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Old 11-17-13, 10:51 PM   #3
MN Renovator
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The losses seem to be mostly from the time it takes for the heat exchanger to warm up and the losses from heat lost into the ductwork in places that its located where you don't want the heat to go.

For my house, I have all rigid metal ductwork and it takes 10 minutes of burning time before the delta T settles on its way out of a supply register across the house from the furnace. I don't occupy the basement in my house, it is storage space and laundry for me so I don't intentionally need it any warmer than it needs to be. If the ductwork was in an attic or in an unconditioned crawlspace, things get worse since the heat escaping that ductwork is lost to the outside. Of course if it takes 10 minutes to settle on the delta T, I should probably be a little upset that my thermostat kicks the show off at 10 mins of burner time even with the max temp span setting. With the expensive Honeywell units, I'd expect the duct losses to multiply, they have setting in those to cycle multiple times per hour. Mine never cycles twice per hour(as in every 30 minutes or less), no matter what load, even with design load yet the Honeywell's are set to something like 5 or 6 cycles for a condensing furnace. Not sure how that makes sense.

Manual J says that in my situation with supply air above 120f, winter design below 15f, with an enclosed unvented basement that duct loss is 25%.
This feels a little exaggerated to me but every time the system fires up it needs to warm up the ductwork before the full BTU output that is possible(subtract losses with what is still getting lost with hot ducts) gets to the living space, and once the furnace is off any extra heat in those ducts will go to where those ducts are. I think there is more cycling loss through the ductwork than inside a condensing furnace in most cases unless the cycle times are long and the ductwork is insulated.
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