Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsBefore cold weather arrives, homeowners can check thermostat settings, power, the air filter, airflow, exterior openings, and carbon monoxide alarms. Schedule a qualified technician to inspect and service the furnace annually, ideally in fall. Do not attempt repairs involving gas, combustion, internal electrical parts, or mechanical components.
What you can check before heating season
These checks can catch common issues and help you notice when the furnace needs professional attention. They are not a substitute for service or permission to open the furnace cabinet and work on its components. Follow the furnace manual and the instructions for your thermostat and alarms. If you are unsure whether a check is safe or appropriate for your system, stop and contact a qualified technician.
As an Amazon Associate I earn from qualifying purchases.
A useful approach is to make one basic check at a time and note what you observe: whether the thermostat calls for heat, whether the furnace starts, and whether heat reaches the rooms. That information can help a technician understand the problem. Do not use these checks to diagnose or repair a fault inside the furnace.
Quick wins for a faster PC:
Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →1. Confirm the thermostat is calling for heat
- Make sure the thermostat has power and is set to Heat.
- Set the temperature above the current room temperature to call for heat.
- If the thermostat uses batteries, check or replace them as its instructions specify.
If the thermostat appears to be working but the furnace does not respond, continue with the basic power check. If the thermostat display is blank, check its instructions before doing anything beyond the battery check, if applicable. Do not open the thermostat or furnace to inspect wiring.
#1 Best Overall
- 20x25x1 AIR FILTER FOR FURNACE & AC SYSTEMS - Filtrete MERV 5 furnace filters designed for standard HVAC systems. Each 6-pack includes filters with actual dimensions 19.69" x 24.69" x 0.81." Compatible with furnace and air conditioner units requiring 20x25x1 size filters for whole air filtration
- MAKE YOUR HOME YOUR SANCTUARY: 6-pack of 3-month pleated Filtrete 20x25x1 MPR 300 AC Furnace Air Filter helps capture unwanted particles from your household air to contribute to a cleaner, fresher home environment
- DIFFERENT FROM NOMINAL SIZE: Slightly different from the nominal size, this air filter’s exact dimensions are 19.69 x 24.69 x 0.81
- EXCLUSIVE TECHNOLOGY: Designed with exclusive Filtrete 3-in-1 technology to pull in and trap unwanted particles, allowing cleaner air to flow through
- COUNTLESS PARTICLES: With a 300 MPR and MERV 5 rating, your Filtrete Filter will help capture many particles from the air passing through it
After making a setting change, allow the system a chance to respond and observe whether it starts. If it does not, avoid repeatedly changing settings or attempting internal adjustments; move on to the simple power check and then call for service if the issue remains.
2. Check the furnace switch and breaker
If the system is not running, check that its furnace switch is on and that the circuit breaker has not tripped. If a breaker trips repeatedly, stop resetting it and arrange professional service. Do not open electrical panels or furnace electrical components.
This is a basic status check, not an invitation to troubleshoot electrical wiring. If the furnace switch is on and the breaker appears normal but the system still does not run, report that to the technician rather than removing a panel or reaching into the equipment.
3. Inspect the air filter
A dirty or incorrectly installed filter can restrict airflow. Check the filter and replace or clean it according to its label and the furnace manual; turn off the furnace as directed before changing it. Carrier gives a general interval of every 1–3 months for filter replacement or cleaning, while some thicker media filters may last 6–12 months when the manufacturer specifies that interval. The correct schedule depends on the filter and the home, so do not treat either interval as universal. When selecting a replacement furnace air filter, match the existing dimensions and use a type permitted by the furnace manufacturer.
Use the instructions for the particular filter: some are replaced and others are cleaned. Do not assume a filter can be washed or reused unless its label says so. If you cannot determine the appropriate filter type or interval, check the furnace manual or ask a technician. The general 1–3 month interval is a starting point, not a guarantee that every household should follow exactly the same calendar.
Thicker media filters may have a longer stated life, including 6–12 months, but only follow that interval when the filter manufacturer specifies it for the filter being used. A longer interval is not a reason to ignore the filter’s instructions or the furnace manual. If replacing a filter, do not choose a different type based on an assumed benefit; no single rating or style is established for every furnace.
Rank #2
- MADE IN USA: UL 900 Classified Material designed for use in HVAC systems - Polyester Washable Furnace / Air Conditioner Replacement Filter Media.
- CUT TO FIT: Simply cut to any size needed from this 20"x30"x1" pad (20" x 30" x .75" actual)
- WASHABLE: Rinse with low pressure water, allow to dry and reinstall.
- PERFORMACE: Proper balance of Air Flow and Performance
- DESIGNED TO FIT: Many brand name furnaces / air conditioners - Measure existing filter and cut to fit.
4. Clear supply and return airflow
Check that supply registers, which deliver heated air, and return grilles, which draw air back to the system, are open and unobstructed. Move furniture, rugs, curtains, or stored items that block them. If some rooms remain cold after airflow is clear, note the pattern for the technician rather than trying to adjust internal dampers or ductwork.
Recommended Free Tools
Look at both sides of the airflow path: the supply register in a room and the return grille that lets air travel back to the system. An open register alone may not resolve a problem if something else is blocking the return. Avoid closing vents or making changes to ductwork as a substitute for diagnosis. If weak or inconsistent heating continues, tell the technician which rooms are affected and whether the problem is constant or intermittent.
5. Look at accessible outdoor intake and exhaust openings
If your furnace has outdoor intake or exhaust openings, look for obvious blockage only where they are safely accessible. Clear snow from outside vents for fuel-burning appliances, as the U.S. Consumer Product Safety Commission (CPSC) advises. Do not dismantle venting, climb into an unsafe area, or attempt to repair a vent. Call a technician if an opening appears damaged or remains obstructed.
This is a visual check of accessible openings, not a vent inspection or repair. Do not remove covers, take apart venting, or reach into an opening. If snow or another visible obstruction returns, or you cannot safely reach the area, contact a professional rather than taking a risk to clear it.
6. Check carbon monoxide alarms
Make sure CO alarms are installed on every level of the home and outside sleeping areas. Test them regularly according to the manufacturer’s instructions; CPSC’s January 23, 2026 heating-safety guidance recommends testing smoke and CO alarms monthly. An alarm with battery backup is one possible type, but follow the alarm maker’s guidance on placement, power, testing, and replacement. Alarms supplement safe appliance maintenance; they do not replace it.
Do not assume that an alarm on one floor covers the whole home. Check placement against CPSC guidance and the alarm maker’s instructions, and make sure you know how your particular alarm signals a test or an alert. A carbon monoxide alarm with battery backup is a relevant option to consider, but the right alarm and its placement should follow the manufacturer’s directions. Testing and alarm placement are safety steps; they do not make it safe to use a poorly maintained appliance.
Rank #3
- DIFFERENT FROM NOMINAL SIZE: Slightly different from the nominal size, this air filter’s exact dimensions are 19.63 in x 24.75 in x 4.31 in, This item is a 2-pack of 12-month Filtrete Air Filters
- DESIGNED TO FIT: Lennox brand media air cleaners
- MAKE YOUR HOME YOUR SANCTUARY: 1000 MPR + MERV 11 rating helps capture up to 2X more microparticles than MERV alone(2)
- EXCLUSIVE TECHNOLOGY: Designed with exclusive Filtrete 3-in-1 technology to pull in and trap unwanted particles, allowing cleaner air to flow through
- CERTIFIED: This furnace filter is CERTIFIED asthma and allergy-friendly (2) and helps capture dust, lint and other allergens in your household air
Why and when to schedule professional service
Arrange furnace service annually, ideally in fall before the heating season. CPSC recommends annual inspection and service of home heating systems by a trained technician; Carrier and Trane also recommend annual furnace maintenance. Booking before cold weather gives you a chance to address problems before you rely on the system every day.
Annual service is different from the homeowner checks above. Checking a thermostat, filter, vents, or alarms does not inspect the furnace’s combustion, heat exchanger, electrical components, or other internal parts. CPSC’s recommendation is for inspection and service by a trained technician, and the work should be performed by a qualified professional.
A technician’s work may include checks of the venting and intake or exhaust, condensate drainage, burner flame, and thermostat operation. A broader service inspection can also cover the heat exchanger, blower, gas valve, electrical components, flue, ductwork, and fuel supply. The appropriate steps depend on the furnace model and its condition; ask what the visit includes rather than assuming every provider performs an identical checklist.
Free tools Windows power users keep installed
One-click scans. No signup required.
When you arrange the visit, ask whether the provider checks combustion and venting, what components are included in the inspection, and whether the technician is qualified for the work. Ask whether the provider serves your area and what the appointment covers. These questions make it easier to compare the scope of service without assuming that every tune-up includes the same procedures.
Do not treat a brief filter change or visual check as a full inspection unless the provider explains what was inspected. A service visit should be understood by its stated scope, not just by the word “tune-up.” The manufacturer’s maintenance guidance and the furnace’s model and condition matter, so a technician may not perform every listed check on every system.
When choosing a service provider, ask about technician qualifications, whether combustion and venting checks are included, the inspection scope, and whether the provider serves your area. No single provider or service plan is specified here; verify availability and terms directly before booking.
Rank #4
- DIFFERENT FROM NOMINAL SIZE: Slightly different from the nominal size, this air filter’s exact dimensions are 15.63 in x 24.74 in x 4.31 in, This item is a 2-pack of 12-month Filtrete Air Filters
- DESIGNED TO FIT: Lennox brand media air cleaners
- MAKE YOUR HOME YOUR SANCTUARY: 1000 MPR + MERV 11 rating helps capture up to 2X more microparticles than MERV alone(2)
- EXCLUSIVE TECHNOLOGY: Designed with exclusive Filtrete 3-in-1 technology to pull in and trap unwanted particles, allowing cleaner air to flow through
- CERTIFIED: This furnace filter is CERTIFIED asthma and allergy-friendly (2) and helps capture dust, lint and other allergens in your household air
When basic checks are not enough
Call a qualified HVAC professional if the simple checks do not restore heat, the furnace repeatedly shuts off or short-cycles, heating is weak or inconsistent, or unusual noises or persistent smells continue. Share what you observed, when the problem occurs, and any changes you made to the thermostat or filter. Do not remove panels or try to diagnose a combustion or electrical fault yourself.
Windows Errors? Fix Them Before They Spread
Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallCrashes, No Sound, or Screen Glitches?
Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteFor example, tell the technician whether the furnace does not start at all or starts and shuts off, whether the problem affects the whole home or particular rooms, and whether it continues after you clear an obstruction or check the filter. These observations can help describe the issue, but they do not establish its cause. If basic settings and airflow checks do not resolve the problem, the next step is professional service—not repeated resets or opening the furnace.
Leave work involving gas supply, burners and combustion, the heat exchanger, internal electrical parts, and mechanical components to a qualified professional. Homeowner maintenance is limited to accessible settings, filters, clear airflow, and obvious exterior obstructions—not furnace repair. Even if a problem seems minor, do not attempt an internal repair based on a symptom alone.
What to do if you suspect gas or carbon monoxide
If you smell or hear escaping gas
Leave immediately and contact the local gas authority from outside. Do not stay inside to search for the source or attempt to fix the furnace. Follow the instructions of emergency responders or the gas authority before returning. This is an emergency response, not a reason to continue the tune-up checklist.
If a CO alarm sounds or someone has possible symptoms
- Get everyone outside to fresh air immediately, including pets if you can do so without delaying your exit.
- Call emergency services or the fire department from outside and account for everyone in the home.
- Do not reenter to investigate, silence the alarm, or retrieve belongings. Wait until responders say it is safe.
- If anyone may have CO-related symptoms, get them outside and seek emergency help.
Do not try to identify the source by inspecting the furnace while people remain inside, and do not dismiss an alarm because the furnace seems to be operating normally. CPSC’s guidance is direct: “Never ignore an alarming CO alarm! It is warning you of a potentially deadly hazard.” Wait for responders to say it is safe before reentering.
Frequently Asked Questions
What furnace maintenance can I do myself?
You can check that the thermostat is powered, set to Heat, and calling for heat; check the furnace switch and breaker if the system is not running; inspect and replace or clean the filter as directed; clear blocked supply registers and return grilles; look for obvious blockage at safely accessible outdoor intake and exhaust openings; and test CO alarms according to their instructions. Do not open the furnace or attempt gas, combustion, internal electrical, or mechanical repairs.
Best Value
- Genuine AprilAire 213 Filters are engineered to the exact specifications of your AprilAire Air Purifier, delivering superior filtration, a perfect airtight fit and seal, and the best overall system protection. To maintain peak performance, always choose certified AprilAire Replacement Filters.
- MERV 13 Filtration – The AprilAire 213 removes up to 97% of common allergens and 92% of viruses* *Virus performance results based on controlled testing in accordance with ASHRAE 52.2 test standard using the MS-2 virus
- Advanced Purification – The AprilAire 213 Replacement Filter removes 15% more harmful particles compared to minimum MERV 13 requirement* *Harmful particles defined as 0.30 to 1.0μm. Testing was verified by 3rd party lab. All results follow ASHRAE 52.2 2017 Testing Method.
- Energy Efficiency – The AprilAire 213 lasts up to 2X longer* for fewer replacements and less strain on your HVAC system, saving you money *Performance results based on controlled testing in accordance with ASHRAE 52.2 2017 test procedure. MERV 11 and 13 Filters only.
- Buy with Confidence – Genuine AprilAire 213 Air Filters are designed and made in the USA by AprilAire, the leader in Indoor Air Quality solutions for the home since 1954
How often should a homeowner change a furnace filter?
Carrier gives a general interval of every 1–3 months for filter replacement or cleaning. Some thicker media filters may last 6–12 months when the filter manufacturer specifies that interval. Follow the filter label and furnace manual, since the right schedule depends on the filter and home.
How often should a furnace be professionally serviced?
Schedule inspection and service annually, ideally in fall before the heating season. CPSC recommends annual inspection and service of home heating systems by a trained technician, and Carrier and Trane also recommend annual furnace maintenance.
What does a professional furnace tune-up include?
The scope depends on the furnace model and condition. Technician checks may include venting and intake or exhaust, condensate drainage, burner flame, and thermostat operation. A broader inspection may cover the heat exchanger, blower, gas valve, electrical components, flue, ductwork, and fuel supply. Ask the provider what is included, including whether combustion and venting checks are performed.
What should I do if my carbon monoxide alarm sounds?
Get everyone outside to fresh air immediately, call emergency services or the fire department from outside, account for everyone, and do not reenter until responders say it is safe. If anyone may have CO-related symptoms, get them outside and seek emergency help. Do not investigate the source while people are inside.
Where should carbon monoxide alarms be installed, and how often should I test them?
CPSC guidance calls for alarms on every level of the home and outside sleeping areas. Follow the alarm maker’s placement and testing instructions. CPSC’s January 23, 2026 heating-safety release recommends testing smoke and CO alarms monthly.
When should I stop troubleshooting and call a professional?
Call a qualified HVAC professional if basic checks do not restore heat, the furnace repeatedly shuts off or short-cycles, heating is weak or inconsistent, or unusual noises or persistent smells continue. If you smell or hear escaping gas, leave immediately and contact the local gas authority from outside. If a CO alarm sounds, follow the emergency steps rather than troubleshooting.
The Bottom Line
Before heating season, check the thermostat, power, filter, airflow, safely accessible outdoor openings, and CO alarms. Arrange professional furnace service every year, ideally in fall, and leave gas, combustion, internal electrical, and mechanical work to a qualified technician. If you suspect escaping gas or a CO emergency, leave and contact the appropriate emergency help from outside.
What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
