Switching From AC to Heat: The Fall Transition Without a Service Call

Somewhere in the next few weeks, you'll wake up cold.

Not dangerous cold. Just the kind where the house has dropped to 63 overnight, the floor is chilly, and you stand at the thermostat wondering whether you're really going to turn the heat on in September. Then by two in the afternoon it's 82 out and the AC is running again.

That's a West Michigan fall. We don't get a clean handoff from summer to heating season. We get five or six weeks of the two seasons overlapping, and that overlap is when I get a specific kind of call: not a broken furnace, but a furnace that was fine, and an AC that was fine, and a homeowner who did something perfectly reasonable in between that turned into a no-heat morning.

Most of those calls are avoidable. This is the order I'd walk through the switch in, so you don't have to make one.

Test the Furnace on a Warm Afternoon, Not the First Cold Night

This is the whole post in one sentence, so I'll put it first.

Pick a mild day in September, when nobody in the house cares whether the heat works. Set the thermostat to heat, bump the setpoint five degrees above the room temperature, and let it run for fifteen minutes. Listen to it start. Feel for warm air at a register. Then set it back.

If it doesn't fire, you've found that out on a Tuesday afternoon with the windows open, and you can call and get a normal appointment. If you find out on the first 38-degree night in October, you're calling at the same time as everyone else in Ottawa County who also waited, and every shop in the area is booked. Same repair, worse week.

The test costs you fifteen minutes of a warm afternoon. I don't have a stronger argument than that.

Change the Filter Before the Season, Not During It

Your filter has been catching cottonwood, pollen, pet hair, and whatever else the AC pulled through the house all summer. It's tired. Change it before the first heat cycle so the furnace isn't starting its season starved for air.

Two things to know. First, a clogged filter is the single most common reason a furnace overheats and shuts itself down on a high-limit switch — the furnace fires, gets too hot because air can't move through it, quits, cools, and tries again. That looks like a broken furnace and it's a nine-dollar fix. Second, if you have a whole-house humidifier with a water panel, this is the time to swap that too. It sat dry all summer and probably has a crust of minerals on it.

I wrote a whole piece on how often to change your filter. The short version for fall: change it now, then check it monthly once the furnace is running.

Walk the House and Open the Registers

Every fall I find at least one furnace working too hard because somebody closed off registers in the summer, or a couch got pushed over a return, or the basement supplies got shut to force more cooling upstairs.

Open them back up. All of them, including the basement. A furnace is designed to move a certain amount of air across its heat exchanger, and every closed register pushes it toward the overheating problem I just described. If you're closing registers to fix rooms that are too hot or too cold, that's a balancing problem worth solving properly rather than working around, and I'd rather see you get it looked at than choke off half the system.

While you're walking around, check that nothing is stacked against the furnace itself, and that the vent pipes on the side of the house (the white PVC on a high-efficiency unit) are clear of anything you leaned there over the summer.

Get the Thermostat Right, Then Stop Touching It

The overlap weeks cause the most trouble at the thermostat.

Auto mode is fine. Bouncing by hand isn't. Most thermostats have an auto setting that will switch between heat and cool on its own. People are suspicious of it, and I understand why. But used with a sensible gap between the heat and cool setpoints — say heat at 66, cool at 76 — it works well and it prevents the thing that causes wear, which is a person flipping the mode back and forth several times a day.

Why the flipping matters. Every switch is a stop and a start for the outdoor compressor. That start is the hardest thing the system does all day, and it's why most equipment builds in a five-minute lockout before it will restart. If your thermostat is only a few degrees apart on heat and cool, or you're setting the cool point at 70 and the heat point at 69, the system can end up cooling, then heating, then cooling, chasing a one-degree window. That's short cycling with extra steps. Widen the gap.

Check the batteries. If your thermostat takes them, this is the time. A thermostat that goes dark on the first cold night is a genuinely common no-heat call, and I always feel a little bad charging for it. Our guide to what to check when the thermostat isn't responding covers the rest.

Rewrite the schedule. If you set a summer schedule that runs the AC hard in the afternoons, that schedule doesn't know it's October. Smart thermostats handle this gracefully; a programmable one from 2009 needs to be told. Our summer thermostat settings guide has the cooling side of this, and if you've been thinking about upgrading, the honest case for smart thermostats is mostly about exactly this kind of shoulder-season juggling.

Leave the Outdoor Unit Alone for Now

I know the urge. Labor Day comes, the nights get cool, and it feels like time to put the AC to bed. Cover it, throw the disconnect, done.

Don't. Not yet.

You are going to get an 84-degree afternoon in the last week of September, and probably one in October, and you'll want that unit running. If it's covered, you'll be out there pulling a tarp off in the dark. Worse, if someone throws the disconnect and then forgets, the next warm afternoon the thermostat calls for cooling, the blower runs, the outdoor unit sits dead, and the house fills with room-temperature air until somebody decides the AC broke. I've driven out for that one. It was supposed to be a helpful thing to do.

The right time to shut down and cover the AC is after the last real warm stretch, and it comes with its own short checklist. I'll be posting that separately. For now, the whole instruction is: leave it on, leave it uncovered.

The Humidifier Damper, If You Have One

If your furnace has a bypass humidifier — the box on the ductwork with a small round damper on it, usually labeled Summer and Winter — turn that damper to Winter when you make the switch. All summer it's been closed so the AC didn't have to fight moist air being recirculated. Once heat's on, it needs to be open for the humidifier to do anything.

Not every home has one. If yours doesn't, skip this. If it does, this is one of the small things that separates a house that feels comfortable at 68 in January from one that feels dry and staticky at 71.

The Furnace Started. Now What?

If the test run went well, filter changed, registers open, thermostat set, first-cycle smell came and went, you're ahead of most of the county.

There's a limit to what you just did, though. A test run tells you the furnace fires. It doesn't tell you the heat exchanger is sound, the flame sensor is clean enough to make it to January, or that the safety switches would trip when they need to. Those are the reasons a fall tune-up exists, and why we push people to book one in September rather than November.

If the furnace is a few years old and was serviced last fall, I'm not going to tell you it's urgent. If it's over ten, or it's been a couple of years, or it did anything odd during the test — slow to light, a bang on ignition, a smell that lingered — get furnace maintenance on the calendar now. Same reason as the test run: the mild weeks are when the repair is easy.

When to Call Instead of Fiddling

A short list. Call if:

• The furnace doesn't fire on the test, or fires and shuts off within a minute or two, more than once. Check the filter and the switch on the side of the furnace first, then call.
• You smell gas at any point. Leave the house, then call from outside. Don't test anything, don't flip switches.
• A CO detector sounds. Same answer: everyone outside, then call.
• The burn-off smell is electrical or lasts beyond the first hour.
• The AC ran fine all summer, you touched the thermostat, and now nothing runs in either mode. That's usually a thermostat or a low-voltage issue and it's a quick fix, but it's not one to guess at.

Anything else — a slightly cool room, a schedule you can't figure out, a noise you're not sure about — is a normal appointment. You can book online and pick a day that isn't the first cold one.

I was in a basement in Georgetown Township one September on a "furnace won't start" call. The homeowner had done everything right: new filter, registers open, tested it on a warm day just like I tell people to. It wouldn't light. Turned out the switch on the side of the furnace — the one that looks exactly like a light switch — had been bumped off by a box of Christmas decorations somebody stacked in July. Flipped it, furnace ran. I didn't charge for that one, but I'd rather you check it before I get there.