A hard freeze, in three stages

Do the work while it is still comfortable outside

Everything on this list is easier now than it will be at any later point, and most of it is free.

• Change the filter. A restricted filter is the commonest reason a heating system cannot keep up on the one night it needs to, and it is the cheapest thing on this page.
• Run the heat now, hard, for an hour. If something is wrong you want to find out on a Tuesday when the whole town is not ringing the same four companies.
• Clear the outdoor unit. Leaves, grass clippings, anything stacked against it. A heat pump has to breathe to defrost.
• Find out where the water shut-off is and whether it actually turns. Do this in daylight.
• Identify every pipe in an unheated space — attic, crawl space, exterior wall runs, an unheated back room. That is where the split will be.
• Decide the overnight temperature now and stop setting the thermostat back. Recovery costs more than holding, and in a heat pump it costs a great deal more because it brings the strip heat on.

Hold steady, and do not chase it

• Leave the thermostat alone. Turning it up will not make the building warm faster; on a heat pump it simply switches on the most expensive heat you own.
• If the outdoor unit is a block of ice and is not clearing, switch to emergency or auxiliary heat and ring us. Do not chip at it, do not pour hot water over it, and do not run it iced.
• Let a tap drip on the runs you identified as being in unheated space. Moving water is a great deal harder to freeze than standing water.
• Open the cabinet doors under sinks on outside walls.
• If it stops entirely, check the breaker once. Once. If it trips again, leave it off and ring. A breaker that keeps tripping is telling you something and resetting it repeatedly is how a small fault becomes a large one.
• Never heat a building with an oven, a grill or an unvented burner. Carbon monoxide kills people during exactly these weeks, and it does it to people who were cold and thought they were being resourceful. If a CO alarm sounds, get everyone outside and ring 911 from outside.

Water first. Then equipment. In that order.

• The walk-round above comes before anything else. Restarting into an undiscovered split is the single most costly mistake of the week.
• Then look at the outdoor unit. Ice that sat on a fan blade for two days sometimes leaves a bent one, and a bent blade means vibration and a bearing that will not last the summer.
• Check the condensate route, which may have frozen and may still be blocked further along even though everything indoors feels normal.
• Watch what the system does over the following week. Compressors that were asked to start repeatedly in extreme cold sometimes fail a fortnight later, and the failure gets blamed on age.
• Write down what actually happened. Which unit struggled, what time, what you did. That page is worth more than any survey we could do in April.

Where a house fails

Three places, and it is nearly always one of them. The attic, where the ductwork and often the air handler live, and where any water line runs with nothing warming it. The outdoor unit, which ices up, tries to defrost, and cannot clear because it is smothered or because the defrost control has been faulty since last winter and nobody knew. The strip heat, which quietly becomes your entire heating system when the heat pump stops contributing, and which turns up as a bill three weeks later.

The fourth thing, which is not equipment, is the garage or the back room that was never really heated and now has a washing machine and a water line in it.

Where a small commercial unit fails

On a roof, in a kitchen, or at a door. Rooftop packaged units have their gas section, their condensate and sometimes their water supply all exposed on a roof with wind across it, and a unit that has been running in economiser mode can pull freezing outside air straight through itself if the damper is stuck open — which is why the economiser is checked at the autumn visit rather than in March.

A kitchen extract running with no make-up air holds the whole unit at negative pressure, which in a freeze means it is actively sucking outdoor air in around every door and window and through the front entrance every time it opens. That is the building that cannot be warmed no matter what the units do.

And the water is usually in the worst place — a mop sink on an outside wall, an ice machine line, a line to a soda system running through an unheated store room.

Where a large shed fails

Volume is the problem. A tall building with unit heaters stratifies badly — you can be holding a comfortable figure at the thermostat on a wall at head height while the roof space is warm and the floor, where the pipework and the people are, is close to freezing. Destratification fans are cheap next to a burst main.

Suspended unit heaters and their flues are the other failure. They are high up, they are rarely serviced because getting to them needs a lift, and a flue that has partially separated is both a heating failure and a carbon monoxide problem in a building where nobody is near it.

Then the ordinary ones: dock doors left open, which turn a heated building into an unheated one in about ten minutes, and a fire sprinkler system in an unheated section, which is not our trade but is worth knowing about before the week rather than during it.