Why is my older furnace running long cycles?
HVAC inspectors often recommend replacing an older, otherwise safe furnace simply because it runs long heating cycles. The actual cause can be a basic mismatch between the replacement thermostat's factory settings and the furnace's gas block amperage. Because technicians misdiagnose or ignore this simple electrical setting, homeowners are left to troubleshoot the issue themselves to avoid buying an entirely new system.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Replacing the entire furnace unit as recommended by inspectors
- 2Trial and error cleaning, filter replacement, and searching for obscure control setting mismatches
- 3Asking HVAC professionals online to interpret the thermostat data
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“When I bought this home last year, I had a couple HVAC inspectors insist I replace it... No cracks in exchanger, zero C02 readings. Their main complaint was long cycling.”source ↗
“Dug back into it today and found the issue nobody else could (or wanted to). I replaced the thermostat with a NOS one last year and it was set exactly like the old nasty one I removed. That was the problem, the gas block is 2 amp and these old thermostats were factory set at .45 thus the long cycle confusion.”source ↗
Where this came up
People with this problem also raised
- 4Why is my furnace blowing cold air with a red blinking light?
- 2Why does my tankless water heater keep short cycling?
- 13Furnace lights up then immediately goes out
- 3Why is water pooling inside my furnace?
- 2Why does my AC keep tripping the breaker?
- 2Why does my HVAC fuse blow immediately when I turn the power on?