FAQs on a forced warm air heater troubleshooting: blower fan stops too soon:

This article series describes what to check if the warm air heating blower fan seems to stop sooner than it should. Some conditions that cause unexpected furnace fan operation may be dangerous, risking overheating of the furnace heat exchanger which in turn risks cracks and even carbon monoxide leaks.

Q&A: How to Fix a Blower that Stops Before Thermostat is Satisfied

These questions & answers about fixing a furnace that shuts off before the call for heat is satisfied were posted originally at FURNACE FAN STOPS EARLY - be sure to review the diagnosis and repair steps given there.

On 2018-11-27 by (mod) - watch the fan limit control dial to see what's wrong

Tomas,

Watch the dial on The Fan limit switch as the system heats up.

If you see the dial turned to the High when the fan turns off you'll know the system is overheating and that's why it's shutting off.

That could be as simple as a dirty filter or other blockage of air flow through the heat exchanger. Otherwise I would suspect a bad limit control switch.

Watch out: when the furnace fan shuts off early that may indicate overheating at the supply plenum: a condition that risks expensive damage to the heating system and that can even make it unsafe to use.

On 2018-11-27 by Tomas

I have a fairly old Comfortmaker furnace, on which I have replaced a few parts. This heating season, it started doing this: If I set the fan mode to ON, then turn the furnace on, the furnace will light, but once it gets hot, it turns the blower off.

If l set the fan mode to AUTO, the furnace will light, and keep cycling thru lighting and gas going on, then turn itself off, but the blower will never turn on. If I do either of these, it takes a few minutes to reset before the fan will turn on with the ON setting.

The blower capacitor tested bad and I replaced it, but still does the same thing. Took squirrel cage out and cleaned it thoroughly (and oiled bearings), and new air filter. I attempted to take out the flame sensor, but it was fairly rusted and in whooping on it, I broke the brittle ignitor, which I obviously must replace now.

Any idea of what is shutting OFF the blower once it gets up to temp? Seems to be opposite of what it should be doing. A limit or pressure switch? Thank you.

On 2018-11-23 by (mod) - Discharge Blocking warning on a heating furnace

JBook

Discharge Blocking warning on a heating furnace

Watch out: This sounds like a safety issue: a blocked exhaust vent; check for blockages in the vent or near its exit.

Also check for a dirty filter or blower squirrel cage and a reduced airflow problem

On 2018-11-23 by Jbook

Forced air furnace cycles ok until the lower door is closed......then will get 3 blink code about discharge blocking.....when running water discharged normally.

On 2018-10-09 by (mod) - Furnace fan shuts off before desired heat is achieved on thermostat

Anon
Thanks for asking.
My best advice is to check the diagnostic suggestions in the article FURNACE FAN STOPS EARLY. From just your note we've nothing else to go-on.

On 2018-10-08 by Anonymous

Furnace fan shuts off before desired heat is achieved on thermostat

On 2016-12-23 by (mod) - fan shuts off, burner stays on

Zac:

Barring a loose wire or failing control, if the blower cycles OFF during a heat cycle and before the thermostat is satisfied, the burner will keep running; when the supply plenum is warm enough the blower will turn back on.

That design is to avoid blowing cold air on occupants.

It could be that the fan speed is set too high and it's cooling down the heat exchanger.

It could be that the heat exchanger is dirty or sooty and so not enough heat is being transferred fast enough into the to-be-heated air. In this case the system is UNSAFE.

it's time for a service call and a heat exchanger inspection.

Keep me posted.

On 2016-12-23 by Zac

Similar problem as above thread but slightly different. Furnace fires up as normal. Randomly the circulation blower will turn off.

The burner will stay lit. The Circulation blower will then come back on, off, on, random amount of times and then stay on until the thermostat is satisfied.

Burner stays lit during the whole cycle. I replaced the filter which was only slightly dirty. It did this a week ago, seemed fine for a few days, and then did it again today. Any help is greatly appreciated. Thanks

On 2016-11-29 by Rachelle

I live in a single wide mobile home with forced air heat. The furnace has a black reset button between two wires.

The thermostat is a digital programmable set at 59 (away) or 61 (home) with a 1 degree differential. The furnace is not responding to the heat demand until I push the reset button, after which the pilot light "blooms" and the plenum heats, followed by the blower motor starting.

A few minutes later the blower stops and the pilot light returns to standby mode before reaching the programmed temperature and will not start again until the reset button is pressed. Repeat ad infinitum. The landlord refuses to have a professional look at it, and insists it's the solid state thermostat controller.

I disagree and want him to get someone out who knows what they're doing. Any thoughts on cause?

On 2016-01-1 by Esther

My furnace comes on forbout 3 minutes,blower quits. Burner still lit then blower comes on 3 minutes shuts down,burner stays on

On 2015-10-28 by Steve

Rheem oil furnace; just replaced limit switch; on calling for heat- it runs (both blower and fan) then shuts off after few minutes. After all cools down (several minutes) it goes back on again only to turn off again

On 2015-10-17 by Mod - Blower fan will not cut off

Dennis, see the diagnosis and repair at FURNACE FAN WONT STOP

On 2015-10-17 by Dennis

Blower fan will not cut off

On 2014-03-30 by (mod) - Tripping off - usually means that a safety control is shutting down the system. T

Tripping off - usually means that a safety control is shutting down the system.

There could be a failing control or relay, but because safety is involved it makes sense to ask for a check by your heating company.

On 2014-03-29 by Scott - Clare furnace, model # HEMB 70-B

I have a Clare furnace, model # HEMB 70-B and looks to be about 25ys old! The issue I have is that it keeps tripping off. I assume its overheating and shutting off. If I reset the power to the furnace it starts up no problem and heats the house up to the set temp.

For the last week we have woke up to a cold house! I have changed the filter and I pulled the evaporator to clean it but that was fine as it was not dirty at all

in the summer the suction line frosts up as well!

Scott-29/03/14

...

Continue reading at FURNACE FAN STOPS EARLY - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

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  • FURNACE FAN STOPS EARLY
• FAN, AIR HANDLER BLOWER UNIT
• FAN LIMIT SWITCH

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