Go Back  DoItYourself.com Community Forums > Heating, Cooling, Air Conditioning, Ventilation and T-Stat Controls > Gas and Oil Home Heating Furnaces
Reload this Page >

lennox G40 uh id fan works, call for heat status ok, burner fires, blower starts

lennox G40 uh id fan works, call for heat status ok, burner fires, blower starts


  #1  
Old 11-24-12, 06:07 PM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Question lennox G40 uh id fan works, call for heat status ok, burner fires, blower starts

I have a friend here in good old Buffalo NY with a lennox G40UHx series furnace. In the call for heat, ID fan turns on. would operate for about 2-1/2 minutes or so then shut off. LED 1 is off with Led 2 on slow flash, indicating pressure switch open. Would that not be correct since the ID blower is off? Cycle power to reset watchguard. ID fan starts, glow igniter comes on, burners ignite, Blower motor starts. Here's the dome scratcher! the burner stays on for one minute, then turns off. The blower motor then shuts down as it should. Recycled power again and the same one minute flame off. Any good ideas out there?
 
  #2  
Old 11-24-12, 07:39 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
The manual for your furnace is at:

http://www.heatingandairparts.com/do...%20SERVICE.pdf

If you don't have a copy of it, you should print it out and keep it for reference at your furnace.


A furnace doesn't just turn on. It follows a series of events called the sequence of operation which ultimately leads to heat if everything is working properly, or it will stop at some point if there is something wrong.

The sequence of operation for your furnace is described beginning on page 31 of the manual.

Read through that sequence and see which of your questions you can answer.

If you get stuck, we'll help.
 
  #3  
Old 11-25-12, 11:57 AM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Since it was quite windy, and LED 1 was off, and LED was on low flash, it appears that the vacuum effect of the chimney may be the culprit. With less wind today, all is well. Would it be possible to allow some air into the flue pipe without compromising safety? Any suggestions would be appreciated. Thank you for the info on the service manual. It has much more info than the user guide. I initially thought the flame detector was at fault, but why would the burner stay on for one minute, then shut off?
 
  #4  
Old 11-26-12, 03:41 PM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Lennox G40UH was working fine for two days. Now goes through sequence. Heat demand, LED 1& 2 fast flash, ID fan runs, then glow igniter turns on, burners light, indoor blower starts, burner turns off after 1 minute, indoor blower turns off, led 1 off, led 2 slow flash indicating pressure switch problem. If there was an actual pressure switch problem, all indications are that it would not light at all. Recycle power and the same thing happens over & over. I'm completely baffled right now and at my wits end. any assistance would be greatly appreciated. This is a 911 for the 411!
 
  #5  
Old 11-26-12, 07:24 PM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Is there anyone out there that can help me with this problem? I'm getting desperate!
 
  #6  
Old 11-26-12, 09:15 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
Beginning on page 41 of the manual, the various diagnostic codes and their possible causes are listed.

The code you are experiencing with #1 off and #2 slow flash has possible causes listed as 2.1-2.3 and 3.1.

Use a multimeter to understand exactly when the pressures witch is closing and opening during the operation of the furnace.

You need to be able to measure 24 VAC. Connect one test lead to furnace sheet metal ground or the C terminal on the ignition control board. Use the other test lead to test for voltage.

You should find 24 VAC at one connection of the pressure switch whenever the thermostat is calling for heat. The other side should go to 24 VAC whenever the inducer motor is on and comes up to speed.

Observe when the pressure switch closes during a call for heat and if it opens when the burners shut off after a minute of operating or when the burners shouldn't be shutting off.

The idea is to check to see if it's the pressure switch that is shutting the burners off.

It's quite possible that the pressure switch is closing and allowing the burners to light up, but is later opening and shutting the burners off.

If we verify that is the problem, we can start looking for the specific cause of that problem.
 
  #7  
Old 11-26-12, 10:14 PM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thank you so much for the switch info, could not find any obstruction of the flu or chimney.
 
  #8  
Old 11-27-12, 12:09 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
What were the results of determining when the pressure switch was open or closed?
 
  #9  
Old 11-27-12, 07:08 AM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Haven't been to my friends house yet today, she's picking me up shortly.
 
  #10  
Old 11-27-12, 10:14 AM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
The pressure switch was the problem. intermittent closing and sometimes not at all. pulled off the stak-con connectors and used an automotive fuse as temporary jumper. Told my friend that when she needs heat, she will need to pull off one wire and reattach after ID fan starts. No Lennox distributor in Buffalo. Will search online for part. Thank you very much for your assistance!
 
  #11  
Old 11-27-12, 11:31 AM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 4,469
Upvotes: 0
Received 1 Upvote on 1 Post
The pressure switch is a vital part of the furnace safety system and must not be disconnected in order to operate the furnace. That could easily cause massive amounts of carbon monoxide to be produced and could injure or kill building occupants.

Replacing the pressure switch is a waste of time and money, at least until the much more likely causes of the pressure switch not closing have been investigated. Replacing the pressure switch has about a 2% chance of being the problem, and therefore it is a waste of time and money to replace the pressure switch.

There are a variety of things which are likely to cause the pressure switch issue. You can have a repairman investigate those for you, or perhaps we can coach you through investigating the likely causes of that problem if you are interested in doing so.
 
  #12  
Old 11-27-12, 05:32 PM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
With the pressure switch disconnected, I used a sphygmomanometer bulb and waited. after 15 minutes the vacuum was no longer there. That gave me further indication that the switch may have something to do with it. Since the engineers that designed the sequence were forward thinking, the ID fan must be operating before the ignition sequence can be reset. The switch closure must be opened and reclosed . that requires human intervention, thereby observing the ID fan as operational. Specific instructions were left to ensure that the fan was running. New part, 49L4001, Lennox oem replacement should be here Thursday. I measured voltage across the across the prove terminals a few times and it did go to zero volts at initial sequence start. after three successive random openings, (when voltage reappeared), I was convinced that the pressure switch was the issue. I am very interested to investigate other possible causes. Any thoughts that you may be able to share?
 
  #13  
Old 11-27-12, 05:49 PM
P
Member
Thread Starter
Join Date: Nov 2012
Location: USA
Posts: 9
Upvotes: 0
Received 0 Upvotes on 0 Posts
Since I agree that safety is of the utmost importance, the "department of redundancy department" could suggest that I disassemble the flue piping that goes into a brick chimney (only about 6 feet long with about .5 rise) and look for any obstructions. Your thoughts?
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: