Not Working After a Power Outage: The Reset Order

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Not Working After a Power Outage: The Reset Order

Short answer

Most of the time nothing is broken. Power coming back does a few specific things to a heating and cooling system, and some of them undo themselves in five minutes if you leave it alone. Here is the order to check, and the one fault a surge really does cause.

The power comes back on, the lights work, the fridge hums, and the heating sits there doing nothing. It is one of the most alarming versions of a breakdown because everything else in the house is fine.

In most cases nothing is broken. A system has a short list of things that go wrong when the power comes and goes, and most of that list costs nothing to put right.

Before you start

What to carry: a torch and a screwdriver. Fifteen minutes.

A gas furnace still needs electricity. This is the assumption that catches most people. Gas pays for the heat, but the board, the inducer, the igniter and the blower all run on power, so a gas furnace is as dead in an outage as an electric one. Only an old standing pilot system with a millivolt valve carries on, and there are very few of those left.

Do the easy half first. Everything in the next section is free and takes a quarter of an hour, and it fixes most of these.

The two things that actually hurt people after an outage. A generator never goes indoors. Not the garage, not the porch, not by an open window, not in a shed attached to the house. Carbon monoxide from a generator kills people every winter storm, and it does it while they sleep. Outside, well away, pointing away from the house, with an alarm indoors.

And never plug a generator into a wall socket to feed the house. It pushes power back out onto the lines and can kill somebody working to restore them. A generator either runs appliances on its own cords or goes through a transfer switch fitted by an electrician.

If anything smells burnt or the air is hazy when power returns, switch it off at the breaker and leave it off.

The reset order

Five things, in this order, and four of them are free. Most systems come back somewhere in the first three.

Check 1Free, fifteen minutes

Work down the list without skipping

1. Wait five full minutes before deciding anything is wrong. Control boards run a self check, and a compressor has a built in delay of about five minutes so it never restarts into pressure. People declare a system dead moments after the power returns, and then a service call turns up to a working machine.

2. The thermostat. Look at the screen. A battery one may have run itself flat while it kept trying. A wired one may have come back on a hold, on Off, or on a factory setting. Set the mode, set the temperature well past room temperature, and put fresh batteries in: how to prove a thermostat is really calling.

3. The breakers. One for the furnace or air handler, one for the outdoor unit. A tripped breaker very often does not look tripped, so push each one firmly all the way to Off and then back to On.

4. The switches. The one on or beside the furnace that looks like a light switch, and the pull out disconnect box on the wall by the outdoor unit. Both get knocked, and the disconnect gets pulled during storm work.

5. The fuse on the control board. Take the furnace door off with the power off and look for a small blade fuse, usually three amps and usually bright colours. It is a cheap part, and a blown one makes the entire system act completely dead.

Why a brownout is worse than a blackout

A clean cut is the easy one. The damaging version is the sag, where the power is there but too weak, and motors keep trying to start on it.

Check 2

Work out which kind of outage you actually had

A clean outage does very little. Everything stops, everything starts again, and the worst of it is a clock to reset.

A brownout is the one that costs money. The voltage sags rather than disappearing, often during a storm or as the supply is restored. A compressor motor tries to start on weak power, cannot get going, draws a huge current instead and heats up.

What that feels like. Lights dim and brighten, the fridge and the outdoor unit hum oddly, and afterwards the compressor will not start at all.

Then give it a few hours. A compressor that has overheated sits on its own internal protection and will refuse to start until it has cooled. Goodman says to allow at least three to four hours for it to cool and reset. That is not a fault and the wait is free.

Flickering power is worse than none. If the supply is coming and going, Trane says to switch the system off at its breakers, and to turn them back on one at a time once the power has been steady for ten minutes. That single move prevents most of the damage on this page.

What a surge really breaks

Check 3

The four parts, in the order to check them

The board fuse. A cheap part, and the first to check. Fit the same rating, never a bigger one.

The low voltage transformer. The little box that makes 24 volts for the thermostat and the controls. If the thermostat screen is blank and new batteries do nothing, this is a strong suspect.

The capacitor. Often already tired, and a surge finishes it. The tell is a compressor that hums and will not turn: why this cheap part causes so much.

The control board. The expensive one, so it comes last. Anybody who reaches for a board first has skipped three cheaper answers.

And on an oil system, the burner will have locked out. Press the reset once and only once: why the second press is the dangerous one.

Stopping it next time

Check 4

What is worth fitting, and what is not

Switch the system off during a long outage. Trane says at the breakers, so it cannot restart into an unstable supply. Free, and it is the single most effective thing on this list.

A whole house surge protector at the panel. An electrician fits it, and it protects the heating, the cooling, the oven and everything else at once. It is the only real answer to surges.

A surge device on the outdoor unit. Cheaper, and it only covers that one machine. Worth it if the panel option is not happening.

Check your thermostat keeps its settings. Some lose the schedule every time and quietly go back to a factory default that heats an empty house all day.

A hard start kit is not surge protection. It helps a tired compressor get going and it is sold as a fix for this. It treats the symptom of a weak start, not the cause.

Questions people ask

Will my gas furnace work in a power cut?

No, unless it is a very old standing pilot system. Everything modern needs power for the board and the blower, which is why a gas furnace and a gas water heater often behave completely differently in the same outage.

My thermostat is blank and new batteries did nothing.

Look at the board fuse first, then the transformer. Both cut the 24 volts the thermostat runs on, and both are common after a surge.

The outdoor unit hums but will not start.

Switch it off and leave it a few hours. If it starts after cooling down, you had a brownout and the capacitor is the thing to check before next summer.

Everything else in the house works fine.

That is normal and it is not evidence of anything. Heating and cooling sit on their own breakers, their own disconnect and their own fuse, and any one of those can be out while every socket in the house is fine.

Should I run the heating off a generator?

A gas furnace can run off a modest generator through a transfer switch, and a whole air conditioning system usually cannot, because the compressor start draws far more than the running figure suggests.

It came back on its own the next day.

Then it was a delay, a lockout or an overheated compressor rather than a broken part. Worth noting the date, because a system that does it after every storm is telling you to fit the surge protection.

Sources: Trane Home "What To Do During a Power Outage" (breakers off, wait 10 minutes), Ready.gov "Power Outages", CPSC safety alert "Portable Generator Hazards", Carrier service manual 24-25-9SM (5 minute compressor delay), Honeywell T6 Pro thermostat instructions (compressor protection), Carrier 58SC0A and White-Rodgers 50M56U instructions (3 amp board fuse), and Goodman's condensing unit service manual (compressor internal overload)