First, the safety questions
- Does anyone smell gas? If yes: everyone out, do not flip switches, and call 911 or the gas utility from outside.
- Has a carbon monoxide alarm gone off, or does anyone have a headache, dizziness or nausea that gets better outside? If yes: everyone out, and call 911.
- Is there a burning smell, smoke, or scorch marks near the furnace? If yes: turn the furnace off at the thermostat and the furnace switch, and keep everyone away from it.
- Is anyone in the home very young, elderly or unwell, and is the house getting cold? That call moves up the schedule.
Narrowing it down: the quick checks
- How long does it run before it shuts off: a few seconds, a few minutes, or does it reach temperature and then start right back up?
- Does the burner light and then go out, or does only the fan come on and off? The tech needs to know which.
- When was the filter last changed? If it is gray and clogged, a new one is safe to put in, and a clogged filter is one of the simplest causes.
- Where is the thermostat? Next to a heat vent, a lamp, a sunny window or an outside door, it can switch the furnace off too early.
- Are the thermostat batteries fresh, if it uses them?
- Are the supply vents in the rooms open and not covered by furniture or rugs?
- Outside, are the white plastic vent pipes clear of snow, ice, leaves or a nest? If something is blocking the end and it is safe to reach from the ground, they can clear it. Do not touch anything that is hot.
- Is there a blinking light visible through the small window on the furnace door? A count of the blinks helps the tech. Do not open the panel to see it.
What the tech needs to know
- Gas furnace, oil, electric, or a heat pump, and the brand if they can read it.
- The age of the system, or a photo of the label so the tech can read the model and serial number.
- When it started: after a filter change, after a power outage, after work on the house, or out of nowhere.
- The pattern: how long it runs, how long it stays off, and whether the house ever gets warm.
- Any blink code they could see, and any error showing on the thermostat.
- Where the furnace is: basement, attic, closet, crawlspace, garage, and whether it is easy to reach.
How it sounds on the phone
Before anything else, do you smell gas anywhere, and has a carbon monoxide alarm gone off?
What is the address?
When it shuts off, does the flame go out, or does just the fan stop? And how long does it run first?
It is probably just the thermostat.
It runs about a minute and the flame goes out, the filter is new and the vent outside is clear, so I am booking the first open slot and telling the tech it sounds like the burner is shutting down on its own.
Try turning it off and on a few more times and call back if it keeps happening.
The three ways this goes wrong
- Treating it as a comfort problem and skipping gas and carbon monoxide. A furnace that keeps shutting itself off can be a safety switch doing its job, so those questions come first, every time.
- Telling the customer to keep resetting it. Restarting a furnace over and over can hide the problem or make it worse, and resetting safety switches is the tech's job.
- Not asking how long it runs and whether the flame goes out, so the tech arrives without the right part and the customer pays for a second visit.
A furnace that keeps turning on and off is gas and carbon monoxide first, then the filter, the thermostat and the vents, then the pattern and the details. Ask in that order, and the family stays safe and the tech arrives ready to fix it on the first visit.
I answer the phones for HVAC contractors, and on every furnace call I ask about gas and carbon monoxide before I book the visit. callbobai.com