InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Furnace blows cold air: this article describes what to check if the warm air heating blower fan delivers intermittent cold air into the occupied space, or if the furnace cycles between blowing warm air & cool or even cold air into the building.
How to Diagnose & Fix a Blower Fan that Blows Alternating Hot then Cold Air
Question: forced air heating furnace blows COLD, HOT, COLD, then OFF - what's up with its fan limit control?
Fan Limit Switch Problem History
Two story 75 year old classic cape cod home. Gas forced air furnace is Lennox G16 24 years old, in good condition. With Honeywell fan limit switch, outside fresh air intake and draft motor. We have owned home 12 years. (has AC). Owner not a technician but understands mechanical/heating concepts.
Furnace operating problem
When heat called for and fan comes on it blows cold air, then heat, then cold, then off. It has always done this. Limit switch is at off 100, on 125 off 150.
Changing set points on limit switch does not seem to change pattern. I do not dare force it. This winter local utility provider replaced burned out fan motor with same HP, number of speeds, etc. Since the draft motor was noisy we replaced that as well. Upon dissection of draft motor is was covered with white crusty minerals probably from the water from the humidifier mounted on the nearby cold air return. When we first owned this house we have removed the humidifier (and installed a whole house water filter).
The fan limit switch sensor which protrudes into the burn area is crusted and therefore "locked" into a former position and now un adjustable.
What do you think? - J.C. 4/15/2013
Reply: stuff to check when the fan is not turning on and off at reasonable temperatures
Thanks for the interesting question and the excellent photo. Unfortunately the photo you sent had a virus - we had to clean it up so what we display here is a bit different and at at lower resolution that your version. I don't know what's happening with your heating system but I can, from your note make a few diagnostic suggestions:
Watch out: you are quite right not to force the set point adjustments on a fan limit switch. Doing so can make the heating system unsafe, even risking overheating, heat exchanger damage, and fatal carbon monoxide poisoning of building occupants.
The furnace fan limit switch assembly is intended to be adjusted within a safe range by the heating service tech by moving the stops, but if you bend something the switch is damaged and unsafe, and if you set the switch incorrectly (for example fan coming on too late or turning off too early, or setting the upper limit too high) there is risk of overheating the heat exchanger, cracking it, risking fatal carbon monoxide poisoning at worst, or perhaps a costly heat exchanger or furnace replacement job.
I like Dan Holohan's expression "Keep your hands in your pockets" - my mom used to make me sit on my own hands. (No not recently.)
Take a look at the duct system itself for things that can make it cool down quickly blowing cold air sooner than one might expect, such as a long uninsulated run, missing insulation, air leaks at the return side.
Check that the air filter is clean and that the ductwork is not obstructed. (This is just on general principle).
A very simple "test" that most fellows do in the field before touching anything is to just watch the dial as the plenum heats up when the furnace is running. If the dial doesn't turn it's jammed. The installation instructions warn about installation mistakes (like a too-long or improperly-located sheet metal screw) or sensor in contact with ductwork etc. - stuff that can bind the switch operation.
Inspect the limit switch temperature sensor for obstruction or clogging (as you suggest) - to do this you have to turn power off and completely remove the switch - carefully again so as not to get cut nor bend anything. If the helix and heat sensor look clean, then you can jump to step 3 which is probably where a less curious tech might start
Replace the switch with a new unit, at its factory settings.
Keep us posted - what you learn will help other readers diagnosing a fan limit switch problem with their own furnace.
Other Causes for a Heating Furnace to Blow Cool or Cold Air into the Occupied Spaces
Check for air duct leaks
Check for an air duct leak, particularly, leaks from cold areas such as an un-heated crawl space or basement into the return air duct system. Such leaks not only increase heating costs unnecessarily, they also are unsafe.
Check for deliberate introduction of cold air into the HVAC system
In some buildings we like to run the blower fan continuously for more even heat distribution and improved air filtration - improvements to building indoor air quality. But in some buildings where heating ducts run through cold spaces and are not well insulated, or in installations in which the heating system is inducing cold outdoor air into the duct system, when the furnace burners are off the system will indeed blow cold air onto building occupants.
Check the following two switches to make sure that the furnace blower fan is not simply set to run continuously:
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Ask a Question or Search InspectApedia
Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones
Support InspectApedia.com & See Fewer Advertisements
From Google's Contributor website: Contribute a few dollars each month. See fewer ads. The money you contribute helps fund the sites you visit.