How to change a furnace filter

How to tell this is your problem

• Weak airflow from the vents
• The system runs noticeably longer to reach temperature
• Dust settling quickly after cleaning
• Higher gas or hydro bills with no change in habits
• The filter is visibly grey rather than white

Should you do this yourself?

Yes, entirely. Slide the old one out, slide the new one in the same way round. The only genuine requirements are buying the right size and noting the airflow arrow.

What you need

Step by step

1. 1

   Turn the system off

   Set the thermostat to off, or use the switch on the furnace itself. Changing a filter while the blower is running pulls dust straight into the system.

2. 2

   Find the filter and note its size

   Usually in a slot where the large return duct meets the furnace, sometimes behind a return air grille in a wall or ceiling. The dimensions are printed on the cardboard edge.

3. 3

   Note the airflow arrow before removing it

   Every filter has an arrow on the frame that must point toward the furnace, in the direction air travels. Fitted backwards it restricts airflow and collapses under suction.

4. 4

   Slide the new one in and date it

   Arrow the same way as the old one. Write today’s date on the frame with the marker. Close the door fully - a filter door left ajar lets unfiltered air bypass the filter completely.

5. 5

   Set a sensible replacement interval

   Every three months for a basic 1-inch filter, more often with pets or during renovation dust. Thicker 4 or 5-inch media filters last six months to a year. Checking monthly costs nothing and tells you what your house actually needs.

6. 6

   While you are there: the thermostat

   A programmable or smart thermostat is a genuine saving and a straightforward swap on a standard low-voltage system - photograph the existing wiring before disconnecting anything. If your current thermostat has thick wires or a small sealed box, it may be line voltage, which is electrical work and not DIY.

Mistakes that cost money

• Fitting the filter backwards. The arrow points toward the furnace, always.
• Buying the highest MERV rating available. A very dense filter can restrict airflow enough to strain the blower on older systems - mid-range is usually the right answer.
• Leaving the filter door open or missing, which lets air bypass the filter entirely and defeats the point.
• Treating a dirty filter as the explanation for every symptom. If airflow is still poor with a clean filter, something else is wrong.

When to call a professional

Call an HVAC contractor for anything beyond the filter compartment. Yellow burner flames, short-cycling, unusual noises, a system that will not maintain temperature, or any smell of gas all need a TSSA-certified technician - and a gas smell means leaving the house and calling the utility’s emergency line first.