Quick answer
Most central air conditioners should not run when the outdoor temperature is below about 15°C. Below that, refrigerant pressure drops, the indoor coil can ice over, and refrigerant can migrate into the cold compressor oil and slug the compressor at start-up. If a cool September night leaves the house stuffy, use the fan-only setting or open windows instead. Units with a low-ambient kit, and heat pumps in heating mode, are the exceptions.
How cold is too cold to run the AC? For a standard residential condenser, the line sits around 15°C outdoors — most manufacturers put the minimum at roughly that figure, written as 60°F in the manuals, unless the unit was ordered with low-ambient controls. Above that, run it as much as you like. Below it, you risk a frozen coil and, on repeated cold starts, a damaged compressor.
What happens inside the system on a cold day
An air conditioner is designed around a warm outdoor coil. When the air outside is cool, the outdoor coil sheds heat too easily, head pressure falls, and the pressure at the indoor coil falls with it. Low pressure means a low coil temperature, and once the coil surface drops below 0°C the moisture it should be draining freezes into a block of ice instead.
- Frozen evaporator coil — airflow drops to a trickle, then a puddle appears at the furnace when it thaws. Our guide on why an AC freezes up covers the recovery steps.
- Oil thickening — compressor oil is meant to be warm; cold oil lubricates poorly on start-up.
- Liquid slugging — refrigerant that condensed in the cold compressor gets pumped as liquid, which a compressor cannot tolerate for long.
- Nuisance trips on the low-pressure switch, which is doing its job by shutting the unit down.
Shoulder-season weather in Burlington: warm afternoons, cold nights
The tricky weeks are late September and early October, and again in May. A day can reach the mid-20s while the night falls into single digits, and a thermostat left in cool mode will happily start the compressor at 3 a.m. when it is 9°C outside. The lakeshore neighbourhoods — Roseland, Aldershot, Old Oakville, Bronte — hold their warmth longer in fall because Lake Ontario stores summer heat, while Waterdown and the streets up on the Escarpment cool off faster after sunset. Either way, the fix is the same: on cool nights, set the thermostat to fan-only or turn cooling off and open a window.
Watch the setpoint too. Dropping the thermostat to 18°C on a 14°C evening asks the system to run in exactly the conditions it was not built for. If you see white frost on the larger insulated copper line at the outdoor unit, turn cooling off and let it thaw before it runs again.
The exceptions: low-ambient kits and heat pumps
Server rooms, wine cellars and some commercial spaces need cooling in January, so those condensers are fitted with a low-ambient kit — a fan-cycling or head-pressure control plus a crankcase heater that keeps the oil warm. With that kit a unit can run well below freezing. A heat pump in cooling mode follows the same 15°C rule as an AC, but in heating mode it is built for the cold; see at what temperature a heat pump stops working for that side of the question.
Starting up in spring and shutting down in fall
- Restore power to the condenser 24 hours before the first run of the season. Many units have a crankcase heater that needs that time to drive refrigerant out of the oil; our spring prep checklist covers the rest.
- Wait for a day that is at least 15°C before the first test; that is also the minimum for a technician to check refrigerant charge properly.
- In fall, a board or purpose-made top cover keeps leaves and ice out of the fan. Do not wrap the whole cabinet — a sealed cover traps moisture and gives mice a warm home.
- If you switch the disconnect off for winter, remember it kills the crankcase heater; switch it back on a day before spring start-up.
Quick answers to related questions
- Is it okay to run the AC at 10°C? Not without a low-ambient kit. Use the fan or open windows; the house will cool faster that way anyway.
- Will running the AC once in cold weather ruin the compressor? One short cycle usually does no lasting harm. Repeated cold starts across a season are what shorten a compressor's life.
- Can I run just the fan when it is cold? Yes. Fan-only mode moves air through the filter and evens out room temperatures without starting the compressor. More cooling questions are collected in our answers library.
A system that ices up even on warm days has a different problem — low charge, a dirty filter or a failing blower — and that is worth a diagnostic visit, which runs $90–$150 and is folded into the repair if you go ahead. ZK Mechanical handles air conditioner repair across Burlington, Oakville and Hamilton; send us the details or call (647) 801-1252 and we will tell you whether it is weather or a fault.