Why is my thermostat not reaching the set temperature?

When your thermostat isn't reaching the set temperature, it usually means either the heating or cooling system can't keep up with demand, or there's an underlying issue with the equipment, airflow, or the thermostat itself.

Start with the basics. Check your furnace filter first — a clogged filter is one of the most common culprits. A dirty filter restricts airflow so severely that your furnace runs constantly but can't move enough warm air through the home. Filters should be changed every 1-3 months depending on your home and filter type. While you're at it, make sure all your supply and return vents are open and unobstructed by furniture or rugs.

Consider the outdoor conditions. Ottawa winters are brutal, and on days when temperatures drop to -20°C or below, even a properly functioning furnace can struggle to maintain 21°C inside. This is called the "design temperature" limitation — most residential furnaces are sized for average cold days, not extreme ones. If your system only falls short on the coldest days, that may be normal behavior rather than a malfunction.

The thermostat itself could be the issue. If it's poorly located — near a drafty window, in direct sunlight, or above a heat source — it may be misreading the actual room temperature and short-cycling your system. Old or failing thermostats can also lose calibration over time. A smart thermostat upgrade ($150–$350 installed) can often solve this and improve efficiency.

Other common causes include leaky ductwork losing conditioned air before it reaches living spaces, low refrigerant (in summer cooling situations), a failing heat exchanger, or a furnace that's simply undersized for your home.

On the safety side, if your furnace is running constantly but your home stays cold and you notice any unusual smells, yellow or flickering burner flames, or your carbon monoxide detector goes off — turn the system off and call for service immediately. A cracked heat exchanger can cause CO to enter your living space, which is a serious health hazard.

Your next step is to replace the filter, check your vents, and observe whether the problem happens only on extreme cold days or consistently. If the issue persists after those checks, it's time for a professional diagnostic. A technician can measure airflow, check refrigerant levels, inspect the heat exchanger, and test the thermostat calibration. For a free consultation or to book a diagnostic visit, reach out to Ottawa HVAC Pro — we serve Ottawa and the surrounding area year-round.