Hot Air Heating Furnace Operation FAQs

Q&A to help troubleshooting warm air heating systems

Furnace Short Cycling FAQs

What's causing our heating furnace to short-cycle, turning on for just 4 minutes?

Furnace comes on every hour for just 4 minutes, even though the thermostat is set at 20 deg. C and room temp. is 10 degrees C. - Mary

Reply by (mod):

Mary,

I suspect that your blower fan is not working; when that happens the thermostat may turn on the furnace but because the blower fan isn't working the system gets very hot very quickly and is shut down by the high limit safety control on the fan limit switch.

Take a look at FURNACE FAN STOPS EARLY

Also see the diagnostics beginning

at BLOWER FAN OPERATION & TESTING and also see

• FURNACE FAN CYCLES AFTER HEAT
• FURNACE FAN CYCLES DURING HEAT

One of these is probably the problem you're seeing.

Air Handler Blower Fan FAQs

What turns on the blower fan when our furnace warms up?

What turns the fan on, when a heat exchanger gets warm? - Peter

Reply by (mod) -

The fan limit switch monitors temperature inside the furnace and turns the fan on when the heat exchanger is warm enough.

See FAN LIMIT SWITCH - home

Why does my furnace fan cycle back on briefly at the end of a heating cycle?

I have a forced air furnace that just started having this problem.

When the desired temperature is reached, the furnace shuts off as it normally should but then within a couple seconds the fan starts again but only runs for about 5 seconds, then shuts down and stays off. It runs again normally when the temp drops below the set temperature.

Is this a thermostat problem or a bad switch on the furnace? - Kirk

Reply by (mod) -

Kirk:

Extra residual heat in the heat exchanger will sometimes re-heat the air in the plenum after the blower fan has shut-off, causing it to run again to cool down the system - thus avoiding possible heat damage to the heat exchanger.

You might stop the symptom by having the system cleaned, including the blower fan assembly and of course the air filter. Be sure that there are no duct obstructions.

See our detailed explanation and recommendations

at FURNACE FAN CYCLES AFTER HEAT

Can we use the furnace FAN-ON control to draw cool air out of our basement?

Is there any way to use the fan-on position to draw cold air out of the basement?

My mom used to do something at the furnace to do this, older furnace of course. Gas furnace, forced air...thanks - Betty.

Reply by (mod) -

Betty,

IF your forced warm air heating system ductwork includes a return air inlet in the basement, then yes running the furnace or just the fan will pull air out of the basement, through the furnace (air handler or blower) and push it out of supply ducts including upstairs ducts.

But

Watch out: a duct system that gets return air close to the furnace itself can be unsafe as it can draw combustion products or flue gases out of the burner combustion chamber or flue vent connector and blow them into the occupied space.

That can kill building occupants with carbon monoxide.

So it's worth taking a closer look at the location of the return air ducts on your system.

Use the ARTICLE INDEX found near the end of this article to find the live link to

ADDING RETURN AIR at the AIR HANDLER

 

Furnace blower runs continuously but there's no warm air

I have a forced air gas furnace (high eff.) that blows cold air, no heat.

Blower fan stays running also. What is the problem - On 2012-02-14 by eric

Reply by (mod) -

Eric, check the fan limit switch and the thermostat switch to see if one of them has the fan in "ON" manual mode.

What turns the burner and fan on and off in a two-stage furnace?

What controls the on/off of the burner and fan in a two stage furnace?

That is, when does the high burner turn on vs. the low.

In the morning when heating up the house the high burner and fan and low alternate about every 10-15 minutes depending on the outside temp. Are they controlled by the stat, temp, or time?
Thanks - Rick

Reply by (mod) -

Typically the control board is sensing air flow rate, temperatures, heating demand, and switches the blower speed up as more heat is needed.

(Two speed fans are also used to provide different air flow rates for cooling vs heating. )

Can I Use our Gas Wall Heater if the Fan Isn't Working?

I have a wonderful 50 year old long style vented gas wall heater. The fan motor is out at the moment. Can I still use the heater? - Lisette

Reply by (mod) -

It depends: some heaters have two operating modes, with and without a blower; that would be safe;

Watch out: but if your heater requires the motor for proper cooling or venting it would not be safe to operate it. I can't tell which case applies from just your e-text.

Furnace & Warm Air Heat Operating Temperature FAQs

What's the warm air supply register temperature supposed to be when the fan shuts off?

What should the temp be at the vent in the room with the stat be at the end of the run, just before the fan shuts off. - Rick

Reply by (mod) -

Rick

What is the usual supply air temperature for forced air heating?

A typical supply air temperature measured at a supply register of a forced air heating system is around 110 degF. But there is not a single "right" number for the situation you describe, since the length of vents, insulation, leaks and many other factors will affect supply air temperature and velocity at various supply registers.

Typically a service tech looks at the temperature rise between the temperature of the return air and the supply air measured AT THE FURNACE at installation. You may also find the rated furnace temperature rise on your heater's data tag.

FAN LIMIT CONTROL SETTINGS

gives is the basic operating temperatures measured at the air handler / furnace heat exchanger.

Factory settings at the fan limit switch typically will

1. FAN ON: not let the fan turn on until temperature reaches 20F above the FAN OFF - that's typically around 130F

2. FAN OFF: the low off number is typically 90F

3. FAN LIMIT OFF: the high limit is typically factory set at 200F - a safety level to avoid overheating

Measured downstream in the ductwork the temperature will thus always be lower than right at the heat exchanger / supply plenum.

WARM AIR SUPPLY IMPROVEMENT - at https://inspectapedia.com/heat/Warm_Air_Supply.php
will help you diagnose a too-cool temperature at the supply registers.

Furnace Maintenance FAQs

Should we turn off our furnace at the end of the heating season?

Do you turn gas furnace and gas off at end of season? On 2017-06-08 by Debra

Reply by (mod) -

Debra:

I think most people do not bother to turn off the gas supply to a gas-fired heating furnace at the end of the heating season.

However if I were leaving the home unattended for several months I'd consider that as an added safety step;

Also, at the end of the heating season is a good time to schedule annual service: the heating tech will be less busy and more available.

Furnace Diagnosis & Repair FAQs

Furnace does not turn on when thermostat calls for heat

When the thermostat calls for heat the furnace won't come on. Frank

Reply:

Frank

See NO HEAT - FURNACE for a step by step guide in diagnosing your no heat problem. Often there is a simple problem that a homeowner can fix, such as confirming that the furnace has both power and fuel.

 

Lennox 12R Furnace Only Runs if I hold down a Relay Switch

Hello,
I have a Lennox 12R series unit. It works but only if I push and hold down two "internal" compression switches.

But, as soon as I release pressure on either both simultaneously the system closes down. Can someone tell me what the problem is?

Regards David in Australia - David Drake

Reply by (mod) -

David, sometimes the problem you describe is attributed to a bad relay but that would be a hasty conclusion;

- Without a circuit diagram or IO manual for your specific heating furnace, we can't don't know what each relay is doing

- You will want to confirm that the thermostat is in fact sending a "call for cool" signal to the control board & its relays

Watch out: forcing any heating equipment to run is potentially unsafe, risking fire or other hazards. I'd stop forcing the system to run and get a service tech on site.

Furnace Safety FAQs

CO Alarm goes off after I replaced our GE Furnace Gas Valve

Recently replaced gas valve in a 50 year old GE gas hot home system,now our CO-2 alarm went off 2 times.

Is it possible too much gas coming through the valve? Other reasons?/Could the plenum affect it? - errol bier

Reply by (mod) -

Errol

Almost certainly you meant CO - Carbon monoxide, a fatal gas at high or prolonged exposure. Not CO2 (Carbon Dioxide).

Watch out: I would take the CO alarm VERY seriously as there could be a fatal carbon monoxide hazard in the home.

Start by turning off the suspect gas fired heater including shutting off its gas supply - if the alarm stops then you may have found the problem source.

CO detectors respond to carbon monoxide, not directly to fuel gas in the air, but yes an improperly-adjusted or fueled gas heater could produce dangerous levels of carbon monoxide.

It's time for a visit by a trained service tech.

Why is smoke coming through our air ducts

Smoke coming through the heating air ducts, enough to make ceiling dirty looking why? - Anonymous

Reply by (mod) -

From just that brief text, I don't know. Maybe a dirty filter, a missing filter, or a more serious leaky heat exchanger sending soot through the system.

Watch out: That would be quite dangerous especially if your heating fuel is LP or natural gas.

Safest would be to turn off the system and call for immediate repair.

At the very least, make sure you have working smoke and carbon monoxide detectors properly located, installed and tested.

Furnace Noise FAQs

Constant Furnace Noise Complaint

My system seems to be constantly noisy. I've had the furnaces checked and filters replaced.

Is there such a part as the recheck valve or return valve? Thanks for your help. On 2014-11-02 by kozakda@gmail.com

Reply by (mod) -

Ko

On a warm air furnace, I don't recognize the part names you cite.

Since there are many different possible noise sources - burner, fan motor, duct damper, other parts, I'd start

at HEATING SYSTEM NOISE DIAGNOSIS

Furnace Smells or Odors FAQs

What's the source of heating oil odors when my oil furnace starts

Every time the furnace starts up I get a fume smell? Anonymous said:

Reply:

See the diagnostics and suggestions at OIL BURNER NOISE SMOKE ODORS

Also check out oil leak smell sources at OIL ODOR SOURCES

We Smell Fumes from our Oil Furnace

My hot air oil furnace keeps fuming every now now ands again. I don't know why. - Anonymous<

Reply by (mod) -

Anon with no other information it would be specious for us to pretend to offer a diagnosis,

Watch out: but if there are flue gases escaping from your heating system the system should be serviced promptly, and depending on severity, may need to be shut down immediately - for example if you see smoke, detect intense fumes, if CO or smoke alarms are sounding, or if you see signs of loud rumbling or a puffback (search InspectApedia for PUFFBACK to read those details)

See OIL BURNER SOOT & PUFFBACKS

Miscellaneous Furnace Operation & Use FAQs

Emergency backup heat for our heat pump won't come on

I have a YORK- forced air furnace - the emergency heat does not come on - I have to shut the breaker on and off , the hot air comes back for a bit but then stops.

Repair guy changed my thermostat and charged me for labor - 2 weeks later still same problem. Last week happened I opened and shut breaker and it worked.

Repair guy is now saying he needs to change the whole circuit board + 2 relays pieces $600 and work $300. Can it not be something obvious - - On 2012-02-12 by Claudio - /p>

Reply by (mod) -

Claudio, for backup heat diagnostics see BACKUP HEAT for HEAT PUMPS

where we describe diagnostic steps when backup heat isn't working.

Thermostat is "dead" after I cleaned our air filters

After cleaning the filters my thermostat is frozen. Cannot adjust temp or go to menu. Temperature gauge stays at about 70 degrees with little fluctuation. I have a Lennox heater. - On 2011-12-11 by Dave Koch

On 2012-06-05 by (mod) -

Dave Koch, check first to see that your system has power on.

If power was turned off at the furnace that same circuit may be powering the low voltage transformer that provides power to your room thermostat.

 

Is forced warm air heat (furnaces) bad for the environment?

Is warm air heating bad for the environment? - On 2011-05-05 by Kelsey

Reply by (mod) - no

If you are referring to the outdoor environment in general, warm air heating is not bad for the environment, any more than any other heating method.

If you refer to the indoor environment, warm air heat permits use of air filtration that can improve the indoor air quality in some buildings.

But each heating method has its pros and cons.

The burning of fossil fuels (oil or natural gas) to heat a building does, of course have important implications for the outdoor environment, world and local economies, and world politics.

Those huge concerns are, however, beyond our expertise, but my OPINION is that the hydrocarbon emissions into the environment from a modern high-efficiency furnace burning oil or gas will be extremely small as long as the equipment has been properly adjusted and maintained. And electric heat generally is cleaner still.

Driving out to the movies in a gasoline powered automobile would have a greater impact.