Fall Indoor Air Quality: What Changes When You Close Up

Closing up the house for fall changes indoor air quality in three ways at once: fresh-air exchange drops sharply, humidity swings from too high to too low, and combustion appliances start running daily. Together those turn a house that felt fine in September into one with dry air, lingering odours and rising dust by November.

None of it is dramatic on any single day, which is exactly why it goes unnoticed until someone’s throat is dry every morning. Here is what actually changes, what matters for safety, and the short list of things worth doing before the windows stay shut for six months.

What Actually Changes When You Close the House for Fall?

Air exchange collapses. Through summer, open windows and doors replace indoor air many times a day. Once the house is sealed and the furnace takes over, that exchange drops to whatever leaks through the building envelope — and in a reasonably tight modern home, that is very little.

Everything generated indoors then accumulates instead of dispersing: cooking moisture and odours, cleaning product vapours, pet dander, dust from carpets and bedding, and moisture from showers and laundry. The house has not become dirtier. It has stopped flushing itself.

Why Indoor Air Quality Often Gets Worse in Heating Season

Because people spend more time indoors precisely when ventilation is lowest. The US Environmental Protection Agency’s introduction to indoor air quality notes that indoor pollutant levels can be higher than outdoor levels, and that people spend the large majority of their time indoors — a proportion that rises further in a Canadian winter.

The heating system also becomes the main air-moving device in the house. Every cycle pulls air through returns, past the filter, across the heat exchanger and back out through the supply registers. Whatever is in the ductwork and on the filter is now circulating on a schedule.

The Humidity Swing: From Too Much to Too Little

Summer indoor humidity in Hamilton frequently sits above 55%, which is the muggy end. Once heating starts, the same house often falls below 30% — cold outdoor air holds very little moisture, and heating it drops the relative humidity further. The comfortable band for a heating season is roughly 30–45%.

The upper limit matters more in winter than people expect. Push humidity too high while it is genuinely cold outside and moisture condenses on the coldest surfaces — window glass first, then frames and exterior wall corners. Humidity that is comfortable at 0°C outdoors can cause visible condensation at −15°C, which is why a whole-home humidifier needs adjusting through the season rather than setting once. Our furnace humidifier guide covers settings and maintenance.

What Running the Furnace Adds to the Air

A well-maintained, correctly venting furnace adds nothing to your indoor air — combustion gases leave through the flue. What running it does is redistribute what is already in the house and in the ducts, and burn off dust that settled on the heat exchanger over summer.

That first-run burning-dust smell is normal and should clear within an hour or so. A smell that persists for days, or any electrical, plastic or gas odour, is not normal and means shutting the system down. Those distinctions are covered in detail in our guide to first-startup smells and sounds.

Combustion Safety: The Part That Is Not Optional

Sealing a house tighter while running fuel-burning appliances daily is the one fall change with a genuine safety dimension. Carbon monoxide is colourless and odourless, and the conditions that produce it — a cracked heat exchanger, a blocked flue, poor combustion air — give no warning you can detect unaided.

Two things address this properly. Working CO alarms, tested with the button, with fresh batteries and checked against the expiry date printed on the back — CO sensors retire after about 7–10 years regardless of battery state. And an annual professional inspection that includes combustion testing and a heat exchanger check, which is what the furnace tune-up checklist covers. Natural Resources Canada’s home heating and cooling maintenance guide puts gas connections, burner combustion and the heat exchanger among the core items of a fall check-up for exactly this reason.

Filters: What the MERV Rating Actually Does

A furnace filter’s MERV rating describes how small a particle it captures. Higher is not automatically better — a filter too restrictive for your system starves airflow, which makes the furnace work harder and can trip high-limit safeties. The right filter is the highest rating your system is designed to handle.

Two practical points beat any rating debate: change the filter on schedule — every one to three months through heating season — and make sure it is seated properly with no air bypassing around the edges. A premium filter fitted loosely performs worse than a basic one fitted correctly.

Ventilation in a Sealed-Up House

Tighter houses need mechanical ventilation, because they no longer leak enough to exchange air on their own. A heat recovery ventilator brings in fresh outdoor air and exhausts stale indoor air while transferring heat between the two streams, so ventilation does not simply throw away the heat you paid for.

If your home has one, fall is the moment to check it: clean or replace the filters, confirm the core is clean, verify the exterior intake and exhaust hoods are clear, and make sure it is actually set to run rather than switched off since spring. If your home does not have one, running bathroom and kitchen exhaust fans properly — during and for a while after showering and cooking — handles the largest single moisture sources.

What Helps, and What Is Mostly Marketing

Ranked by real-world impact: source control first (fix the moisture, reduce what you introduce), then ventilation, then filtration, then air cleaners. Most homeowners buy in reverse order, starting with a device and ending with the leak that caused the problem.

• Works: exhaust fans used properly, correct filter changed on schedule, humidity kept in the 30–45% band, an annual furnace inspection, fixing basement moisture at the source.
• Works in the right situation: HRV or ERV in a tight house, whole-home humidifier in a dry one, portable HEPA units in a specific problem room.
• Overrated: ozone-generating “air purifiers”, scented plug-ins that mask rather than remove, and routine duct cleaning as a general air-quality measure.

Duct cleaning deserves the nuance rather than a slogan. It is genuinely warranted after renovation dust, visible contamination, or a rodent or moisture problem — and it is not a routine annual necessity for a normal home. Our duct cleaning guide sets out when it is worth doing and when it is not.

Your Fall Indoor Air Quality Checklist

Six items, most of which take minutes: replace the furnace filter, test every CO and smoke alarm and check expiry dates, set humidity into the 30–45% band, service the HRV or ERV if you have one, confirm exhaust fans actually move air, and book the annual furnace inspection.

1. Fresh filter at the start of heating season, correct size, seated with no bypass.
2. Alarm check — button test, batteries, and the expiry date on the back of each unit.
3. Humidity — measure it rather than guessing; a basic hygrometer costs very little.
4. Ventilation — HRV filters and core cleaned, exterior hoods clear.
5. Exhaust fans — confirm they pull, and use them during and after showers and cooking.
6. Professional inspection — combustion, CO and heat exchanger, before daily running starts.

When to Bring in a Professional

Call someone when you have symptoms that track with the heating system running, persistent condensation on windows, a musty smell that returns, or any suspicion about combustion. Those are diagnosable conditions with instruments, not problems to solve by buying a device.

Our indoor air quality services in Hamilton cover assessment, filtration, humidification and ventilation, and our complete guide to HVAC and indoor air quality goes deeper on the equipment side. If the furnace has not been inspected this year, start there — it addresses the safety question and the air-quality question in the same visit.

FAQs

What humidity should I keep my house at in winter?
Between 30% and 45% for most Ontario homes. Lower brings static, dry skin and shrinking woodwork; higher risks condensation on cold window glass and frames. In deep cold you may need to lower the setting further, because the coldest surfaces condense moisture sooner.

Does closing vents in unused rooms improve air quality?
No, and it usually makes things worse. Closing supply vents raises pressure in the duct system, which increases leakage and can reduce airflow across the heat exchanger. It also leaves those rooms colder and more prone to condensation on exterior walls.

How often should I change my furnace filter in heating season?
Every one to three months for a standard one-inch filter, more often with pets, allergies or ongoing renovation dust. Thicker media filters generally last longer. Check it monthly at first — a look at the filter tells you your own home’s interval better than any rule.

Is duct cleaning worth doing every year?
Not as routine maintenance for a normal home. It is genuinely worthwhile after renovations, or where there is visible contamination, mould, or evidence of rodents or moisture in the ductwork. Regular filter changes address ordinary dust far more effectively than periodic cleaning does.

Why does my house smell stuffy only in winter?
Because ventilation has dropped. Odours that dispersed through open windows in summer now accumulate, and the furnace circulates them through the whole house. Improved exhaust fan use, correct humidity and mechanical ventilation address the cause; air fresheners only mask it.