Short cycling is when your furnace turns on, runs for a minute or two, shuts off, and then starts again — over and over. It’s one of the most common Ottawa winter complaints, and it matters because it does three bad things at once: it wastes gas, it shortens the life of your furnace by years, and it leaves your home unevenly heated even when the thermostat reads 21°C.
The good news is that the two most common causes are things you can check in five minutes before calling anyone. Start here, in order.
1. Start with the air filter
A clogged air filter is the single most common cause of short cycling, by a wide margin. When the filter is dirty it chokes the airflow your furnace needs to carry heat away from the heat exchanger. The furnace overheats, a high-limit safety switch shuts it down before the cycle finishes, and once it cools off it fires right back up — over and over. Homes with pets, a recent renovation, or a furnace running hard through a cold snap can clog a filter faster than you’d expect.
Pull the filter out and hold it up to a light. If you can’t see light through it, it’s clogged. Replace it with the same size and MERV rating (most Ottawa homes run MERV 8–11). If the short cycling stops within an hour, you’ve fixed it.
2. The furnace thinking something’s wrong
If a fresh filter doesn’t solve it, the next most likely reason is the furnace shutting itself down because a sensor or the thermostat is telling it to. The usual culprit is the flame sensor — a thin metal rod that confirms the burners are actually lit. Over time it gets coated in carbon, stops “seeing” the flame, and the furnace shuts the gas off as a safety measure, then tries again a moment later. A failing thermostat, or one with weak batteries, can cause the same rapid on-off by toggling the call for heat.
Both are routine fixes for a technician — but they involve your furnace’s safety and combustion controls, and that’s exactly where do-it-yourself troubleshooting should stop. Poking around burners and gas components isn’t worth the risk.
If you smell anything off or your CO detector activates, shut the furnace off and call us immediately.
When to call us
If you’ve changed the filter and the short cycling hasn’t stopped within 24 hours, give us a call. Our flat-rate diagnostic is $169+tax — we find the actual cause, tell you in writing what it costs to fix, and you decide. No guessing, no commission-driven upselling.