What normal looks like
A properly sized furnace in moderate weather typically runs in cycles of roughly ten to fifteen minutes, a few times per hour. On the coldest Buffalo days it may run nearly continuously — that is normal and correct, not a fault. Cycles consistently under about five minutes, especially in mild weather, are the red flag.
Causes, from simplest to most structural
| Cause | What's happening | Correction |
|---|---|---|
| Dirty filter | Airflow restriction causes limit trips | Replace filter; verify size and rating |
| Blocked registers or returns | Same effect as a dirty filter | Open and clear them |
| Dirty blower wheel or coil | Hidden airflow restriction | Professional cleaning |
| Thermostat location | Sun, a register, or a drafty wall gives false readings | Relocate or recalibrate |
| Flame sensor degradation | Flame drops out and the board retries | Clean or replace; measure microamps |
| Blocked flue or intake | Pressure switch opens mid-cycle | Clear terminations, inspect venting |
| Oversized furnace | Satisfies the thermostat too quickly | Structural; addressed at replacement |
The oversizing problem in Buffalo homes
A surprising number of Western New York homes have furnaces considerably larger than the house needs. Some were sized generously decades ago; others became oversized after the owners added attic insulation, replaced windows, or air-sealed. An oversized furnace blasts the thermostat's area up to setpoint quickly, shuts off before heat has distributed to the far bedrooms, and repeats. The result is a house that is simultaneously too warm near the thermostat and cold at the edges.
You cannot fix oversizing with a part. What you can do is confirm it with a heat-loss calculation and correct it at replacement time — which is one reason properly sizing a new furnace matters more than picking a bigger one.
What short cycling costs
- Higher fuel use, because startup is the least efficient part of every cycle.
- Faster wear on ignitors, gas valves, and blower motors, all of which are cycle-rated.
- Uneven room temperatures and persistent comfort complaints.
- Additional stress on the heat exchanger when short cycling is caused by limit trips.
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