What happens when the thermostat calls for heat?
Modern furnaces will not light until each safety check passes in order. Understanding the order is genuinely useful: the step where the furnace stalls tells a technician which part failed, which is why we ask what you heard before the heat stopped.
The seven-step ignition sequence
- 1. The thermostat closes a low-voltage circuit and signals the control board.
- 2. The inducer motor spins up and clears the flue of any leftover combustion gas.
- 3. The pressure switch senses that draft and proves the flue is open, not blocked.
- 4. The hot-surface igniter glows — roughly 1,800-2,500°F — for about 30 seconds.
- 5. The gas valve opens and the burners light across the burner assembly.
- 6. The flame sensor confirms an actual flame within seconds, or the valve shuts.
- 7. The blower starts after a delay, once the heat exchanger is warm.
Why do the combustion gases stay separate from my air?
The heat exchanger is the wall between the two. Combustion happens inside it and vents outdoors through the flue; your household air passes over the outside of that metal and never touches the flame. That separation is the single most safety-critical thing about a furnace.
When a heat exchanger cracks — usually from years of thermal cycling, or accelerated by a restricted filter that overheats the metal — those two air streams can mix, and carbon monoxide can enter the home. This is why a real tune-up includes a heat exchanger inspection, and why we will red-tag a furnace rather than relight it if that inspection fails. Every Iowa home with gas appliances should also have working CO alarms on each level.
What do AFUE and staging actually change?
AFUE (Annual Fuel Utilization Efficiency) is the share of fuel that becomes usable heat. An 80% AFUE furnace sends about a fifth of its fuel up the flue; a 96% condensing furnace extracts so much heat that the exhaust condenses to liquid water and vents through PVC pipe instead of metal.
Staging changes how the furnace runs rather than how efficient the burn is. A single-stage furnace is either full-blast or off. A two-stage runs at roughly 65% capacity most of the time, and a modulating furnace adjusts continuously. In a Des Moines winter the practical difference is comfort: longer, gentler cycles hold a steadier temperature and move less cold air across your feet than short full-power blasts.
Which step fails most often in Iowa homes?
In our field experience the flame sensor is the most common single failure. It is a thin metal rod that must carry a tiny electrical current through the flame; a light coating of oxidation interrupts that current, the board reads no flame, and the gas valve closes for safety. The symptom is distinctive — the furnace lights, runs a few seconds, then shuts off, often repeating several times before locking out.
The second most common is airflow starvation from a clogged filter, which trips the high-limit switch. Both are inexpensive to correct, and both get caught during annual maintenance before they leave you without heat in January.
Frequently asked questions
Why does my furnace start then shut off after a few seconds?
That pattern almost always means the flame sensor is not proving the flame. The burners light, the sensor fails to detect current through the flame, and the control board closes the gas valve as a safety measure. Cleaning or replacing the sensor is a routine, low-cost repair. If it keeps recurring after cleaning, the underlying cause is usually a combustion or grounding issue worth diagnosing properly.
How long should a furnace run per cycle?
In moderate weather, roughly 10 to 15 minutes per cycle, two or three times an hour, is normal for a single-stage furnace. Cycles shorter than about 5 minutes suggest short-cycling from an oversized system, a restricted filter, or a failing sensor. On the coldest Iowa nights a correctly sized furnace may run nearly continuously, which is expected and not a fault.
Is a 96% AFUE furnace worth it over an 80%?
It depends on how long you plan to stay and whether the venting change is straightforward. A condensing furnace needs PVC venting and a condensate drain, which adds installation cost in some homes. Over a full Iowa heating season the fuel savings are real, but we would rather show you the actual payback for your house than push a tier — that comparison is part of every quote we write.
What does a cracked heat exchanger mean for my family?
It means combustion gases, including carbon monoxide, can mix with the air circulating through your home. It is the one furnace fault we will not work around: we shut the unit down and explain the options rather than relight it. If you have gas appliances, keep working CO alarms on every level of the house regardless of your furnace's age.
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