Key takeaways
- A gas furnace keeps combustion gases separate from your indoor air. The heat exchanger is the part that makes that possible, and it is the part you most want inspected.
- Replace or clean the filter on schedule and have the furnace inspected before the first cold stretch, not during it.
- Install carbon monoxide alarms on every level and near sleeping areas. If one sounds, get everyone outside first.
- Age, repair history, efficiency, safety, and comfort all matter when deciding between repair and replacement. No single one of them settles it.
How a gas furnace heats your home
A gas furnace runs the same sequence every cycle. The thermostat calls for heat, and the inducer fan starts to clear the combustion chamber. A pressure switch confirms that air is moving through the vent. Then the igniter heats up, the gas valve opens, and the burners light.
As the heat exchanger warms, the blower starts and pushes house air across the outside of it and into your ducts. The exhaust gases stay inside the heat exchanger and leave through the vent pipe. They are not supposed to mix with the air you breathe. That separation is why the heat exchanger is the most important safety part in the furnace.
When the house reaches the set temperature, the burners shut off and the blower runs a little longer to move the remaining heat out.
Many homes in Southwest Ohio have a furnace in the basement and ductwork that has been extended or altered over the decades through additions and basement finishing. A furnace can be in good shape while the ducts around it cause the complaints, so uneven heat is not always a furnace problem.
Warning signs worth acting on
Most furnace trouble gives notice before it becomes a no-heat call. Watch for a furnace that turns on and off in short bursts, new banging, squealing, or rattling, a burner flame that looks yellow instead of mostly blue, rooms that no longer heat evenly, and heating bills that climb without a change in how you live.
A carbon monoxide alarm is a different category. Treat it as an emergency, not a maintenance item. The steps are in the carbon monoxide section below.
Some of these signs have simple causes, such as a clogged filter. Others point to a cracked heat exchanger, a failing ignition part, or a venting problem, and those are jobs for a HVAC contractor. Opening the burner compartment or adjusting gas components yourself is not safe.
The furnace warning signs article goes through each one and explains what is likely behind it.
Filters and airflow
The filter protects the blower and the heat exchanger from dust, and it is the one part of the system you are expected to handle yourself. A clogged filter makes the furnace work harder, can cause it to overheat and shut off, and shortens the life of the equipment.
Filters come in different thicknesses and MERV ratings. A higher rating catches smaller particles but also resists airflow more. Many residential furnaces are not designed for the densest filters, so a higher number is not automatically better. Check what your furnace manufacturer allows.
How often you change it depends on the filter type, the size of the home, and whether you have pets or someone with allergies. Heating season is also when the house is closed up, and dust and pet hair build up faster.
The furnace filters article covers MERV ratings, change schedules, and the airflow tradeoffs in more detail.
Annual tune-up before heating season
Fall is the right time for a furnace inspection, before the first hard freeze and before contractors are booked with no-heat calls. A good tune-up is more than a quick look. A HVAC contractor should inspect the heat exchanger, test the safety controls, check the burners and ignition, measure the gas pressure, confirm the venting is intact and draws properly, check the condensate drain on high-efficiency models, clean what needs cleaning, and test the blower and the thermostat.
This is also when carbon monoxide risks get caught. A blocked flue, a loose vent connection, or a cracked heat exchanger can be present with no obvious symptoms. Many manufacturers require regular maintenance to keep a warranty valid, so check your paperwork.
Between visits, you can keep the area around the furnace clear, keep supply and return vents unblocked, and check the filter. Leave anything involving gas, venting, or electrical components to a professional.
See maintenance for what a seasonal visit typically covers.
Carbon monoxide safety
Carbon monoxide is a colorless, odorless gas produced when fuel burns incompletely. A furnace in good condition, properly vented, should not put it into your home. A cracked heat exchanger, a blocked or disconnected flue, or poor combustion air can.
Put a carbon monoxide alarm on every level of the home and near sleeping areas, and follow the manufacturer's instructions for placement and replacement. Test them regularly and replace the batteries as directed. Alarms have an end-of-life date printed on them, so check it.
If an alarm sounds, get everyone, pets included, outside into fresh air first. Then call 911 or your gas utility's emergency line from outside. Do not go back in to find the source, and do not silence the alarm and carry on. Have the furnace and any other fuel-burning equipment inspected before using them again.
Headaches, dizziness, nausea, or flu-like symptoms that ease when you leave the house, especially when several people are affected, are another reason to get outside and call for help. If you ever smell gas, leave immediately and call from outside as well.
Repair or replace
Furnaces do not wear out on a calendar, but age matters. Many last somewhere around 15 to 20 years, depending on the equipment, the installation, and how well it was maintained. Past that range, parts get harder to find and failures become more likely.
The questions that help are practical ones. How old is it? How often has it needed repair lately? Is the problem a small part or a major component such as the heat exchanger? Is the home comfortable, and are the bills reasonable? Is there any safety concern?
A single minor repair on a furnace in decent shape is usually worth doing. A cracked heat exchanger, repeated breakdowns, or a furnace that is old and inefficient shifts the picture toward replacement. If you replace the furnace, it is also a good time to look at ductwork, the thermostat, and whether a heat pump would suit your home.
The repair or replace article walks through each factor.
Efficiency ratings explained
AFUE, or annual fuel utilization efficiency, tells you how much of the fuel a furnace turns into heat for your home over a season. A rating of 80 percent means about 80 percent of the fuel becomes usable heat and the rest leaves through the exhaust. Standard-efficiency furnaces are commonly around 80 percent. Condensing furnaces, which pull extra heat out of the exhaust, are rated 90 percent or higher.
Condensing models vent through plastic pipe, usually out a side wall, and produce condensate that has to drain. In a basement that gets cold, the drain line needs to be installed and protected properly so it does not freeze.
A higher AFUE helps most in a home that uses a lot of heat. It will not fix leaky ducts, poor insulation, or a furnace that is the wrong size. Sizing should come from a proper load calculation, not from matching the old unit. A furnace that is too large cycles on and off too often and heats unevenly.
If you have questions about your own equipment, check the FAQ or reach out through contact.
Frequently asked questions
How often should a gas furnace be inspected?
Once a year, ideally in early fall before heating season starts. An annual inspection checks the heat exchanger, venting, and safety controls, which are the things most likely to cause a carbon monoxide problem.
Is it normal for a furnace to smell the first time it runs in the fall?
A light, dusty smell for a short time is common, because dust has settled on the heat exchanger over the summer. A smell of gas, burning plastic, or anything that lingers is not normal. If you smell gas, leave the house and call your gas utility from outside.
Can I put a carbon monoxide alarm near the furnace?
Follow the alarm manufacturer's directions, but the usual advice is to place alarms on each level and near bedrooms rather than right beside fuel-burning equipment. Test them regularly and replace them when they reach their end-of-life date.
Does a higher AFUE always mean lower heating costs?
A higher AFUE means less fuel is lost through the exhaust, but the result depends on the whole home. Leaky ducts, poor insulation, and incorrect sizing can cancel out much of the benefit.
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