle with overtemp.
I don't have any idea on the OP's situation.
greg
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On 1/25/2011 10:59 AM, Mark wrote:

That's the way any furnace works. They never shut the blower off with the exchanger hot.
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Steve Barker
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Steve Barker wrote:

Cycle per hour setting is kinda safety feature. That's max. cycle allowed. Also Honeywell suggests how to set it depending on system you have. Your 'stat is as smart or smarter than you, so don't worry. I have two Vision Pro 6000 in use one at home and one out at my cabin. Since I installed them, only thing I had to do was replacing batteries. Ex-Honeyweller
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It's not a safety feature, it's how many times an hour it cycles and can be set from 1 to 12. What does safety have to do with whether the system comes on 3 times an hour or 6?

It's not a question of worrying. It's a curiousity question of why they would recommend cycling an 80% furnace 5 times, while a 90+% only 3 times an hour.

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I uderstand that. But at whatever temp they do shut off, eg 85, there is still some residual heat left in it that goes into the basement, garage, up the chimney, etc. On an old 80% furnace, 5 mins after it shuts off, it's not unusual for it to still feel warm to the touch. You can't cool off all the heat exchanger mass in just the extra min or so the blower runs after the burner shuts off. Hence, it would seem to me you'd want to run it at least the same number of cycles an hour that you'd run a newer furnace. In the grand scheme of things, maybe it doesn't amount to much energy, but why the difference in cycles an hour then?
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On 1/25/2011 7:28 AM, snipped-for-privacy@optonline.net wrote:

I've got a HW visionpro and it just comes on when the room gets cool and goes off when it hits the set point. I set it all up myself and didn't see any such thing about "cycles per hour"
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