Furnace Maintenance: Cost, Schedule, and What's Included
Richard Trethewey, the longtime plumbing and heating expert on This Old House, walks through what a real furnace maintenance visit actually checks — and shows exactly what can go wrong when a furnace goes years without one.
Furnace maintenance often gets treated as optional, the kind of thing that gets put off when the furnace still seems to be working fine. Richard Trethewey's walkthrough makes the case for why that's a real mistake, using a genuinely alarming real-world example rather than just a general efficiency argument. The HVAC systems guide covers all five major home heating and cooling systems for a broader overview.
If your furnace is showing signs of a real problem rather than needing routine upkeep, the guide to furnace repair cost covers that decision directly. And if maintenance reveals your furnace is beyond the point where repair makes sense, the guide to furnace replacement cost covers what a new system costs.
How a Furnace Actually Works
Understanding what a maintenance technician is actually checking starts with understanding the basic anatomy of a gas furnace. Air returns through the return air duct, and a blower pushes it up through the unit and out through the ductwork to heat the building. Separately, on the combustion side, gas burners and a gas valve ignite into a sealed component called the heat exchanger. The exhaust from that combustion — caustic, containing carbon monoxide and carbon dioxide — vents out to a chimney or outside. The air being heated for the house passes by the heat exchanger and picks up heat from it, but critically, never mixes with the combustion exhaust — as long as the heat exchanger remains intact.
What a Real Maintenance Visit Checks
A proper furnace service call follows a real checklist, not a quick glance. A technician checks the incoming gas pressure from the supply line and the pressure at the gas valve itself, since that pressure determines how much gas mixes at the burners. They check the safety devices throughout the unit to confirm they're all operational. And they run a combustion analysis with a dedicated analyzer that reads temperature, carbon monoxide, and carbon dioxide levels — data that shows whether the burn is properly dialed in, too hot, or not clean enough. This isn't something a homeowner can eyeball; it requires the actual analyzer to know for certain.
Why This Isn't Just an Efficiency Question
The most common assumption about furnace maintenance is that skipping it just means losing some efficiency. Richard Trethewey's walkthrough includes a real example that shows why that assumption can be dangerously wrong: a furnace pulled from a home that had never been serviced, with visible cracks running across every section of its heat exchanger. Those cracks meant that every time the burner fired, carbon monoxide that was supposed to vent outside was instead leaking directly into the living space.
The homeowners in that case only discovered the problem because they called a contractor to add air conditioning during an unusually hot summer, and a combustion analysis performed as part of that unrelated job caught it. A regular annual service call — the kind that includes a combustion analysis as standard practice — would have caught the cracked heat exchanger long before it became a genuine carbon monoxide hazard.
A working carbon monoxide detector on every level of the home is a real, separate safety backstop worth having regardless of maintenance schedule, since it can alert a household to a leak between service visits. It's a backup measure, though, not a substitute for the combustion analysis a proper maintenance visit performs — a detector alerts you after carbon monoxide is already present, while a combustion analysis can catch a developing problem like a cracked heat exchanger before it starts leaking into the living space at all.
The Filter's Role in Both Heating and Cooling
Changing the furnace filter is something homeowners should be doing themselves regularly, not just leaving to an annual service visit. A dirty or removed filter doesn't just reduce airflow — it lets dust, pet hair, and lint travel further into the system than they should, and if there's a cooling coil mounted above the furnace for central air conditioning, that debris clogs the coil's fine fins. A clogged coil impedes airflow for both cooling and heating, and if it gets bad enough, cleaning it means partially disassembling the unit and working carefully around delicate fins rather than a simple wipe-down. Using a higher-quality filter than the bare minimum is worth the modest extra cost specifically to avoid reaching that point.
Furnace-Only vs. Combined Heating and Cooling Systems
A basic furnace handles heating only — gas burners, heat exchanger, blower, and nothing else. Many homes have a more elaborate setup: the same furnace with an air conditioning cooling coil mounted directly on top of it, sharing the same blower and ductwork for both heating and cooling. The combustion and heating components work exactly the same way in either configuration — the coil is simply an add-on box that the same airflow passes through. This is part of why filter maintenance matters for heating performance even in a combined system: airflow problems caused by a neglected filter or a clogged cooling coil affect both sides of the system, not just whichever one happens to be running that season.
How Often to Schedule Furnace Maintenance
An annual service call is the standard recommendation, ideally scheduled before the heating season starts. For a home with a combined heating and cooling system sharing the same air handler, an equivalent seasonal check before cooling season is worth the same consideration, since the same airflow and coil-condition issues apply to the cooling side.
What a Furnace Maintenance Visit Costs
Professional furnace maintenance typically costs $100 to $300 per visit for a standard inspection and tune-up, including filter replacement, safety device checks, and a combustion analysis. Cost varies based on region and whether the visit is a standalone maintenance call or bundled into a service plan that also covers priority scheduling or discounted repairs.
What to Look for When Hiring a Furnace Maintenance Provider
Ask specifically whether a combustion analysis is a standard part of the visit, not an optional add-on — this is the single test that would have caught the cracked heat exchanger in Richard Trethewey's example, and a maintenance visit without it is missing the most important safety check a technician can perform.
Ask what safety devices are checked as part of the visit, and confirm the technician tests rather than simply glances at each one.
Ask how the technician handles a filter that's clearly overdue for replacement — a provider who treats this as routine, expected maintenance rather than an awkward surprise is giving you a more complete picture of your system's real condition.
If your home has a combined heating and cooling system, ask whether the cooling coil gets inspected during a furnace-season visit, since airflow problems on that coil affect both sides of the system.
Final Thoughts
A furnace maintenance visit is genuinely more than a formality — the combustion analysis alone is capable of catching a hazard that a homeowner would have no way of detecting on their own. Skipping it risks more than losing some efficiency, and the real-world example of a cracked heat exchanger leaking carbon monoxide into a home for who knows how long is exactly the kind of problem an annual service call exists to catch before it becomes serious.
When you are ready to get estimates from licensed contractors in your area, contact Home Services Pros to connect with professionals who offer free no-obligation assessments.
Frequently Asked Questions
Why is furnace maintenance important if the furnace seems to be working fine?
A furnace can appear to be working normally while still developing serious safety issues, like cracks in the heat exchanger that let carbon monoxide leak into the home. A proper maintenance visit includes a combustion analysis specifically designed to catch this kind of hazard before it becomes dangerous, not just an efficiency check.
What does a professional furnace maintenance visit include?
A real maintenance visit includes checking incoming gas pressure and the pressure at the gas valve, testing all safety devices, running a combustion analysis with a dedicated analyzer to check temperature and carbon monoxide levels, and replacing or inspecting the filter.
How much does furnace maintenance cost?
Professional furnace maintenance typically costs $100 to $300 per visit, including a standard inspection, tune-up, and filter replacement. Cost varies by region and whether the visit is bundled into a broader service plan.
How often should a furnace be serviced?
An annual service call is the standard recommendation, ideally scheduled before the start of heating season. Homes with a combined heating and cooling system benefit from an equivalent check before cooling season as well.
Can I do furnace maintenance myself?
Homeowners can and should change the filter regularly, since a dirty filter is one of the most common and preventable issues. However, the combustion analysis, gas pressure checks, and safety device testing that make up the core of a real maintenance visit require professional equipment and training that isn't practical for a homeowner to perform.
What happens if a furnace's heat exchanger cracks?
A cracked heat exchanger allows combustion exhaust — including carbon monoxide — to mix with the air being circulated through the home instead of venting safely outside. This is a genuine safety hazard, and it's exactly the kind of problem a combustion analysis performed during regular maintenance is designed to catch.
