St. Louis winters can go from mild to single-digit temperatures with little warning. The furnaces we see fail in January almost always show signs in October — a dirty flame sensor, a cracked heat exchanger, a filter that hasn't been changed since spring. Fall is the right time to catch those issues, before you're depending on the system and emergency rates apply.
Here's a practical checklist: what you can do yourself, what's worth scheduling a tech for, and how to know when your system might not make it through another winter.
What You Can Do Yourself
These tasks don't require any tools or HVAC knowledge. They take 30–60 minutes total and can prevent the most common fall heating problems.
Replace your air filter
If you haven't changed your filter since spring or summer, do it before you fire up the heat. A clogged filter restricts airflow, causes the furnace to overheat, and makes the system work harder than it should. For most St. Louis homes, a 1-inch filter every 1–2 months is about right during active use. Thicker media filters (4–5 inch) last longer but need to be the right fit for your system — check with your tech if you're unsure.
Test your thermostat before you need it
Switch your thermostat to HEAT and set it above the current room temperature. Give the furnace a few minutes to respond. If the system doesn't fire up, check that the fan setting is on AUTO, not ON — and confirm the furnace switch near the unit hasn't been turned off. It's better to find out in September than in November when temperatures drop.
Clear the area around your furnace and vents
Furnaces need adequate clearance to operate safely. Remove any storage, cardboard, or flammable material from within a few feet of the unit. Walk through your home and make sure supply and return vents aren't blocked by furniture, rugs, or stored items — blocked vents cause uneven heating and put strain on the system.
Check your vents and registers
Open and inspect supply registers (where warm air comes out) and return vents (larger grilles where air goes back to the system). Dust them off. Make sure they're open in rooms you use — closing too many vents actually increases system pressure and can cause problems.
Test your carbon monoxide detectors
Fall is a natural time to press the test button on your CO detectors and replace batteries if needed. A cracked heat exchanger — one of the more serious furnace failures — can allow carbon monoxide to enter your living space. You can't detect CO without a detector. If yours is more than 7 years old, replace it.
Check the condensate drain line (high-efficiency furnaces)
If you have a high-efficiency furnace (90%+ AFUE), it produces condensate — water as a byproduct of combustion. Check the drain line near the unit for visible clogs or standing water. A blocked condensate line will shut the furnace down as a safety measure, usually at the worst time.
What a Fall Furnace Tune-Up Actually Covers
A professional tune-up isn't just someone showing up and saying everything looks fine. When it's done properly, here's what gets checked:
- Flame sensor cleaning — a dirty sensor causes short-cycling or a furnace that won't stay lit
- Burner inspection — checking for proper flame pattern and combustion
- Heat exchanger inspection — looking for cracks or corrosion that could allow CO to enter living spaces
- Blower motor and belt check — wear, alignment, and amperage draw
- Flue pipe inspection — making sure exhaust gases are venting properly
- Safety control testing — limit switches, pressure switches, rollout sensors
- Gas pressure and manifold check
- Filter check (or replacement if provided)
- Thermostat calibration and operation test
A tune-up takes about an hour. The goal is to find what's worn, dirty, or about to fail — and address it before it causes a breakdown. It also gives you an honest picture of whether your system is likely to make it through another season or whether replacement is the better financial call.
Signs Your Furnace Might Not Make It Through This Winter
Not every older furnace needs to be replaced — but some shouldn't be repaired again. Here are the signs worth paying attention to:
The furnace is 15–20+ years old
Most gas furnaces have a 15–20 year useful life with proper maintenance. If yours is in this range and starting to have problems, repair costs versus replacement cost needs to be evaluated honestly.
You've had two or more repairs in the past two years
Multiple failures in a short period usually means the system is degrading across the board — not isolated incidents. Each repair is buying less time.
Your heating bills have increased without a clear reason
As furnaces age, efficiency drops. If your gas bill has gone up noticeably without a change in weather patterns or usage, it may be the furnace working harder to produce the same heat.
The heat isn't consistent across rooms
Uneven heating can be a duct issue, but it can also indicate the furnace is struggling to maintain proper output — especially in the rooms farthest from the unit.
You hear banging, rattling, or popping that wasn't there before
New noises on an older furnace are worth having looked at before heating season. Some are minor; some point to components that are about to fail.
If your furnace is showing two or more of these signs, a tune-up is still worth doing — it gives you a clear picture of what you're working with. But go in knowing that replacement may be the more cost-effective conversation.
Don't Forget Your Outdoor Equipment
If you have a central AC or heat pump with an outdoor unit, fall is the right time to give it some attention before winter:
- Clear debris: Remove leaves, grass clippings, and any buildup around the outdoor unit. Keep at least 2 feet of clearance on all sides.
- AC-only units: You can cover the top of a central AC condenser (not the sides) with a breathable cover or a piece of plywood weighted down — this keeps leaves and ice out of the unit over winter. Don't cover a heat pump — it runs in winter.
- Heat pump owners: Don't do anything to winterize it — heat pumps run year-round. Keep the area clear and make sure nothing is blocking airflow.
- Check refrigerant lines: The insulation on the lines running between your indoor and outdoor units should be intact. Damaged insulation can reduce efficiency.
When to Schedule Your Tune-Up
September and October are the ideal window for St. Louis homeowners. By November, heating season is underway and most HVAC companies are backed up. If something turns up during a tune-up — a failed part that needs ordering, a heat exchanger that requires replacement — you want time to handle it before it's urgent.
Booking early also means you're not competing with emergency calls. In St. Louis, when temperatures drop suddenly in late October, service schedules fill up fast.
Ready to schedule your fall furnace tune-up?
We've been servicing furnaces across St. Louis County, St. Charles County, and Jefferson County since 2010. A tune-up from Lolich means an honest assessment of what your system actually needs — not a list of add-ons. If we find something, we'll tell you what it is, what it costs to fix, and whether it's worth fixing. No pressure.