The short answer

On the coldest Northern Virginia nights, a furnace that runs almost nonstop is doing its job. It’s sized for a design temperature of about 12 to 14°F, so near that it should run long. Running nonstop on a 45 degree day, or running all evening and never reaching the temperature you set, is a different story.

The part most people don’t know

A right-sized furnace isn’t meant to have lots of spare capacity on the coldest night. It’s sized to the local design temperature, the cold an area only drops under for about 1 percent of the year. Here that’s 12°F in Loudoun and 14°F in Fairfax and Prince William on RESNET’s county table, and 14°F at Dulles in NOAA’s data. Near that, long runs are by design.

On milder days it should cycle. A Building America furnace guide says a well-sized furnace can recover from a night setback within an hour even on the design day, and that a furnace cycling more than 6 to 10 times an hour may be too big. Honeywell puts a typical mid-efficiency forced-air system at about five cycles an hour. The opposite problem, quick on-off runs, is short cycling.

An opened gas furnace in a Northern Virginia home, showing the blower, inducer motor and wiring
The blower can run without the burners. Cool air from the vents and a fan that never stops is the fan, not the furnace heating.

The $67 Heating Check

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When long runs mean something’s wrong

  • Leaky ducts. ENERGY STAR says 20 to 30 percent of the air moving through a typical home’s ducts is lost to leaks, holes and bad connections. Heat lost in the attic or crawlspace is heat the furnace has to make again. More on leaky ducts.
  • A clogged filter. Less air across the heat exchanger means less heat delivered each minute, so it runs longer. A badly clogged one can trip the high limit switch, which shuts the burners off while the fan keeps going.
  • Where the thermostat sits. DOE says keep a thermostat away from sun, drafts, doorways, skylights and windows. One by a drafty door keeps calling for heat the rest of the house doesn’t need. See thermostat not working.
  • The fan set to ON. Then the blower runs all day by design, heating or not: fan ON or AUTO.
  • A blower that won’t shut off. If the fan runs nonstop on AUTO, a stuck fan relay or a tripped limit switch is a common cause, and many furnaces flash a code for it. Our error codes and warning lights guide covers the blink codes.
  • Closed vents. Shutting registers in empty rooms doesn’t help and can make the furnace work harder: does closing vents save money.
  • Age and dirt. An older furnace or a dusty blower wheel delivers less heat for the same run time. See old furnace efficiency and how long a furnace lasts.
  • One cold room. If it runs long because one room never warms up, the problem is usually airflow to that room, not the furnace: cold room in the house.
A gray, clogged return air filter a SwiftPro technician pulled from a Northern Virginia home's HVAC system
A clogged return filter from a Northern Virginia home. Less air through it means the furnace runs longer to deliver the same heat.

Does it raise the bill?

Yes, when it runs long for a bad reason. Heat leaking into the attic is gas you paid for. And the blower runs on electricity the whole time: an older standard blower motor drew about 500 watts on average in one Wisconsin field study, so long run times show up on the electric bill too.

What we check

Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade, fit a new filter, set your dampers for winter and tune the furnace. If the ducts look like the problem, our duct cleaning starts with camera photos of your ducts, free, so you can see what’s in there before you pay for anything. The cleaning is $598 instead of $798 for one system in a home up to 2,000 square feet.

Holding a quote already? Our free second opinion sends a tech to look at the system you were quoted on and tell you what it actually needs, with our price if you want it. You don’t owe us anything for looking. If it is time, the estimator shows installed prices with $1,500 off a new heating and cooling system already in them, and if you get your estimate this week, a basic single-stage gas furnace comes free with a new AC and its matching coil, installed together.

Book it

Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.

Questions people ask

Is it normal for a furnace to run constantly when it’s very cold?

Yes. Near the design temperature, about 12 to 14°F in Northern Virginia, a right-sized furnace runs long and may barely stop. On a mild day it should cycle on and off.

How often should a furnace cycle on and off?

A typical mid-efficiency furnace cycles about five times an hour on a mild day. More than 6 to 10 times an hour can mean it’s oversized or something is shutting it off early.

Why does my furnace keep running after the house reaches temperature?

Either the fan is set to ON or the blower isn’t shutting off, often a stuck relay or a tripped limit switch. If the air from the vents feels cool, it’s the fan running, not the burners.

Does a furnace running constantly raise my bills?

It can. Leaky ducts and a clogged filter make it run longer for the same heat, and the blower runs on electricity, so long run times show up on the gas and electric bills.