How JustAnswer Works:

  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.

Ask Tom Your Own Question

Tom
Tom, HVAC Technician
Category: HVAC
Satisfied Customers: 770
Experience:  16+yrs HVAC/R service. Residential Commerical Industrial
18550064
Type Your HVAC Question Here...
Tom is online now
A new question is answered every 9 seconds

Hi. We have a Lennox G23 series furnace about 11 years old.

Customer Question

Hi. We have a Lennox G23 series furnace about 11 years old. The unit will work for awhile before the burners will go out and the unit will not turn off. Therefore, the tempature will keep decreasing and the house becomes cold. We must shut off the therostat and then start it again. Sometimes the burners will not come on but the fan does and it blows out cold air. We just changed the therostat because thats what a hvac tech told us to do. We also had the unit cleaned and they could find no problem. Please help. This still happens even with the new therostat.
Submitted: 4 years ago.
Category: HVAC
Expert:  Tom replied 4 years ago.

Hi, Tom here,

 

Does the main blower start when you restart the furnace??

 

 

Customer: replied 4 years ago.
Hi Tom. Is the main blower the second fan that lets out the heat? If so yes. However, sometimes the burners will not come on before the heat comes out.
Expert:  Tom replied 4 years ago.

Ok, just to clarify,,

 

If you remove the front panel on your furnace, you should see a small blower motor about 3 1/2 inches in diameter, connected to the exhaust pipe on your furnace. It is the first thing to run on a call for heat and should run the entire time the burner is on.

 

The main blower is at the bottom of the furnace and is quite large and supplies air to the vents in your house. It should start after the burner lights and run for a minute or two after the burner shuts off.

 

  1. Does the main blower continue to run even after the burner shuts down?
  2. If you look at your thermostat is the fan set to auto or fan on?
  3. Does the house get up to setpoint?

Just need a little more info...I think we'll get it figured out.

Tom

 

 

 

Customer: replied 4 years ago.

Tom,

The main blower continues to run after the burner shuts down. The fan is set to auto. The house sometimes gets up to set point. Thanks.

Expert:  Tom replied 4 years ago.

Ok, here is what it sounds like is going on.

 

IF the heat exchanger in the furnace gets too hot, a hi limit sensor shuts off the main burner AND energizes the blower relay on the control board to keep your fan running until the heat exchanger has returned to a safe temperature.

 

The hi limit control looks something like this:

 

It is located in the front section of the furnace behind the gas valve.

The problem is usually a lack of air flow through the furnace.

 

This can be caused by a dirty furnace filter, too many vents shut off in the house, or a dirty A coil. (The A coil is the A/C coil that sits on top of the furnace that provides cold air in cooling mode, assuming you have central air). Over the years dirt and pet hair (if you have one) can accumulate on the bottom side of the coil and reduce air flow to the point where the high limit acts up.

 

Check the airflow out of your vents to see if you have a strong airflow or seems weak, you have to get your hand close to vent to feel airflow.

 

The other possibilities that cause this could be a slight crack or hole in the heat exchanger causing flue gas to go into the air stream around the limit sensor. This is a serious problem and should be checked out by a tech. One way to tell if you have a crack is to look at the burner flames on your furnace. You should have stable blue flames shooting into the heat exchanger. If one or more of them suddenly change, burn back or become disturbed when the main blower kicks in you probably have a problem in the heat exchanger and need to have it checked immediately.

 

Last, you may simply have a defective hi limit sensor. For testing purposes ONLY, you can remove the two wires, (usually red) and temporarily hook them together to see if it turns the fan off. Although it is only 24 volts going to the wires, make sure power is off if you try jumping them and make sure your jumper doesn't touch any metal in the furnace or you'll likely cause more problems.

 

If you are not comfortable trying any of these suggestions, by all means get a qualified furnace guy back out there. A hi limit fault is serious as it indicates an overheat condition and needs to be rectified.

 

Check the simple stuff first. (filter, airflow).

 

I hope it turns out to be minor. Feel free to reply with follow up questions. Thanks.

Tom

 

Tom, HVAC Technician
Category: HVAC
Satisfied Customers: 770
Experience: 16+yrs HVAC/R service. Residential Commerical Industrial
Tom and other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

tom,

 

Thanks for your suggestions. We don't have any questions now. Thanks for your help!

Expert:  Tom replied 4 years ago.

OK, thank you for the accept. Let me know if you have more questions. You can reply to this thread without clicking on accept.

 

Thanks, Tom

JustAnswer in the News:

 
 
 
Ask-a-doc Web sites: If you've got a quick question, you can try to get an answer from sites that say they have various specialists on hand to give quick answers... Justanswer.com.
JustAnswer.com...has seen a spike since October in legal questions from readers about layoffs, unemployment and severance.
Web sites like justanswer.com/legal
...leave nothing to chance.
Traffic on JustAnswer rose 14 percent...and had nearly 400,000 page views in 30 days...inquiries related to stress, high blood pressure, drinking and heart pain jumped 33 percent.
Tory Johnson, GMA Workplace Contributor, discusses work-from-home jobs, such as JustAnswer in which verified Experts answer people’s questions.
I will tell you that...the things you have to go through to be an Expert are quite rigorous.
 
 
 

What Customers are Saying:

 
 
 
  • You did one super job of explaining to me everything there is to know about this fridge. I'm looking forward to asking you questions in the future. Jimmy Bagley, IA
< Last | Next >
  • You did one super job of explaining to me everything there is to know about this fridge. I'm looking forward to asking you questions in the future. Jimmy Bagley, IA
  • Wonderful service, prompt, efficient, and accurate. Couldn't have asked for more. I cannot thank you enough for your help. Mary C. Freshfield, Liverpool, UK
  • This expert is wonderful. They truly know what they are talking about, and they actually care about you. They really helped put my nerves at ease. Thank you so much!!!! Alex Los Angeles, CA
  • Thank you for all your help. It is nice to know that this service is here for people like myself, who need answers fast and are not sure who to consult. GP Hesperia, CA
  • I couldn't be more satisfied! This is the site I will always come to when I need a second opinion. Justin Kernersville, NC
  • Just let me say that this encounter has been entirely professional and most helpful. I liked that I could ask additional questions and get answered in a very short turn around. Esther Woodstock, NY
  • Thank you so much for taking your time and knowledge to support my concerns. Not only did you answer my questions, you even took it a step further with replying with more pertinent information I needed to know. Robin Elkton, Maryland
 
 
 

Meet The Experts:

 
 
 
  • gotoman for ac

    HVAC Technician

    Satisfied Customers:

    4657
    51 years of experience install design and repair of all makes and models of AC
< Last | Next >
  • http://ww2.justanswer.com/uploads/SE/ServiceTechRus/2012-6-7_2119_056.64x64.JPG gotoman for ac's Avatar

    gotoman for ac

    HVAC Technician

    Satisfied Customers:

    4657
    51 years of experience install design and repair of all makes and models of AC
  • http://ww2.justanswer.com/uploads/AL/alumalite/2012-5-10_234018_shutterstock506249951forJA500.64x64.jpg Phil's Avatar

    Phil

    Mechanical Engineer

    Satisfied Customers:

    3888
    Retired HVAC/ Electrical & Boiler contractor. Industrial
  • http://ww2.justanswer.com/uploads/skibum0101/2008-12-16_015819_just_answer.jpg Rick Mather's Avatar

    Rick Mather

    HVAC Technician

    Satisfied Customers:

    3132
    35 years experience, HVAC
  • http://ww2.justanswer.com/uploads/RY/Rygis20/2011-5-7_3143_DSC5324.64x64.JPG Matthew's Avatar

    Matthew

    HVAC Technician

    Satisfied Customers:

    1994
    14 years experience in the HVAC field of service, installation, Manual J & D calculations
  • http://ww2.justanswer.com/uploads/KE/kentguidry/2011-9-26_214316_0059.64x64.jpg Kenton Guidry's Avatar

    Kenton Guidry

    HVAC Technician

    Satisfied Customers:

    962
    12 years install/service experience.
  • http://ww2.justanswer.com/uploads/solarman1947/2009-12-13_223915_phpkKBcXq_c2PM.jpg Rick Ridley's Avatar

    Rick Ridley

    UK Heating Engineer

    Satisfied Customers:

    871
    Retired But have had 30 years on the tools, Ex CORGI
  • http://ww2.justanswer.com/uploads/Ironheadfan/2009-12-18_222416_pump_005.jpg Tom's Avatar

    Tom

    HVAC Technician

    Satisfied Customers:

    770
    16+yrs HVAC/R service. Residential Commerical Industrial
 
 
 
Chat Now With A HVAC Technician
Tom
Tom
Journeyman HVAC/R service tech
770 Satisfied Customers
16+yrs HVAC/R service. Residential Commerical Industrial