Heating furnace operation:

This article lists the sequence of steps in the operation of a modern warm air heating furnace as well as older heating furnaces such as the "octopus" furnace that may have little or no duct work.

Understanding exactly what happens from the moment that the room thermostat calls for heat until the call for heat is satisfied can help troubleshoot a heating furnace that is not working properly.

How does a Warm Air Heating Furnace work? Sequence of Operating Steps

These questions and answers about the operating sequence or steps of heating furnaces - forced air or warm air heating systems - were posted originally at x - be sure to review that article.

Sketch at page top courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

On 2018-07-10 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

On 2018-07-10 15:34:34.806630 by Betty

Is the r e 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

On 2018-02-01 01:44:56.931876 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. )

On 2018-01-28 13:29:30.930177 by Rick

Thanks Dan.
Another question we have is 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

On 2018-01-27 23:57:09.725382 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 at https://inspectapedia.com/heat/Fan_Limit_Switch_L4064B_Settings.php

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.

On 2018-01-27 21:30:49.475794 by Rick

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.

On 2017-06-08 21:16:58.350234 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.

On 2017-06-08 21:15:33.483873 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.

On 2017-06-08 13:19:27.483032 by Debra

Do you turn gas furnace and gas off at end of season?

On 2017-01-27 00:33:44.554109 by Kirk

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?

On 2017-01-17 20:04:17.083545 by Al

When my hot air system blower turns off I hear a clunk or bang after each cycle. What causes this as it is very annoying

On 2016-10-06 19:43:22.740911 by Woodford F Jones

I have a new propane warm air furnace and I'm getting only a 40 degree temperature difference from return air to discharge air. I should be getting 60 degrees for the fan speed. What could be the problem, because it won't warm up house.

On 2016-06-01 00:16:29.796606 by (mod) -

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

- I 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

On 2016-05-31 07:34:17.076430 by David Drake

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

On 2016-04-28 15:48:23.677490 by (mod) -

The fan limit switch. Search InspectApedia.com for FAN LIMIT SWITCH to read details.

On 2016-04-28 14:05:34.423031 by peter

what turns the fan on, when a heat exchanger gets warm?

On 2016-01-29 01:54:15.742165 by joe

Lost heat in gas fired hot air furnace no fire in chamber help

On 2016-01-24 01:32:10.575873 by (mod) -

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

On 2016-01-14 02:45:06.493760 by Lisette

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?

On 2015-12-28 15:35:40.789801 by (mod) -

Errol

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.

On 2015-12-28 11:52:19.487502 by errol bier

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?

On 2014-11-04 22:21:00.773820 by (mod) -

Ko

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

On 2014-11-02 07:54:02.780890 by kozakda@gmail.com

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 2014-06-30 19:22:54.613140 by (mod) -

Jill,

A simple inspection from below or from chimney top is useful but usually incomplete as it can be a bit like peering into a black abyss, even with a strong light, and because chimneys with bends can't be inspected directly. FOr those cases the most thorough inspection is by a professional chimney sweep using a camera.

Details are at

https://inspectapedia.com/chimneys/Chimney_Inspection_Camera.htm

On 2014-06-29 22:29:52.414650 by Jill

How can I ck chimney see if blocked or not

On 2014-05-03 by (mod) -

Sounds as if you may be describing a bad fan limit switch.

On 2014-05-03 by SY W

MY PLENUM HAS A BOX WITH NO RESET AND IS HOLDING UNIY OFF . iNSIDE IT HAS A FLAT DIAL WITHTWO MOVIABLE PINS

On 2014-03-05 by (mod) -

Don't know. Maybe a dirty filter, or a more serious leaky heat exchanger sending soot through the system. That would be quite dangerous esp. If its gas heat.mbe sure you have working smoke and canon monoxide detectors or turn off the system.

On 2014-03-04 15:09:03.451490 by Anonymous

smoke coming through air ducks enough to make ceiling dirty looking why?

On 2014-02-10 15:08:38.625450 by (mod) -

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

On 2014-02-1 by Anonymous

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

On 2013-04-05 by john

i made chamber natural gas fired furnace with ceramic board and ceramic wool to prepare jewelry casting flasks at temperature 750c ..the furnace is preparing my 20 flasks in 5 hours with 2 injector without blower or air but problem is when i close the door tightly furnace does not increase temperature quickly .. The chimney size is 2 inch in center the roof and i made 6 holes in furnace bottom to take oxegen .when i opens the door some temperature increase quickly plz help me about it if can u do?

On 2013-03-01 by Anonymous

a forced warm air furnace eqipped with an atmoshperic burner is over firing. how can the problem be correctd?

On 2013-02-01 by vince

my furnace blows cold air wen it comes on but when i turn it up it blows hot air

On 2013-01-31 by Wendy

our gas furnace makes a slight roar every four minutes during operation and with it comes a 'bubble' of exhaust-like smelling air - has the local gas company here twice and there's no leak of gas or CO which is good but this bubble smells noxious nonetheless - no chimney - just pipes exiting the basement to exterior with one intake pipe and 3 intakes across 3 apartments - any ideas? thank you.

On 2012-06-05 by (mod) -

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

On 2012-06-05 by (mod) -

Claudio, for backup heat diagnostics see BACKUP HEAT for HEAT PUMPS (article links in our ARTICLE INDEX)

On 2012-06-05 19:37:07.229597 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.

On 2012-06-05 19:36:34.636011 by (mod) -

Jo, when the climate in an area means that the split system is the only heat source, we test at the same temperature range as you describe. If you're in a cold-winter climate you may be looking at backup heat in some designs.

On 2012-06-05 14:05:13.588465 by jo

Its inspector should chek split heating sistem on any temperarure or its same standard like heat pump ower 60 below 80 F thanks

On 2012-02-14 14:10:22.774227 by eric

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 2012-02-12 16:21:14.255582 by Claudio

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 2011-12-11 03:46:46.866852 by Dave Koch

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 2011-09-22 12:53:34.576262 by Huckelberry

where does i get one of these im cold

On 2011-05-25 12:44:22.129204 by prasath

any more tips?

On 2011-05-05 17:01:22.322057 by (mod) -

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.

On 2011-05-05 16:39:51.195588 by Kelsey

Is warm air heating bad for the enviroment?

Question: source of heating oil odors when furnace starts

(Dec 5, 2014) Anonymous said:
Every time the furnace starts up I get a fume smell?

Reply:

See the diagnostics and suggestions at OIL BURNER NOISE SMOKE ODORS

Also check out oil leak smell sources at OIL ODOR SOURCES

Question: furnace does not turn on when thermostat calls for heat

7 Feb 2015 Frank said

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

Reply:

Frank

In the ARTICLE INDEX found among the Continue Reading links at the end of this article select the article titled NO HEAT - FURNACE for some help diagnosing your no heat problem.

...

Continue reading at FURNACE OPERATING TEMPERATURES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• NO HEAT - FURNACE

Suggested citation for this web page

FURNACE OPERATING FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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