Quote:
Originally Posted by Servicetech
Back to the original topic, it does appear that duct losses are significant in the cycling loss calculations. Not sure hot much the 45 second burn time to heat up the exchanger costs in efficiency, and how much is recovered during the 180 second blower off delay after the burner shuts off. 90%+, 80% and old school pilot light furnaces have different cycling losses.
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My furnace runs a blower off delay of 75 seconds and in that 1 minute 15 seconds at the recommended blower speed, it is still putting out 125 degrees of heat through the ductwork when it powers off and my thermometer will actually slowly climb after the airflow stops as the heat in the metal is hotter than the air passing through it and some of that hot slowly rises out of the ductwork but I'd imagine most of it ends up conducting off the surfaces of the ductwork into the basement where I do not need it. I ran the blower for 2 minutes and that seems appropriate but my furnace(and many others) are not adjustable in their blower off delay.
My original thermostat, which was installed from when the house was built in 1985 until 2010 when I swapped it out, used to run the furnace with a 5 minute gas burn time. The new thermostat that I installed runs the gas for about 10 minutes. I can only imagine that cutting the cycles in half has helped a ton but I have a feeling that I still have room for improvement if I could allow for 1 or 2 more degrees in temperature spread before it turns on. I think the key may be to get a thermostat that allows you to adjust the spread or anticipation settings to the maximum comfort/efficiency level compromise for the people living in the house. ..along with making sure that the minimum size furnace is installed in the space.