--- title: "How a Propane Furnace Works: Interactive Cutaway | T10" url: "https://t10.net/learn/propane-furnace/" description: "Watch a propane furnace prove its draft, light and send warm air through the ducts, read its blink code, and see what a dirty filter, a cracked ignitor or a snow-buried vent does." lastmod: "2026-10-16" --- [Learning Center](https://t10.net/learn/)Forced-air heat # How a propane furnace works A furnace heats air and a blower sends it through ducts to every room. Watch a propane furnace go through its light-off step by step, read its blink code, then break it the ways it breaks on the coldest nights. Light-off ## Every step has to prove itself before the next one. A modern furnace won't open the gas until it has proven the vent is clear, and won't keep it open unless it can prove there's a flame. That's why most no-heat calls come down to one step that couldn't prove itself, and the control board's blink code says which one. 1. 0 s ### Call for heat The thermostat sends 24 volts to the control board. 2. 5 s ### Draft proven The inducer runs and the pressure switch closes once air is moving through the vent. 3. 20 s ### Ignition The ignitor glows orange, the gas valve opens and the burners light. 4. 25 s ### Flame proven The flame sensor confirms the flame, or the gas shuts off and the board tries again. 5. 1 min ### Blower on Once the heat exchanger is warm, the blower sends warm air to the rooms. Blink codes differ by brand; the drawing uses a typical set. The code chart is usually printed inside the furnace's lower door. Heat and air conditioning ## The same ducts in summer. Many New Hampshire houses with a furnace also have central air: an indoor coil sits on top of the furnace, and the furnace's blower pushes air across it in summer. It shares the filter and ducts, and only the source of heat or cold changes. Some pair the furnace with a heat pump instead, and use propane only on the coldest days. [How the air conditioning side works](https://t10.net/learn/split-system/) Before you call ## Five safe checks. 1. 1The thermostat has power, is set to Heat, and is set above the room temperature. 2. 2The furnace's service switch, which looks like a light switch, is on, and the breaker hasn't tripped. 3. 3The filter is clean and every register is open and clear. 4. 4The two white pipes outside are clear of snow and ice. 5. 5Count the blinks on the control board's light through the little window, and write the number down for the technician. If you smell gas or a CO alarm sounds, skip the checks: get out and call from outside. What breaks ## Seven common calls, one of them an emergency. Pick one to watch the sequence stop where it stops, and see what the blink code and the chart show. - ### Dirty filter "The furnace runs and runs, the air coming out is hot but there isn't much of it, and it keeps shutting off and on." A clogged filter starves the furnace of air. The heat exchanger runs much hotter than it should, the house takes longer to warm, and on a cold day the high limit starts tripping. Over years the extra heat stresses the heat exchanger. - ### Blower not running "I can hear the furnace fire up, then it shuts off after a minute. No air comes out of the vents." The blower's capacitor or motor fails. The burners light and the heat exchanger heats with no air moving across it, so within a minute the high limit opens and shuts the gas off. It cools, lights again, and trips again, all night. - ### Cracked ignitor "The furnace starts, hums for a while, then gives up. There's a blinking light." The hot surface ignitor is a thin ceramic element that glows orange to light the gas. It's brittle, and after a few years it cracks. The gas valve opens, nothing lights, and after three tries the board locks out. - ### Dirty flame sensor "It lights for a few seconds and then goes out, over and over." The flame sensor is a metal rod in the flame that proves it's lit. A thin coating builds up on it over a season, and the board stops believing the flame. The burners light for a few seconds, shut off, and try again, until the board locks out. - ### Pressure switch open (blocked vent) "The fan in the furnace runs but it never lights. We just had a big storm." A condensing furnace vents through PVC pipes out the side of the house. When a snowbank or ice buries them, or the condensate trap clogs, the inducer can't move enough air, the pressure switch won't close, and the board refuses to light the burners. It's one of the most common no-heat calls after a New Hampshire snowstorm. - ### No call for heat "The furnace isn't doing anything at all. It's completely quiet." The furnace is fine, but it's never asked to run. Dead thermostat batteries, a thermostat set to Cool or Off, the furnace's service switch turned off, a tripped breaker, or the blower door not pushed tight against its safety switch. - ### Carbon monoxide at the registers "Our carbon monoxide alarm is going off." A heat exchanger that has cracked, often after years of overheating, can let combustion gases into the air the blower sends through the house. What people notice are the signs: the CO alarm sounds, the flame flickers or rolls when the blower starts, soot shows around the burners, and people in the house get headaches or feel sick that ease when they leave. Only a technician can confirm the cause. For heating contractors ## Where AI helps the office and the trucks. The morning after a snowstorm, every furnace customer calls at once. The ones with a buried vent or a dead battery can be fixed on the phone, the lockouts need a truck, and the CO alarms need a person right now. Sorting those quickly is where software earns its place. - ### Blink codes and snowbanks sorted on the phone After a storm, half the no-heat calls are a buried vent or a dead thermostat battery. An intake agent asks the questions on this page, walks the owner through the safe fixes, and books a truck for the rest with the blink code already on the ticket. [Custom AI agents](https://t10.net/services/custom-ai-agents-model-training/) - ### A CO call goes straight to a person Anything about a carbon monoxide alarm skips the script: the agent tells the caller to get out and call 911, then pages the on-call tech. Some calls should never wait on software, and the agent is built to know which ones. [Custom AI agents](https://t10.net/services/custom-ai-agents-model-training/) - ### Tune-ups and filters on schedule Each customer's furnace age, filter size and last tune-up, with reminders before the heating season and visit notes drafted from the tech's readings: temperature rise, gas pressure, flame signal, CO. [AI Cowork training](https://t10.net/services/ai-cowork-training/) [Book a free review](https://t10.net/#book) This page shows a typical condensing propane furnace, simplified for learning. Timings, temperatures and blink codes are typical values, not any particular model. If you smell gas or a carbon monoxide alarm sounds, leave the house and call 911 or your propane supplier from outside. Keep CO alarms on every floor, and have the furnace serviced every year by a licensed heating technician.