Homeowner answer library
Furnace answers for your home.
Find questions about furnace sounds, controls, comfort, care, and repair decisions.
Browse all equipment topicsStart with your question.
Choose the question closest to yours. Read the full answer for the details and next steps.
100 answers
Safety and urgent help
My furnace smells like gas each time it starts. Is that normal?
The furnace gives a strong whoosh after gas seems to flow before ignition.
I bypassed furnace limit switches while chasing repeated shutdowns.
Why do I smell gas outside near the furnace exhaust on very cold days?
The utility found no leak. Why does the furnace exhaust still smell unusually strong?
The gas utility and HVAC company disagree. Who decides whether my furnace is safe?
The furnace runs poorly and we get headaches, but our CO alarm reads zero. Can we keep using it?
A new furnace still has a strong gas-like smell. Is that a normal new-equipment smell?
Reusing the fan wire powered the thermostat but I lost independent fan control.
A strange furnace noise starts before heating in extreme cold and advice conflicts.
I turned down the blower speed to reduce noise, but it barely helped.
A repair stopped freezing by closing the combustion-air duct. Is that safe?
Snow buries the furnace intake and the house loses heat
Power came back after a winter storm, but the furnace will not heat
Are pressure switches with the same printed rating interchangeable?
Bypassing a pressure switch caused forceful ignition and a gas smell
I came home from vacation to no heat despite leaving the thermostat on hold
Diagnosis and repair
The gas smell returns after four service visits and a replaced gas valve.
A failed furnace blower also stopped my air conditioning before a heat wave.
A working furnace is called defective because it was built with bad parts.
The furnace fails for hours or days but works when the technician arrives.
The furnace stops below target but shows a normal status light.
A new two-stage furnace cycles every few minutes from its first day.
Heating trouble returned after a condensate-related fault was cleared.
The burners go out before the blower starts. Why does the furnace keep retrying?
A technician said the heat exchanger was safe because the flame looked fine. Is that enough?
Do these photos prove my furnace has a cracked heat exchanger?
My furnace light keeps blinking red, but the heat works. Is it a warning?
The furnace hits its temperature limit after I install a higher-rated filter. Is the filter the problem?
My furnace intake freezes solid again soon after I clear a small opening.
A new furnace repeatedly locks out because frost covers its intake
I replaced the igniter and flame sensor, but the furnace still will not start
The small exhaust fan runs, but the igniter never glows
My furnace makes startup noises, but the inducer does not seem to start
Burners light, but the main blower never starts and heating shuts down
Controls and operation
The blower keeps running after the heat shuts off on a very old furnace.
The furnace shuts down only when the thermostat calls for second-stage heat.
The furnace fan ramps up repeatedly even after the thermostat is disconnected.
The furnace starts again briefly after reaching the thermostat target.
There is a C terminal at the furnace but no common wire at my thermostat.
My smart thermostat's remote temperature sensor stopped connecting after installation.
The furnace works, but my smart thermostat keeps using up batteries.
Heating passed a thermostat installation test months ago but fails when winter arrives.
My electric heating equipment has no labeled common terminal for a smart thermostat.
The thermostat reports loss of power around a utility-installed control switch.
Why are the two wires that run my old furnace not enough to power a smart thermostat?
Will a new thermostat work with my existing equipment interface module?
The furnace keeps heating after the thermostat is set to off
Repair versus replacement
Costs, quotes and choosing help
Two furnace replacement quotes differ sharply for comparable models.
How much of an installed furnace quote is equipment versus installation work?
Can I replace two heating systems in stages rather than all four HVAC units at once?
Financing seems to increase the quoted installed-system price.
How do I tell a real heating diagnosis from a replacement sales pitch?
A furnace problem turned into a quote for a furnace, AC, and water heater. Do I need all three?
My warranty heat exchanger will take months to arrive. Do I have to buy a furnace now?
Is two-stage heat worth paying for if we may move in five years?
Furnace bids include different filter cabinets, warranties, and first-year care
Comfort and hot-water delivery
Design, installation and home fit
The furnace in a home I may buy was made in 1988. What should I investigate?
The high-altitude furnace manual does not match the gas heat-value number I found online.
Should a heating replacement be sized larger for Colorado cold snaps?
The home inspector said the furnace was healthy. Why did a later technician condemn it?
Does the furnace need a different filter from the AC?
How do I compare a two-stage standard furnace with a single-stage high-efficiency one?
The contractor quoted a smaller furnace than an online calculator suggests
Contractors give widely different furnace sizes for the same home
Should a replacement match the old furnace's BTU input or its heat output?
A larger furnace quote is cheaper. Is oversizing still a problem?
Adding AC came with a recommendation for a larger furnace
Incentives, permits and warranties
Does a Colorado emissions-law change mean I must replace my working furnace?
Are older furnace models being priced higher because of new Colorado rules?
The heat exchanger is under warranty. Why am I being charged for the repair?
The manufacturer says my furnace is registered at another address. How do I fix that?
Care and maintenance
We bought the house, and I cannot find the furnace filter. Where should I look?
Are modern air filters only suitable for high-efficiency furnaces?
I did not know the furnace filter needed changing. Have I damaged the system?
Can my older furnace use a thick media-filter cabinet?
Which way should the arrow on my furnace filter point?
Should I change the filter monthly or every three months?
What care does a brand-new furnace need?
A replacement filter with the same printed size does not fit the cabinet.
What does a professional furnace check include that I cannot do myself?
A maintenance visit lasted only minutes and included no tests. Was it useful?
Should a furnace check specifically inspect combustion safety on an old unit?
Is a maintenance membership better value than individual visits?
Correcting the filter direction made the furnace suddenly much louder.
Is a high-efficiency furnace more complicated to maintain?
Leaks, odors and noise
A loud furnace sound starts when heating ends and lasts about a minute.
A boom at the vents appeared after a larger replacement furnace was installed.
The furnace is so loud we have to shout to talk at home.
How can I tell whether the loud motor noise is the inducer or the blower?
The furnace squeals and rattles as its fans slow down.
Can I quiet the furnace closet without blocking the air it needs?
A condensate drain froze inside a warm basement.
Wind pushes furnace exhaust toward the intake and creates frost
Water seems to leak from furnace exhaust or intake pipe joints
No matching questions.
Try a shorter phrase, clear the search, or browse the full answer library.
Browse all answers ↗When your question needs a closer look.
An answer can help you prepare. For a concern about your own equipment, explore the service and what to share before requesting a visit.