Why is one room in my house always colder than the rest?

In a house with forced-air heat, every room gets its share of warm air through one supply duct and gives air back to the furnace through a return. A room that gets less air, loses its heat faster, or cannot push its air back will always run a few degrees behind. October and November are when people notice, because the furnace starts running for long stretches and the coldest room stands out against the rest.

Why is one room colder? Narrow it down

Hold your hand over the register while the furnace runs, then compare it with a warm room. What you feel, and when the room is cold, points to the cause.

How do I check the register and the dampers?

Start with the register: make sure the louvres are fully open and nothing sits on top of it or within a hand's width in front of it. Then look in the basement. In many houses each branch duct has a balancing damper close to where it leaves the main rectangular duct, and NRCan says you can usually spot it by a small lever on the outside of the duct; the lever's angle shows the position of the plate inside. If the dampers were set for air conditioning in summer, the upstairs ones may be partly closed now.

1. Find the branch that feeds the cold room. Following it from the room's register is easiest with two people, one tapping on the duct.
2. Open that damper fully, so the lever runs in line with the duct.
3. Partly close the dampers that feed the warmest rooms, a little at a time. If your ducts have no dampers, use the louvres on those rooms' registers instead.
4. Wait a few days before changing anything else, as NRCan advises, because the house takes time to settle.
5. Do not close many registers at once. Too little total airflow overheats the furnace, and NRCan suggests having the temperature rise checked by your service technician after balancing.

Does a dirty furnace filter make one room colder?

Yes. A clogged filter cuts the airflow for the whole house, and the rooms at the end of the longest duct runs lose out first, so a cold back bedroom is often the earliest sign. Pull the filter and hold it up to a light. If you cannot see through it, replace it with one of the same size, which is printed on the frame, and note the date. A filter that is far more restrictive than the furnace was designed for can do the same thing, so match the type your furnace manual recommends.

Why is the room colder when the door is closed?

Because the air pushed into the room needs a way back to the furnace. With no return grille in the room and the door shut, pressure builds and the supply air slows to a trickle. NRCan notes that many houses have poor cold-air returns and that adding returns in living areas, particularly bedrooms, can improve both comfort and how well the furnace runs. The quick test is to leave the door open overnight. If the room evens out, the fix is a return duct, a transfer grille through the wall or above the door, or a slightly larger gap under the door. Cutting into ducts and framing is a job for an HVAC technician.

Why are rooms over a garage and upstairs bedrooms always cold?

They lose more heat and get less air. NRCan lists two main causes of uneven heat: warm air leaking out of joints in the ducts, and ducts that pass through unheated spaces such as a garage, an attic or a crawl space. Rooms upstairs or far from the furnace also fight friction in long runs and sharp bends. A room over an attached garage sits on a floor that is cold underneath, and its duct often runs through that cold space. The fixes, in order: seal the duct joints with duct mastic rather than tape, which NRCan says tends to come loose over time; insulate ducts that run through cold areas; then rebalance. If the garage ceiling is poorly insulated, that is often the bigger job.

Running the furnace fan continuously also evens out temperatures between floors. NRCan notes it raises the electricity bill unless the furnace has an efficient variable-speed motor.

In Toronto and York Region homes, what is most common?

• Forced-air gas heat is the norm: Enbridge Gas supplies natural gas in Toronto, Vaughan, Richmond Hill, Markham, Newmarket and Aurora, so most cold-room questions here are about ducts and dampers, not radiators.
• Older houses: 29% of Toronto's homes were built in 1960 or earlier (Statistics Canada, 2021 Census). Older duct systems often have few or no returns upstairs, and older windows leak more air.
• Newer subdivisions: about half of Vaughan's homes and about 44% of Richmond Hill's were built in 2001 or later. In these two-storey houses the bedroom over the garage and the room at the end of the longest second-floor run are the ones people most often ask about.
• Finished basements: ducts above a new basement ceiling are hard to reach, so dampers there are often left wherever the installer set them.

When is a cold room really a furnace problem?

When it is not one room. If several rooms got cold at the same time, the furnace starts and stops every few minutes, the air coming out is cool, or a light on the furnace control board is blinking a code, the problem is the furnace, and adjusting dampers will not fix it. Gas furnaces in Ontario are serviced by technicians certified by the Technical Standards and Safety Authority (TSSA). On Doyo, Furnace Maintenance is a fixed $201.25, the full price with Doyo’s fee included, and Doyo checks the technician's TSSA certificate before they are offered heating work. Furnace Repair is priced by written quote once the technician has seen the problem.

What to tell the pro

• Which room, which floor, and whether it sits over a garage or has outside walls on two or more sides.
• How much colder it is, measured with a thermometer, and whether it is better with the door open.
• What the airflow at its register feels like compared with a warm room.
• The age of the furnace and the date the filter was last changed.
• What you already tried: dampers, registers, fan setting.
• Photos of the register, the basement ductwork near the furnace and the furnace label.