What a Missouri winter asks of your furnace
St. Louis winters are not consistently severe, but they swing hard. A 55-degree Tuesday followed by a 5-degree Thursday is a normal January here, and that pattern is harder on equipment than a steady cold. Systems idle, then run at full load for 72 hours straight, which is exactly when neglected parts fail.
The practical implication: do your maintenance in September or October, not during the first cold snap when every contractor in the metro is triaging no-heat calls.
Fall checklist, before the first hard freeze
Replace the filter and note the size on the furnace with a marker. Vacuum the blower compartment and the return grilles. Test the thermostat by running heat for a full cycle and confirming the burners ignite within about a minute of the fan. Confirm the furnace door switch seats properly, because a loose door panel is a surprisingly common no-heat cause.
Then walk outside. On a high-efficiency condensing furnace, find the PVC intake and exhaust pipes on the exterior wall and clear leaves, mulch, nests, and dryer lint from the openings. Check that they sit above expected snow depth. On an older 80 percent furnace, look at the metal flue for rust streaks or separated joints and have a technician evaluate anything you find.
Carbon monoxide checks that are not optional
Install a CO alarm on every level of the home and within about 10 feet of each sleeping area, and replace the units themselves every 7 to 10 years, not just the batteries. The National Fire Protection Association's guidance on CO alarm placement is the standard most local codes follow.
Warning signs worth an immediate service call: yellow rather than blue burner flames, soot around the furnace or vent, a strong stale odor when the unit runs, condensation on windows near the furnace, or headaches and nausea that improve when you leave the house. If an alarm sounds, leave the home and call 911 or the gas utility from outside.
During the season
Check the filter monthly during heavy use. Keep supply and return registers clear of furniture and rugs. Listen for new sounds: a rhythmic squeal usually means a blower bearing, a boom at ignition points to delayed burner ignition and deserves prompt attention.
On condensing furnaces, watch for water at the base of the unit, which usually means a plugged condensate trap. Keep the area around the furnace clear of paint, solvents, and stored chemicals; combustion air pulls those fumes through the burner.
What a professional tune-up should include
A real tune-up in this market runs about $100 to $200 and should include a combustion analysis, heat exchanger inspection, gas pressure and temperature-rise measurement, safety control testing, blower amperage draw, and a written report with the numbers on it.
Ask for the readings. A tune-up receipt that says only "system operating normally" is a receipt, not an inspection. Temperature rise outside the manufacturer's stated range on the data plate is a specific, checkable finding you can act on.
Frequently asked questions
How often should a furnace be professionally serviced?
Once a year for gas furnaces, ideally in early fall. Systems in dusty environments, or those over 15 years old, benefit from a mid-season check as well.
What temperature should I set when leaving town in winter?
Do not go below 55 degrees, and open cabinet doors under sinks on exterior walls. Missouri cold snaps arrive quickly, and frozen pipes cost far more than the heating savings.
Do high-efficiency furnaces have problems specific to Missouri?
Yes. Their PVC intake and exhaust terminate outside at low height, so wet snow, ice, leaves, and nests can block them and trip a pressure switch. Checking those pipes after a winter storm prevents a lot of no-heat calls.
Sources and further reading
Price ranges in this guide come from quotes and invoices shared with STL Home Pros by St. Louis area homeowners and contractors. Technical and climate references are listed below.
About the author
Nick Cascani is a New York native who has called St. Louis home for the last few years. He believes getting professional home service should be simple and convenient, and he writes these guides to help local homeowners make confident decisions about their HVAC, plumbing, and foundation projects.
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