A typical heating cycle lasts about 15 to 20 minutes with the blower normally running a little longer to move all of the warm air into the home. If your furnace starts then stops, there are several potential causes. Some of the most common are a problem with the high limit switch or your furnace is short cycling, explained below. As we walk through gas furnace troubleshooting, a few other potential causes when the furnace blower starts then stops will be discussed.
Why Does My Furnace Start Then Stop?
Following are some of the most common issues that cause your furnace blower to start and then stop. Some are the result of bad parts that need replacement. Others are related to factors outside the furnace itself.
We offer solutions too along with links to other Pick HVAC FAQ and Troubleshooting articles that give more detail on the topics.
Faulty High Limit Switch
Also called a fan limit switch or limit switch, the high limit switch is a control system that allows the blower to turn on, but shuts down the gas valve and burner if the furnace’s internal temperature gets higher than it should be – enough to damage the control board or wiring, crack the heat exchanger, etc.
A limit switch should be inspected by an HVAC professional to determine if the switch itself is malfunctioning, the most common reason, or if there are other issues causing the limit switch to turn off the furnace and blowers.
Replacing a high limit switch is a fairly easy DIY job, and the part costs less than $40 for most furnaces – and as little as $15. These details are discussed in our FAQ guide Furnace High Limit Switch Keeps Tripping.
Problems with the Flame Sensor
The flame sensor rod is part of the high limit switch and the burner assembly. The flame sensor rod makes sure there is a flame burning in the combustion chamber while the gas is on in the furnace. If the flame sensor doesn’t recognize a flame, it will shut down the furnace.
Usually, the issue can be corrected by cleaning the flame sensor rod. Remove the sensor rod from the furnace and clean it by rubbing it with a mildly abrasive pad or fine grade sandpaper or Emery cloth. Here is more on a Dirty Flame Sensor and How to Clean It.
A Bad Flame Rollout Switch
When the flame is rolling back toward the gas line – out of the burn chamber – the flame rollout switch senses it and shuts down the furnace.
First, check the switch for continuity using a multimeter. If it doesn’t have continuity – that is, if the electrical circuit isn’t completed as it goes through it – then the switch is bad and should be replaced.
If the switch is good, then it’s possible that something is blocking the exhaust vent. When there isn’t a good flow of air into the burn chamber and out the flue vent, then the flame won’t be drawn into the burn box, but might rollout into the furnace cabinet, and the switch will be tripped. Make sure the vent is open, not plugged by debris.
Can you bypass a flame rollout switch? Yes, but only for test purposes - to troubleshoot the issue.
A Problem with the Hot Surface Ignitor
These can also be spelled igniter. When the thermostat calls for heat, the hot surface ignitor heats up to 2500°F and lights the gas. If your blower suddenly stops, it could mean there’s a problem with the ignitor not working properly.
Ignitors last about 7 years and as they age, they can warp or crack and become unable to heat up properly or remain hot enough to light the gas.
You can check your ignitor with a multimeter or by removing the furnace panel and watching as your furnace turns on. In a gas furnace the ignitor will heat up and begin to glow just as the gas turns on creating the flame. If the ignitor is faulty, it will not heat up and glow, or if it does, it might not remain hot enough to maintain flames and your furnace will shut down.
An ignitor, or HSI (hot surface ignitor) can be cleaned. Gently remove it by unscrewing the screw holding it. Don’t touch the ignitor stick, and gently clean it with Emery cloth.
There are more details about this issue in our FAQ guide How to Tell if Your Furnace Ignitor is Bad & Where to Buy One.
Bad Control Board
If the control board isn’t opening the circuit to send power to the igniter, the blower might start but will quickly stop when gas doesn’t release and ignite.
This issue is tricky to diagnose. If the other parts seem to be working, then the board could be the issue. We don’t recommend DIY board replacement unless you’re sure the board is the problem. The cost of a board ranges from $250 to more than $500, and once a board is installed, it cannot be returned.
Issues with Your Thermostat
If your thermostat is in direct sunlight or near a heat generating appliance or light source, like a lamp, it will reach the desired temperature much quicker than the rest of your home causing the furnace to shut off before the house is fully warmed. In this case you will need to move the thermostat to a location away from appliances, sun, and other light sources.
Occasionally the wiring to the thermostat can cause a short and quickly shut off the furnace. You can test your thermostat to see if this is the case. Your thermostat might be old and need replacing or the batteries may simply need changing.
Of course, your thermostat could be old and need replacement.
Furnace is Overheating
This is a slightly different issue than the high limit switch, which will also trigger the furnace to shut down. Your furnace is engineered to turn off when it becomes too hot therefore preventing the kind of damage just described. Overheating is often caused by restricted air flow. Have you changed your furnace filter lately? That’s the most common cause of furnace overheating.
Overheating of the furnace can also be caused by very dirty or clogged internal parts including the blower motor.
The motor or motor bearings wearing out and creating excessive heat might cause overheating too. Is your blower motor making strange noises?
A broken gas valve, which is a rarer cause of the blower starting then stopping can allow too much gas into your furnace, increasing the size of the flames, and causing your heat exchanger to overheat.
Restricted Air Flow
Let’s dig a little deeper into this reason for a blower starting then stopping.
Air flow problems can cause your furnace blower to turn on and then stop. If your furnace doesn’t get enough air flowing through it, the fuel can’t burn, and the furnace will turn off. Restricted air flow into the burn chamber and out of your furnace can be caused by a number of things. The most common cause of restricted air flow is a dirty air filter. A standard furnace filter should be changed every 3 months, but if you have an active household, especially with a pet, your filter may need changing every month.
If your furnace is in a small space like a utility closet or surrounded with items in a storage area, air flow may also be restricted. The general recommendation is to allow 30 inches of clear space on all sides of the furnace.
Objects or debris in the exhaust vent will prevent the combustion gases from exiting the furnace causing the furnace to quickly overheat and shut off. Check the exhaust vent for animal nests, leaves, ice, and snow. A blocked fresh air intake vent will cause overheating and the high limit switch will shut down the furnace.
Closed or blocked room registers or vents can also restrict air flow and cause your furnace to overheat. Make sure the room registers are free and the dampers are open. At least 75% of the dampers on the room registers should be open during the heating season.
Also check all ductwork for blockages and collapsed areas to assure air can flow through them properly.
An Oversized Furnace
This problem will show up immediately after installation – or during the first few weeks of cold weather after the furnace was installed. If that’s not the case with your furnace, this isn’t the issue.
Your furnace can actually be too large for your home. An over-sized furnace will quickly heat areas closest to the thermostat then turn off. Rooms farther away from the thermostat or furnace will not receive enough warm air. The furnace will also go through very quick heating cycles or short cycles. Short cycling creates wear and tear on your furnace, shortens its life span, and causes it to consume more energy raising your heating costs.
If the thermostat setting is being reached very quickly, but the house cools quickly too and the furnace restarts, this could be the issue. Consult a qualified furnace contractor about doing a load calculation designed to determine how much heat your home needs – and whether the furnace is delivering too much of it.
If it turns out it is time to replace your furnace, start your research with our comprehensive Gas Furnace Guide.