Furnace Problems and Repair Guides

How a gas furnace makes heat

A furnace warms air and sends it through ducts to your rooms. Most furnaces burn natural gas, and some run on oil, propane or electricity.⁠[1] Most furnaces that earn the ENERGY STAR are condensing models. They pull so much heat from the exhaust that water forms as a byproduct, which happens above 90 percent efficiency.⁠[1]

A gas furnace does not light the moment the thermostat calls. It runs a short safety sequence, and each step must pass before the next one starts. In one Bosch manual, the inducer fan starts first and the control waits for a pressure switch to prove the draft. A 15-second pre-purge follows. Then the igniter heats up, the gas valve opens and a flame must be sensed within 4 seconds.⁠[2] After a 45-second delay, the main blower starts and pushes house air across the heat exchanger.⁠[2]

If the control senses no flame, it shuts the gas, waits and tries again. That Bosch control makes two retries and then locks out.⁠[2] Timings differ by brand and model, so read these figures as an example.

This sequence explains most no-heat problems. A furnace that hums, clicks or runs its small fan and then stops can be a safety check doing its job. The step where the sequence stalls tells you which part to look at next. The diagram on this page shows where the filter, blower, burners and vent sit inside the cabinet.

Safety basics for furnace owners

Put a CO alarm in every sleeping room, outside each sleeping area and on every level, including the basement. Test each alarm at least once a month.⁠[3] Keep anything that can burn at least 3 feet away from the furnace, and keep the area around it clean.⁠[3]

Owners can change the filter, check settings and switches, and look at the burner flames as the manual describes. The Bosch manual says only a qualified service technician should clean the flame sensor or the burners or inspect the heat exchanger.⁠[2] If a limit switch opens, it says to call a technician to fix the cause.⁠[2] This site does not publish gas line, gas valve or venting repairs. Those jobs go to a licensed pro.

Common furnace problems and the first check

Each first check below needs no tools. Do it before you call anyone, and write down what you saw. The guide in the last column goes deeper.

A healthy gas flame is stable, quiet and soft blue with slightly orange tips. Yellow flames, flames that curl or lift off the burner, or heavy soot need a qualified service agent at once.⁠[2] If the furnace breaker trips again after a reset, leave it off and have a licensed electrician find out why.⁠[3]

Furnace maintenance that prevents no-heat calls

The filter is the one job every owner should keep. ENERGY STAR says to check it every month, especially in winter and summer. Change it when it is dirty, or at least every three months.⁠[1] The Bosch manual calls poor filter care the most common cause of weak heating or cooling. It says to replace a disposable filter with one of the same type and size, with the airflow arrow pointing toward the blower.⁠[2]

Book a yearly inspection. NFPA says a licensed professional should inspect and service a furnace at least once a year.⁠[3] ENERGY STAR suggests the heating check in the fall, before contractors get busy.⁠[1] On that visit the technician checks the gas connections, gas pressure, burner combustion and heat exchanger.⁠[5] On a condensing furnace the condensate trap and drain get checked and cleaned too.⁠[2] A plugged condensate drain can cause water damage in the house.⁠[5]

Keep the outside vent clear. NFPA says to keep furnace vents free of snow and debris.⁠[3] Check the vent after every heavy snowfall, and keep firewood, leaves and stored items away from it.

Plan for age before the furnace forces the decision. A 2007 NAHB study put the life of a gas warm-air furnace at about 18 years.⁠[6] ENERGY STAR suggests weighing a replacement once a furnace is more than 15 years old, needs frequent repairs or comes with rising energy bills.⁠[7] Before you approve a large repair on an older unit, run the numbers in the repair-or-replace calculator. Heat pumps and boilers follow different rules; all heating repair guides cover those systems too.