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Unsafe return air input at furnace © D Friedman at InspectApedia.com Furnace Blower Fan Blows Cold Air or Alternates Blowing Warm then Cold Air

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.

Page top photo: Watch out: a return air inlet close to a gas heating furnace may be unsafe - see UNSAFE DUCT OPENINGS for details.

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How to Diagnose & Fix a Blower Fan that Blows Alternating Hot then Cold Air

Fan limit control diagnosis suggestions (C) InspectAPedia JC

Discussed here: what to check if the forced warm air furnace blows cold air out of the supply registers, or cycles between blowing warm air & 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.

Recent information

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).

My Theory

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 sitOn 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).

Now about that pesky fan limit control, you could

  1. Test the operation of the fan limit switch following the instructions in the article above. Start reading
    at FAN LIMIT SWITCH TROUBLESHOOTING.

    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.
  2. 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
  3. 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:

If you need to introduce outdoor air into a building for air quality reasons, consider an air to air heat exchange system to save on heating costs. Check out

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-12-16 by (mod) - furnace blows cold air - what to check first

Deb:

I would start by looking at the furnace itself: is this an electric, gas or oil unit? If gas or oil you can certainly verify whether or not the burner is shutting down when you'd expect it to keep running.

That can be caused in turn by something as simple as a dirty air filter, or there may be a need for cleaning and service of the heater.

Let us know what you find as that will help others.

More diagnostic details are at FURNACE FAN CYCLES DURING HEAT

On 2020-12-16 by Deb

Our 22 year old furnace just started having intermittent problems. Once every few days, the furnace will kick on as normal, send out heat, but after a few minutes we’ll notice that it’s blowing cool air. If left, it will just keep blowing cool air. When we shut it off, then restart(sometimes up to 5 times)

it resets and works properly. We’ve replaced the flame sensor, the filter is clean. But still the same issue. Help!

On 2019-02-12 - by (mod) -

Lyra

As all the ducts were "fully opened" and now you have hat distribution problems, help me out here, why not simply re-adjust the duct dampers to re-balance the system;

partly-close dampers feeding air into too-hot areas so that more heated air reaches too-cool areas.

On 2019-02-12 by Lyra

Help, we have an up/down duplex with forced air natural gas, installed in 2010. Used to work fine and then during a repair in 2016 all the ducts were fully opened- my mistake.

Since then the downstairs apartment gets cold (like 62-64) with the thermostat set to 68. If we increase the thermostat to 70 the downstairs overheats up to 78 degrees in about 15 minutes-so we turn it down again, and then he is cold.

Should we try more duct adjustments or is this something else? As I said all was working well before the repairs as far as I know (maybe other tenants just didn't complain to me?)

We live in central NY-temps during the winter are from 30 to minus 15. Oddly he does not complain during the really cold weather. There is one furnace in the basement and one thermostat -in the upstairs apt. It seems to me if we open the ducts more fully to the thermostat area then heat will shut off sooner, and downstairs will be even colder..

If we close them the heat will stay on longer and it will get too hot. Am I missing something here?

At my request a few days ago the tenant with thermostat set the fan to be ON all the time -and assures me it's still on. But according to downstairs tenant the fan is not on.
Would it matter if the return/supply "loop" for each apt were separate somehow? I don't know if they are-just thought of it.
So why does the downstairs get so cold when thermostat is 68 and way way too hot at 70? It seems like something else besides ducts is the problem.

Can you say what we ought to check? Two different people worked on the 2016 repair. Is it possible one made an adjustment (or work around) that he should not have? One safety bypass he made was found during the repair and corrected. I don't remember what it was but had to do with wiring.
Any help would be appreciated. My tenant is angry and I want to fix this for him.

On 2019-01-27 - by (mod) -

Tim

The normal operating temperature for a forced air furnace heating in residential use depends on where you measure it.

At the supply plenum (right at the furnace) you may see tempertures between 140F and 170F, lower than that (by about 30F) for a newer high-efficiency unit.

BUT if you measure air temperature right at the supply register in the occupied room it's going to be much lower - maybe 80F. Just how much cooler the air temperature will be at the supply register depends on your ductwork: its routing through cold spaces, the quality of its insulation, air leaks, and other factors.

On 2019-01-26 by Tim

My furnace blows out 80 degrees are is this normal

On 2018-03-11 - by (mod) -

Richard

That could be as simple as a dirty air filter causing the system to overheat.

Or in more detail: the heater elements are not staying on - if there is no air obstruction then that sounds like a control or heater failure.

On 2018-03-11 by Richard

I have an electric furnace when running it blows hot air then you hear a click and cool air comes out,then it will click again and hot air will com out. It runs foir a long time until it reaches temperature and then will turn off. It does this constantly day and night?

On 2015-12-21 - by (mod) -

Check the settings on the fan limit switch - or replace it.

On 2015-12-20 by Len

blower comes on imediately before temp for on is reached

On 2015-01-11 - by (mod) -

Lee the fact that you sometimes get hot air out of the electric furnace suggests that the problem is a wiring or control failure.

On 2015-01-11 by Lee

I have a Nordyne electric furnace the blows hot air when first turned on then cold air at the next cycle so I have to turn it off for hours then I turn it on and get the hot air again and the cycle repeats it's self cold air the next time it cuts on so I have to turn it off for a good while and hope it work later when I turn it back on what could be the problem please help! Thank You


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