Why is my furnace not heating my house?

The most common reasons a furnace runs but does not heat are a thermostat set wrong, a clogged air filter, a closed gas valve or power switch, a failed ignitor or flame sensor, or a safety switch shutting the system down. A few of these you can check yourself in ten minutes. If none of them solves it, the problem is inside the furnace and a technician should look at it.

When a furnace fails in a Tulsa cold snap, the house cools fast, so it helps to work through the likely causes in order. Start with the free, safe checks and stop if you smell gas.

First, what should you do if you smell gas?

If you smell gas, leave the house, do not flip switches or use a phone inside, and call your gas utility or 911 from outside. Do not try to troubleshoot. A furnace problem is an inconvenience. A gas leak is an emergency.

Also, if a carbon monoxide alarm sounds, get everyone outside and call for help before doing anything else.

Is the thermostat the problem?

It sounds too simple, but it is the first thing to check, because it is the most common cause.

• Mode and setpoint. Confirm the thermostat is set to heat, not cool or off, and that the target temperature is several degrees above the current room temperature.
• Fan setting. If the fan is set to on instead of auto, the blower can push unheated air through the vents between heating cycles, which feels like the furnace is not heating.
• Batteries. Many thermostats run on batteries. A blank or dim display usually means they need replacing.
• Schedule or hold. A programmed setback can hold the temperature low when you expect it to be warm.

Is the air filter clogged?

A dirty filter restricts airflow. With too little air moving across the heat exchanger, the furnace overheats and a high-limit safety switch shuts the burners off before the house warms up. The blower may keep running, so you feel little or no warmth. Check the filter, and if it is gray, matted or you cannot see light through it, replace it. In a house with pets or dust, a filter can clog faster than the package says.

Is the furnace getting power and fuel?

Next, confirm the basics are on.

• The switch. Most furnaces have a switch that looks like a light switch on or near the unit. It can get flipped off by accident, especially after someone changes a filter or works nearby.
• The breaker. Check the panel for a tripped breaker for the furnace. If it trips again right away, stop and call a professional.
• The gas supply. A gas furnace needs its supply valve open. If you use propane, check the tank level.
• The front panel. Many furnaces will not run unless the blower door is seated properly, because a door switch confirms it is closed.

Why would the furnace start and then quit?

If you hear the furnace try to start, then stop, these are the usual culprits.

• A dirty flame sensor. The sensor confirms that a flame is present. If it is coated with residue, the furnace shuts the gas off after a few seconds as a safety measure. Cleaning it is often a quick job for a technician.
• A failed ignitor. Modern furnaces use a hot surface ignitor or an electronic spark. When it cracks or weakens, the burners do not light.
• A tripped pressure switch or blocked vent. Furnaces check that exhaust is moving out. A blocked flue, a failed inducer motor or a plugged condensate line on a high-efficiency model can stop the cycle.
• A limit switch trip. Overheating from poor airflow shuts the burners down, which loops back to the filter and blocked vents.

High-efficiency furnaces also make condensate, and if the drain backs up, a safety switch can shut the furnace down. If you see water around the unit, that is worth mentioning when you call.

What if the furnace runs but the air is only lukewarm?

A furnace that blows but does not warm up enough could have a limit problem, a failing heat exchanger section, a gas pressure issue or a duct problem. It could also simply be undersized for the house or losing heat through leaky ducts. These are not do-it-yourself fixes. A heat exchanger crack in particular is a safety matter, because it can allow combustion gases into the airstream.

If rooms heat unevenly, closed or blocked supply vents and return grilles are worth checking. Furniture over a return, or a closed damper, can starve a part of the house.

When should you stop and call a technician?

Call for service if:

• You smell gas or a burning odor that does not fade after the first heating cycle of the season.
• The furnace short cycles, meaning it starts and stops repeatedly.
• You hear banging, grinding or squealing.
• A breaker trips again after you reset it once.
• The filter is clean, the thermostat is correct and the system still does not heat.
• The furnace is old and has been failing more often. In general, past about 10 to 15 years, or after repeated repairs, replacement is worth discussing.

Triad Heating and Air handles furnace repair for homes and businesses in Tulsa and nearby towns, and says it will tell you honestly when a repair makes more sense than replacement. You can see the towns covered on the service areas page.

How can you prevent a no-heat call next winter?

Most no-heat problems build over time, and a few habits help.

• Change the filter on schedule. Check it monthly during heavy use.
• Keep vents and returns clear.
• Get the furnace inspected in fall. A general recommendation is to service heating equipment each fall and cooling equipment each spring. A technician can clean burners and sensors, check the ignitor, test safety controls and inspect the heat exchanger before you depend on the system.
• Test it early. Turn the heat on during the first cool evening instead of the first freezing night.

Call Triad Heating and Air at (918) 992-2105 if your furnace is not keeping up. Describe what you see and hear, since the symptoms often point toward the cause.