Furnace Running Constantly? When It's Normal in a Minnesota Cold Snap and When It's Not

A furnace that runs almost nonstop on a subzero January night, while the house stays at the temperature you set, is usually doing exactly what it was designed to do. It becomes a problem when the house can't reach the thermostat setting, when it runs nonstop on a mild day, or when the fan keeps blowing even after the heat has shut off.

Those are three different situations with different causes. Here's how to tell which one you have, what you can safely check yourself, and when constant running is a sign your furnace is near the end of its life.

First, figure out which kind of "constantly" you have

• The furnace runs long cycles, but the house holds the temperature you set. That's often normal, especially in a cold snap or with a two-stage or modulating furnace.
• The furnace runs and runs, but the house never reaches the setting. That's a furnace that can't keep up, and it needs attention.
• The burners shut off, but the blower keeps pushing air through the vents and never stops. That's a fan problem, not a heating problem.

Stand by the thermostat and the furnace for one full cycle. Note the setting, the room temperature, the outdoor temperature and whether the air from the vents is warm. That five minutes tells you most of what you need to know.

When long run times are normal

A furnace is sized to keep your house warm on a typical coldest night of the year. In the Twin Cities, that night is well below zero. A properly sized furnace will run for very long stretches, sometimes nearly nonstop, when the weather gets close to that point. That's not a sign it's too small. It's a sign it was sized correctly instead of oversized. Our post on why bigger isn't better in furnace sizing explains why.

Two-stage and modulating furnaces are also designed to run longer. Instead of blasting at full power for ten minutes and shutting off, they run on a lower setting for much longer, which gives you more even heat and fewer drafts. If you replaced a single-stage furnace with one of these, the new one may seem like it never shuts off. That's the point. Our guide to single-stage, two-stage and modulating furnaces covers the difference.

Other normal reasons for a long cycle: you just turned the thermostat up after a setback overnight, it's windy as well as cold, or a door was left open.

When the furnace runs but can't keep up

If the house never reaches the thermostat setting, or the furnace keeps running long after the cold snap ends, something is limiting how much heat gets delivered. These are the usual causes, roughly from simplest to most serious.

• A clogged filter. Restricted airflow means less heat moves into the house, and the furnace may overheat and cycle off its high-limit switch between long runs. Check it first. Our post on how often to change a furnace filter covers sizes and MERV ratings.
• Closed or blocked vents and returns. Furniture, rugs or closed registers can choke airflow, especially return grilles.
• A thermostat problem. A thermostat in a sunny spot, near a drafty door or above a heat source reads the wrong temperature. Weak batteries or a smart thermostat schedule can also confuse things.
• Leaky ductwork in a cold space. Ducts running through an attic, garage or crawlspace can lose a lot of heat before it reaches the rooms. If one room or one end of the house is always cold, this is a likely cause.
• Burner or gas problems. Dirty burners, low gas pressure or a furnace that keeps stopping on a safety switch and restarting all cut the heat it actually delivers.
• A furnace that was undersized for the house, or a house that changed. A finished basement or an addition added to the same furnace can push it past what it was built to heat.
• An aging furnace. Older furnaces lose capacity over time as parts wear, and repairs start adding up.

When the fan won't shut off

If the burners turn off but the blower keeps running, check the thermostat fan setting first. If it's on ON instead of AUTO, the fan runs all the time by design. Some smart thermostats also run the fan for a set number of minutes each hour, so check the fan or circulate settings too.

If the fan is set to AUTO and still won't stop, even with the thermostat turned off, the cause is usually inside the furnace or the wiring. On newer furnaces, a stuck relay on the control board can hold the blower on. On older furnaces, the fan limit control may be worn or set to manual. A shorted thermostat wire can do it too. These are repairs for a technician. Turning the furnace off at its service switch will stop the fan until help arrives, but the furnace won't heat while it's off, so call before the house gets cold.

What you can safely check

• Set the thermostat fan to AUTO and the mode to HEAT, and replace its batteries if it uses them.
• Check the filter and replace it if it's dirty.
• Walk the house and open every supply register and clear every return grille.
• Feel the air at a vent near the furnace. It should be clearly warm a few minutes into a cycle. If it's cool, see our post on a furnace blowing cold air.
• Look at the status light through the window on the furnace door and write down any blinking code before you open anything.
• After a storm, check the white plastic vent pipes outside for snow and ice.

Don't open the burner compartment or bypass any switches. If a carbon monoxide alarm sounds, or anyone has a headache, dizziness or nausea, get everyone outside and call 911 from outside.

Your gas bill tells part of the story

A furnace that runs more in December and January will use more gas, and a cold month can mean a noticeably higher bill even when nothing is wrong. Compare your use to the same month last year rather than to October. A big jump in use during weather similar to last year, with no change in how you set the thermostat, is worth a service call. Something may be making the furnace work harder than it should.

When constant running means it's time to replace

Sometimes the honest answer is that the furnace has reached the end of the road. Signs that point that way include a furnace in the later years of its expected life, repairs that keep adding up, a house that can't stay warm on cold days even after the filter, ducts and thermostat check out, and rooms that have never heated evenly. Our post on how long a furnace lasts in Minnesota and our guide to whether to repair or replace an aging furnace can help you weigh it.

If you do replace it, the new furnace should be sized with a proper load calculation for your house as it is today, including any finished basement or addition, not just matched to the old furnace's label. That's how you get a furnace that runs long, quiet cycles on the coldest nights and still keeps every room warm.

Furnace running nonstop and the house still cold?

If you've checked the filter, vents and thermostat and the furnace still can't keep up, schedule a furnace repair or call Heating & Cooling Two at 763-428-3677. If it's time for a new furnace, furnace replacement is our specialty. Learn more about our furnace replacement service in Maple Grove and nearby cities, or request a free estimate. Office hours are Monday through Friday, 7:30 a.m. to 4:30 p.m., and after-hours emergency service is available.