Our funance (a 100,000 btu Lennox conservator III installed in 1987) has been cycling on and off in short cycles, apparently for a very long time though we just noticed the problem recently. Our house is a 1956 single story ranch in northern CA (1700 SF).
The service rep who came out concluded that the return air duct is undersized causing the furnace to overheat, hence the short cycling. He said the ingnitor and control panel are OK. He demonstrated the problem to me by removing the panels from the front of the blower & burner compartments (giving the furnace unlimited intake air) and the short cycling stopped.
The existing return duct is 10" and runs through the attic along a central hallway, then steps up to 14" in the garage where the furnace is. The service rep said the return duct should at least be 14" and more likely 20".
I guess my questions are:
1. Does this diagnosis sound correct?
2. Given the age of the furnace and the return duct problem he suggested we consider a new furnance rather than just getting the duct work re-done. He says the existing 100,000 btu unit is oversized for our 1700 SF house and that all the short cycling over a long period has likely taken a toll on the furnace. Does this sound reasonable or should we just get the duct work done?
Thanks very much in advance,
Multiple Furnaces Sharing Return
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Cold Air returns in the up stairs of my 1910 2 story...
HVAC duct work
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