A gas furnace makes heat by burning fuel, but it delivers that heat by moving air. Air from your house is pulled into the furnace, warmed as it passes over the heat exchanger, and pushed back out through the ducts to every room. When furnace airflow is weak, the furnace can’t get rid of its heat fast enough. It runs hot, shuts itself off early, heats the house unevenly and wears out sooner. Most airflow problems come down to a handful of simple causes, and a couple of the fixes are things you can do yourself in five minutes.
How air moves through your furnace
It helps to picture the whole loop:
- Air leaves your rooms through the return grilles. These are usually the larger grilles on a wall or floor, often in a hallway, and they don’t blow air.
- It travels through the return ducts to the furnace and passes through the air filter.
- The blower pulls the air in and pushes it across the outside of the heat exchanger, which the burners are heating from the inside.
- The warmed air goes up into the supply ducts and comes out of the registers in each room.
So air flows toward the furnace on the return side and away from it on the supply side. That is also why the arrow printed on the edge of your filter should point toward the furnace, the direction the air is traveling.
If you have central air, the indoor AC coil usually sits right on top of the furnace, and the same blower moves the air for cooling. That means an airflow problem rarely stays a winter problem. It tends to show up again in July.
What happens when airflow is restricted
The heat exchanger is designed to be cooled by a steady stream of air moving across it. Cut that air down and its temperature climbs.
Every furnace has a safety device called a high limit switch that watches for this. When the furnace gets too hot, the limit switch shuts the burners off to protect it. The blower keeps running to cool things down, then the burners light again, overheat again and shut off again. That on and off pattern is called short cycling, and it leaves you with:
- Rooms that never quite reach the thermostat setting
- Higher gas bills, because the furnace keeps starting over instead of running steadily
- Extra wear on the igniter, the blower motor and the controls
Over the long run, overheating again and again also stresses the heat exchanger metal. Heat exchangers can crack with age and fatigue, and chronic overheating speeds that up. A cracked heat exchanger is one of the most serious problems a gas furnace can have, so good airflow is about safety as well as comfort. Keep a working carbon monoxide alarm on every level of your home, whatever shape your furnace is in.
In summer, the same restriction can let the indoor AC coil get cold enough to ice over, which is how a lot of warm-air calls start in July. Our page on an AC that isn’t cooling covers that side of it.
The usual causes of poor furnace airflow
A dirty or clogged filter
This is the most common one by a wide margin. A filter does its job by catching dust, and as it fills up, less air gets through. With pets, a busy household or a remodel going on, a filter can clog much faster than the package suggests. Check it monthly during the heating season. Thin one-inch filters usually need replacing every one to three months, and thicker media filters last longer.
A filter that’s too restrictive
Filters are rated by how fine a particle they catch, and the finer the filter, the harder the blower has to work to pull air through it. Plenty of residential systems were never designed for the densest one-inch filters sold today. If you want cleaner air without choking your furnace, there are better ways to get it, and our indoor air quality page has the options we install.
Closed or blocked registers and returns
Closing the vents in rooms you don’t use seems like it should save money, but your furnace and ductwork were sized to move air through the whole house. Close too many and the air has nowhere to go. Furniture, rugs and long curtains over registers do the same thing, and a couch pushed against a return grille can starve the system on its own.
Ductwork problems
Some airflow problems are built in. Flexible duct in an attic or crawlspace can get crushed or pull loose at a joint. Ducts in older homes were sometimes undersized from the start, and a finished basement that added rooms without adding return air can leave the whole system short of air.
A dirty blower wheel or coil
Dust that gets past the filter slowly builds up on the blades of the blower wheel and on the indoor AC coil. A caked blower wheel moves noticeably less air, even with a brand new filter. Cleaning either one means opening up the furnace, so this one is a job for us.
What you can safely check yourself
- Pull the filter and hold it up to a light. If you can barely see light through it, replace it.
- Make sure the arrow on the filter points toward the furnace.
- Walk the house and check that the supply registers are open and that nothing is sitting on them or in front of a return grille.
- Make sure the blower door on the front of the furnace is on and seated properly. Most furnaces won’t run with it loose.
- Listen for the furnace starting and stopping every few minutes while the house stays cold.
Please don’t open the burner compartment or take off any inner panels. If the filter is clean, the vents are open and the furnace still short cycles, or you notice a burning smell from the registers, turn it off at the thermostat and give us a call.
How we track down an airflow problem
On a service call we start with the simple things and work our way in: the filter and how it fits, the registers and returns, the blower and its wheel, the indoor coil and the ductwork we can reach. Sometimes the fix is a clean filter and moving a couch. Sometimes it is a blower motor on its way out, or a duct that came apart behind a wall years ago. Either way, we will show you what we found and explain the fix before we touch anything.
A yearly furnace inspection catches most of these problems before they turn into a no-heat call on the coldest night of the year. Ours is $189 per system, and you can see the full checklist on our maintenance and inspections page. If your furnace is older and airflow is only one of its problems, here is how we handle furnace repair and replacement.
Call us at 303-464-7000 or schedule a visit online. You will get Shannon or Dan, not a call center, and if the fix turns out to be a new filter, we will say so.
Furnace giving you trouble?
If the heat is out, start with our no heat checklist. For everything else, see furnace repair and installation. Furnace repairs start with an $89 diagnostic fee, waived with the repair.
