No cooling in a heatwave.

Close the house up: blinds and curtains shut on every window the sun reaches, windows closed while the outside air is hotter than the inside. Then check the filter, the breaker, and whether the outdoor unit is blocked or iced. Those three account for most summer failures.

If anyone in the house is elderly, very young, pregnant, or unwell, getting them somewhere cool is the priority and it comes before diagnosing anything.

First, make the building work for you

• Close blinds and curtains on the sunny side before the sun gets to them, not after. Blocking the sun outside the glass is far more effective than inside it, so exterior shades or an umbrella help disproportionately.
• Keep windows shut during the day if it is hotter outside than in, and open them wide at night when it cools.
• Use fans to move air across people, not to cool a room. A fan cools a person by evaporation and does nothing to an empty room.
• Cross ventilate at night: two windows on opposite sides, and a fan in one blowing out.
• Close off unused rooms on the sunny side.
• Turn off heat sources: the oven, unnecessary lights, and equipment.
• Damp cloths and cool showers work, and so does a bowl of water in front of a fan.
• Drink water steadily rather than in large amounts occasionally.

Above about 95 Fahrenheit, a fan alone stops being enough for a vulnerable person, and moving them somewhere air conditioned is the answer. Libraries, community centers, and shopping centers are the standard advice for a reason.

Then the checks worth doing

1. The thermostat. Batteries, set to cool, set below room temperature, fan set to auto.

2. The breaker. There are usually two: one for the indoor air handler and one for the outdoor unit. Check both. Also look for the outdoor disconnect box next to the condenser.

3. The filter. A blocked filter is the most common cause of both poor cooling and a frozen coil.

4. The outdoor unit. Is it running? Is it clear? Cut back anything growing around it, clear leaves and debris from the fins, and give it a couple of feet of clearance on all sides. A condenser that cannot reject heat cannot cool the house.

5. Ice. If there is ice on the pipework or the coil, switch the cooling off and run the fan only to thaw it. Which is a specific fault with a specific cause.

6. The condensate. Many systems have a float switch that shuts the unit off when the condensate drain blocks, which prevents a ceiling being flooded. If the pan is full or the drain is blocked, that is your fault code.

7. Vents and returns. Open, and not blocked by furniture.

What you cannot fix yourself

Refrigerant. If the system runs but blows warm, or if it is icing repeatedly after the filter has been changed, the likely cause is low refrigerant, and low refrigerant means a leak. Topping it up without finding the leak is a temporary fix for a system that will empty again, and handling refrigerant is regulated work in any case.

Compressors, capacitors, and control boards are also a technician’s job. A failed capacitor is a common and relatively inexpensive fault that presents as a unit that hums but does not start.

While you wait

• Keep the house closed and shaded during the day, open at night.
• Sleep on the lowest floor, since heat rises.
• Check on neighbors who are elderly or unwell.
• Watch pets, who cope worse than people do.
• If the house cannot be kept safe, go somewhere that can. Keep the receipts, because if the property is genuinely uninhabitable there may be cover for it. What additional living expenses pays for.

Getting someone out

The same rules as a winter breakdown. Call early, have the make, model, and any error code ready, say whether anyone is vulnerable, and ask what the service call fee is and whether it is credited against the repair.

What HVAC work costs, and how it is priced.

The short version

Close and shade the house before the day heats it, and move vulnerable people somewhere cool first. Then thermostat, both breakers, filter, and a clear outdoor unit. Ice means switch to fan only. Warm air with the system running usually means refrigerant, which is a technician and a leak rather than a top up.