How to Prepare Your AC for a Toronto Heat Wave

The best way to prepare your AC for a heat wave is to run through a checklist before the forecast climbs, not the morning the GTA hits 35°C. A prolonged heat wave in the GTA is when your AC earns its keep — and when it is most likely to fail if it was already struggling. The weeks between July 1 and the August long weekend account for more than half of all GTA AC service calls each year. Most of those calls are to homes where the system was showing symptoms for weeks and the homeowner waited too long.

This guide walks through what to do before a heat wave to give your AC the best chance of handling it, what to do during a heat wave if the system is struggling, and when a service call cannot wait.

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Pre-heat wave checklist

Replace the air filter

A clean filter is the single most impactful thing you can do before a heat wave. The AC will run extended cycles during extreme heat, and restricted airflow from a clogged filter turns a manageable situation into an overloaded one. Pull the filter out and hold it up to a l ight. If you cannot see light through it, replace it now.

Clear around the outdoor unit

The condenser unit needs at least two feet of clear space on all sides to reject heat effectively. Cut back any overgrown grass or shrubs. Remove any stored items that have accumulated around it over the winter. Make sure the top of the unit is unobstructed — do not cover it with anything even for a day, as the fan on top needs to exhaust hot air freely.

Check the drain line

A blocked condensate drain overflows the drain pan. During heavy AC use, the system generates significant moisture, and a drain that was barely keeping up in spring will fail under peak summer load. Pour a cup of water into the drain pan and watch it drain. If it drains slowly or not at all, clear the line with a condensate drain tablet or a wet-dry vac on the drain opening before the heat arrives.

Program your thermostat

During a heat wave, trying to cool a house from 28C down to 21C at 5 pm is a losing battle. Pre-cool the house. If you know a heat wave is coming, set the thermostat to start cooling at 6 am so the house is at temperature before it gets hot. The AC can maintain 22C far more efficiently than it can recover from 28C in 95 percent humidity.

During the heat wave: what to adjust

Set the fan to run continuously rather than auto. Continuous fan circulation distributes the cool air more evenly and helps with humidity removal. Close blinds and curtains on south and west-facing windows during afternoon hours — solar gain through windows is a significant cooling load that the AC does not need to fight.

Keep interior doors open to allow circulation. Deadend rooms with closed doors become hot spots that the rest of the system has to compensate for. If you have attic hatches, make sure they are sealed. Hot attic air can bleed into upper floor spaces significantly through gaps around hatches.

Signs the system is struggling

Warning signs that the AC is having trouble: (1) The house is not reaching the set temperature even after running all day. A properly functioning system should be able to maintain 22C or cooler even in 35C heat, assuming normal insulation. If it cannot, something is wrong. (2) The outdoor unit is icing up. Ice on the refrigerant lines means the system is not absorbing heat properly. Turn it off and let it thaw before running it again, and call for service. (3) The system is running constantly but the air from the vents does not feel cold. This points to refrigerant or compressor issues.

Managing a heat wave with a marginal system

If you know your AC is borderline and a heat wave is coming, do not wait for it to fail completely. Calling for a service check before the wave hits means you get an appointment at a reasonable time rather than joining the queue of emergency calls on the second day of 35C heat. Most HVAC companies are booked solid within hours of a heat wave starting. Being proactive buys you an appointment window.

If the system fails completely during a heat wave and you cannot get same-day service, move to the lowest floor of the house where it is cooler, use window fans to pull cooler outdoor air in during nighttime hours when temperatures drop, and avoid generating indoor heat from cooking or appliances as much as possible. Community cooling centres operated by the City of Toronto and other GTA municipalities are available at no charge during heat emergencies.

Book AC service before the next heat wave

• Vaughan
• Mississauga
• Brampton
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• Richmond Hill
• Scarborough
• Etobicoke
• Aurora and Newmarket
• Oakville and Burlington
• Ajax, Pickering, and Whitby

Book ac tune-up or repair in the GTA

Do not wait for the heat wave to find out your AC has a problem. NorthWind offers spring tune-ups and same-week repair across the GTA. Free estimates on all AC service calls.

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Written by

Michael T.

Content Writer, Air Conditioning

Michael writes for North Wind HVAC Pro on air conditioning topics, covering central AC, ductless systems, and summer maintenance guides for GTA homeowners. His articles explain common cooling problems and the questions to ask before an upgrade.