Roughly a third of the no-heat calls we take in October have a cause the homeowner can safely check in five minutes. Here they are, in the order a technician would work through them.
Every October, as the first cold snap arrives, our phone starts. A good share of those calls end with a technician replacing a filter or resetting a switch that the homeowner could safely have handled — and paying for a service call to learn that is nobody’s idea of a good afternoon.
Below are nine checks, in the order a technician works through them. They are all things you can do safely without opening anything you should not. If you reach the end and the furnace still will not run, you have not wasted your time: you will be able to tell us exactly what you observed, which frequently means we arrive with the right part already on the truck.
A smell of gas anywhere in the building — leave and call the gas utility first. A carbon monoxide alarm sounding — leave and call the fire department. Visible scorching, melted wiring, or soot around the furnace cabinet. Water standing around the unit with power still on. None of these are homeowner territory.
Obvious, and genuinely the cause often enough to be worth thirty seconds.
A quick test: set the fan to ON. If the blower runs, the thermostat has power and a working connection to the furnace, and the problem is in the heating circuit rather than the controls.
A furnace has more than one thing that can cut its power.
If the furnace starts, runs for a few minutes, then shuts down and repeats, a restricted filter is the first suspect.
Here is the mechanism. The heat exchanger needs a designed volume of air across it to carry heat away. A blocked filter starves that airflow, the exchanger overheats, and a high-limit switch shuts the burner down to protect it. The blower keeps running, the unit cools, the limit resets, and the cycle repeats — short-cycling.
Pull the filter out and hold it to a light. If you cannot see light through it, replace it. It is worth keeping a spare on the shelf; in a cold snap the hardware shops run out.
Related to the filter, and often overlooked. Furnaces are balanced systems that need to get air back as well as push it out.
If other gas appliances in the house work, supply is probably fine. If the range and the water heater are also out, the problem is upstream of the furnace and belongs to the utility.
The furnace has its own manual shut-off valve on the gas line near the unit. The handle is parallel to the pipe when open, perpendicular when closed. Do not force it, and do not disassemble anything downstream of it.
If you use propane, check the tank gauge. A furnace that ran fine last week and will not light now, on a tank showing under 20 per cent, is not a furnace problem.
If your furnace has white PVC venting through a side wall rather than metal pipe into a chimney, it is a condensing unit — and it makes water. Several gallons a day.
That condensate drains through a trap and a line to a floor drain or a pump. Algae blocks the line; the trap fills; a pressure switch detects the backup and refuses to let the furnace fire. It is one of the most common no-heat causes on newer equipment, and it is entirely benign.
Clearing a condensate line properly means removing and flushing the trap, which is a technician job on most units — but knowing this is the fault when you call saves diagnostic time.
This small component causes more no-heat calls than any other single part, and the symptom is highly distinctive.
The pattern: the furnace starts normally, the igniter glows, the burners light — and then, four to seven seconds later, the flame goes out. The system tries again, usually three times, then locks out.
The flame sensor is a thin metal rod sitting in the burner flame. It confirms that a flame is actually present by conducting a tiny current through the ionised gas. Over time it develops an oxide coating, the current falls below threshold, and the control board concludes there is no flame and shuts the gas valve — exactly as it should, because the alternative is releasing unburned gas.
Cleaning it is a genuinely simple job for someone comfortable turning off power and gas, removing a single screw and gently polishing the rod. If you are not comfortable with that, it is a short service call and one of the cheaper visits we make.
If a flame sensor needs cleaning every few months, something else is wrong — poor combustion air, a misaligned burner, or a cracked heat exchanger changing the flame pattern. Repeat sensor failures deserve a combustion analysis rather than repeated cleaning.
The pattern: the inducer motor runs, you hear the sequence begin, but nothing glows and the burners never light. After a few attempts the system locks out.
Most furnaces made since the mid-1990s use a hot surface igniter — a brittle silicon carbide or silicon nitride element that glows bright orange to light the gas. They are consumable; three to seven years is typical, and they usually fail without warning.
With the blower door off and power on, you can often see the igniter through the sight glass during the start sequence. If the inducer runs, the sequence proceeds, and nothing glows, the igniter is the likely fault. They are inexpensive parts we carry on the trucks. They are also fragile — the oil from a fingerprint will shorten one’s life — so this is not a good candidate for a first DIY repair.
Most modern furnaces lock out after three failed ignition attempts and will not retry until reset. A reset is worth exactly one attempt.
To reset: switch the furnace off at the wall switch, wait 60 seconds, switch it back on and watch the start sequence.
If it starts and runs, note what you changed — a new filter, a cleared register — and keep an eye on it. If it locks out again, stop. Repeated resets on a furnace with a real fault accomplish nothing except cycling gas through an appliance that has told you it has a problem.
Most furnaces have a small LED visible through the sight glass, and it flashes a fault code — two flashes, pause, two flashes, for instance. The legend is printed on the inside of the blower door. Telling us that code on the phone is genuinely useful, because it frequently identifies the part before we leave the shop.
If you have worked through all nine and the furnace still will not produce heat, the likely candidates are a pressure switch, an inducer motor, a limit switch, a control board or a gas valve. All involve live electricity and gas, and all are properly a technician’s job.
When you call, tell us: the make and model from the data plate, what the furnace does when it tries to start, any diagnostic flash code, and which of these checks you have already done. That is usually enough for us to arrive with the part.
Call 785-776-5548 — the same number reaches the on-call technician outside office hours. Our heating service page sets out what a repair visit involves, and an autumn tune-up on the $119 Yearly Service Agreement is how most of these faults get caught in September rather than January.
Henton Plumbing & Air Conditioning, Inc. has served Manhattan, Kansas since 1942. Call 785-776-5548 during office hours, or use the appointment form — weekday enquiries before 3 pm usually get a same-day reply.
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