How to Prepare Your Furnace for a Michigan Winter

Do it in October, on purpose

Southeast Michigan gives you a window. The first frost usually lands in mid-October, the first sustained cold in late November, and the first real test, a week of single digits, somewhere between late December and February. Everything in this guide is meant to happen in that window, when a problem is an inconvenience instead of an emergency. The no-heat calls we run at 2 a.m. in January in Ypsilanti and Westland almost always involve a furnace that nobody turned on until the night it got cold.

Step one: the test fire

Pick a cool morning in October, set the thermostat 5 degrees above room temperature, and go stand by the furnace. You are watching for the sequence: the inducer motor spins up with a soft whir, a click as the pressure switch closes, the hot surface ignitor glows orange through the sight glass, the gas valve opens with a quiet thump, the burners light and run blue, and 30 to 60 seconds later the main blower starts. Let it run 15 minutes and feel the air at a few registers.

Things that are worth a call now, not later:

• Burners that light, run for a minute and shut off, then try again. That is the pattern in furnace short cycling causes.
• The ignitor glows but the burners never light, or light with a bang. A dirty flame sensor or a lazy gas valve.
• A burning dust smell is normal for the first cycle of the season. A smell like hot plastic or electrical insulation that persists is not.
• Any scraping, squealing or grinding from the blower or the inducer.
• Nothing happens at all. Check the furnace switch, which looks like a light switch on or near the unit, and the breaker, then read furnace not turning on.

Step two: filter, airflow and the area around the furnace

Put in a new filter and write the date on it. If you use a 1-inch pleated filter, buy enough for the whole winter now, because you will want to change it in January.

Walk the basement or utility room and clear everything within three feet of the furnace. Paint cans, cardboard, the cat litter, laundry baskets. An 80% furnace draws its combustion air from the room, and anything stored against the cabinet is both a fire risk and an airflow restriction. In a lot of 1950s ranches in Garden City and Inkster the furnace sits in the middle of a basement that has slowly become storage, and we have found cabinets with boxes stacked against the burner door.

Go upstairs and confirm every supply register is open and nothing is sitting on the floor returns. Closing registers in unused rooms to "push heat" elsewhere raises static pressure and is one of the quiet causes of high-limit trips.

Step three: the venting

For an 80% furnace vented through a metal flue into a chimney, look at the flue pipe for rust streaks, gaps at the joints and a connection that has pulled out of the chimney thimble. Outside, check the chimney cap for bird nests and leaves. A water heater that shares that chimney depends on it too.

For a 90%+ furnace with white PVC pipes exiting the side of the house, find the terminations outside. Clear leaves, wasp nests and the mulch that has crept up under them. Make sure nothing has been planted or stacked in front of them, and when the snow comes, keep them dug out. A sidewall vent buried by the first big drift is the most common reason a Canton or Saline subdivision furnace shuts down on the exact morning it is needed.

If at any point you smell gas, inside or near the meter, leave the house first. Call DTE at 800-947-5000 and 911 from outside. Do not try to find the leak yourself.

Step four: the safety devices you own

Replace the batteries in every carbon monoxide alarm and every smoke alarm, and press the test button. A CO alarm should live on each sleeping floor and near the furnace. Check the manufacture date on the back; CO sensors degrade and most alarms are rated for 7 to 10 years. If yours is from before 2016, replace it.

Know what the alarm means. If a CO alarm sounds, leave the house, get everyone out including pets, and call 911 from outside. Do not open windows and wait to see if it stops. The full picture is in carbon monoxide and furnace safety.

Find the gas shutoff at the furnace, a quarter-turn valve on the supply pipe, and the main at the meter. You should be able to find both in the dark.

Step five: thermostat and humidifier

Set the thermostat's heating schedule now. If you are on DTE's time-of-day electric rate, the furnace blower is a small load, but a heat pump or electric backup heat is not, and a schedule that preheats before 3 p.m. saves real money. Replace the thermostat batteries if it takes them; a dead thermostat battery is a no-heat call we have run more times than we would like.

If you have a whole-home humidifier, open the bypass damper, which most people close for summer, and turn the humidistat to about 35 percent. Install a new pad, which the humidifier maintenance guide covers. Aim for 30 to 45 percent indoor relative humidity through winter, dropping toward 30 during the coldest stretches so the windows do not frost.

Step six: the professional tune-up

The homeowner steps above catch the obvious. The parts that fail in January, the flame sensor, inducer, pressure switch hose, ignitor and condensate trap, need a technician with a meter to assess. A furnace tune-up in October also produces the CO and temperature-rise readings that tell you whether a 20-year-old furnace is safe to run another winter. The full list of what that includes is in the furnace tune-up checklist.

Before the polar vortex hits. Every few winters the region gets a stretch of minus 10 to minus 20 overnight lows. A furnace that is marginal at 20 degrees fails at minus 15, because it is running nearly continuously with no margin. Before a forecast like that: change the filter, keep the vents clear, open the cabinet doors under sinks on exterior walls so pipes do not freeze, and if the furnace has been doing anything odd in December, do not wait for the cold snap to call. Our line is open 24 hours and we do not charge an overtime premium for nights or weekends, but the best time to fix a furnace is still a Tuesday afternoon in October.