Furnace Maintenance: Cost, Schedule, and What's Included
Richard Trethewey, the longtime plumbing and heating expert on This Old House, walks through what a furnace maintenance visit checks, and shows what can go wrong when a furnace goes years without one.
Furnace maintenance often gets treated as optional, something to put off while the furnace still seems to work. Trethewey’s walkthrough makes the case that this is a mistake, using an alarming example rather than a general efficiency argument. For how a furnace compares with other heating systems, see our complete guide to home HVAC systems.
If your furnace has an actual problem rather than needing routine upkeep, the guide to furnace repair cost covers that decision. If maintenance shows your furnace is past the point where repair makes sense, the guide to furnace replacement cost covers what a new one costs.
How a Furnace Works
Knowing what a technician checks starts with the basic anatomy of a gas furnace. Air comes back through the return duct, and a blower pushes it through the unit and out through the ductwork to heat the house.
Separately, on the combustion side, gas burners and a gas valve fire into a sealed component called the heat exchanger. The exhaust from that combustion, which contains carbon monoxide and carbon dioxide, vents to a chimney or outside.
The air heating the house passes over the heat exchanger and picks up its heat, but it never mixes with the combustion exhaust, as long as the heat exchanger stays intact.
What a Proper Maintenance Visit Checks
A proper furnace service call follows a checklist, not a quick glance:
Gas pressure: the incoming pressure from the supply line and the pressure at the gas valve, which determines how much gas mixes at the burners.
Safety devices: each one tested to confirm it works.
Combustion analysis: a dedicated analyzer reads temperature, carbon monoxide, and carbon dioxide to show whether the burn is dialed in, too hot, or not clean enough.
None of this can be judged by eye. The combustion analysis in particular needs the analyzer to give a reliable answer.
Why This Isn’t Just an Efficiency Question
The usual assumption is that skipping maintenance just costs some efficiency. Trethewey shows why that 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 should have vented outside leaked into the living space instead.
The homeowners found out only because they called a contractor to add air conditioning during an unusually hot summer, and a combustion analysis done for that unrelated job caught it. A regular annual service call that includes a combustion analysis would have caught the cracked heat exchanger long before it became a carbon monoxide hazard.
Carbon Monoxide Detectors Are a Backup, Not a Substitute
A working carbon monoxide detector on every level of the home is a separate safety backstop worth having whatever your maintenance schedule, since it can warn you of a leak between visits.
But a detector alerts you after carbon monoxide is already present. A combustion analysis can catch a developing problem, like a cracked heat exchanger, before it starts leaking into the house at all.
The Filter’s Role in Both Heating and Cooling
Changing the furnace filter is a job homeowners should do themselves regularly, not leave to the annual visit. A dirty or missing filter does more than reduce airflow:
It lets dust, pet hair, and lint travel deeper into the system.
If a central AC coil sits on top of the furnace, that debris clogs the coil’s fine fins.
A clogged coil restricts airflow for both heating and cooling.
Cleaning a badly clogged coil means partly taking the unit apart and working around delicate fins, not a simple wipe-down.
A better filter than the bare minimum is worth the modest extra cost to avoid reaching that point.
Furnace-Only vs. Combined Heating and Cooling Systems
A basic furnace handles heating only: gas burners, heat exchanger, blower. Many homes add an air conditioning coil mounted directly on top, sharing the same blower and ductwork for heating and cooling.
The combustion and heating parts work the same either way. The coil is an add-on box the same airflow passes through. That is why filter care matters for heating even in a combined system: a neglected filter or clogged coil affects both sides, whichever one is running that season.
How Often to Schedule Furnace Maintenance
An annual service call is the standard recommendation, ideally before heating season starts. In a home where heating and cooling share the same air handler, an equivalent check before cooling season is worth the same consideration, since the same airflow and coil issues apply.
Boilers get the same kind of combustion test. The guide to boiler maintenance shows the actual analyzer readings from a well-tuned system.
What a Furnace Maintenance Visit Costs
Professional furnace maintenance typically costs $100 to $250 per visit for a standard inspection and tune-up, including filter replacement, safety device checks, and a combustion analysis.
Cost varies by region and by whether the visit is a standalone call or part of a service plan that also covers priority scheduling or discounted repairs.
What to Look for When Hiring a Furnace Maintenance Provider
A combustion analysis as standard. Ask whether it is part of every visit, not an add-on. It is the test that would have caught the cracked heat exchanger in Trethewey’s example.
Safety devices tested. Ask which safety devices are checked, and confirm each one is tested rather than glanced at.
Routine filter handling. A provider who treats an overdue filter as normal, expected maintenance gives you a more complete picture of your system’s condition.
The cooling coil. In a combined heating and cooling system, ask whether the coil is inspected during the furnace visit, since coil airflow problems affect both sides.
Final Thoughts
A furnace maintenance visit is more than a formality. The combustion analysis alone can catch a hazard a homeowner has no way of detecting.
Skipping it risks more than lost efficiency. A cracked heat exchanger leaking carbon monoxide into a home for an unknown length of time is exactly the problem an annual service call exists to catch.
Book a furnace tune-up before the cold arrives: Home Services Pros matches homeowners with licensed HVAC technicians in their area for free, no-obligation estimates, or call (833) 586-2038.
Frequently Asked Questions
Why is furnace maintenance important if the furnace seems to be working fine?
A furnace can seem to work normally while developing a serious safety problem, like cracks in the heat exchanger that let carbon monoxide into the home. A proper visit includes a combustion analysis designed to catch that hazard before it becomes dangerous.
What does a professional furnace maintenance visit include?
It includes checking the incoming gas pressure and the pressure at the gas valve, testing every safety device, running a combustion analysis 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 $250 per visit, including a standard inspection, tune-up, and filter replacement. Cost varies by region and by whether the visit is part of a service plan.
How often should a furnace be serviced?
Once a year, ideally before heating season starts. Homes where heating and cooling share an air handler also benefit from a similar check before cooling season.
Can I do furnace maintenance myself?
You can and should change the filter regularly, since a dirty filter is one of the most common, preventable problems. The combustion analysis, gas pressure checks, and safety device testing at the core of a proper visit need professional equipment and training.
What happens if a furnace’s heat exchanger cracks?
A cracked heat exchanger lets combustion exhaust, including carbon monoxide, mix with the air circulating through the home instead of venting outside. It is a serious safety hazard, and exactly the kind of problem a combustion analysis during regular maintenance is designed to catch.
