short cycling old gas furnace


  #1  
Old 12-07-08, 02:07 PM
S
Member
Thread Starter
Join Date: Dec 2008
Location: midwest
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
short cycling old gas furnace

I have an ancient gas furnace (Bryant) that is 40 + years. Some repairs done over the years. WAS working fine. This
* has analog t-stat
* had blower & t-stat replaced 10 years ago
* the filter is new and clean
* replaced a water heater next to furnace (no relation?!)
* had the house roof replaced this fall - workers banging around the flue pipe replacing shingles etc.
Now the symptoms:
The flame comes on as t-stat calls for heat.
Flame on 15-20 sec. and then the blower comes on.
All is well for 60+ seconds. Then flame cuts off (blower still going). After few seconds (20, 30 sec) the flame comes on and stays for 20 sec. The blower is still going. After cycling like this for 3 times or so the blower cuts out. The temp has not reached what the t-stat called for (68-70). Inside temp is 65 and outside in the teens. Plannig to clean the flue pipe.
Any insight welcome (I plan to replace furnace next fall)
-Shantosh
 
  #2  
Old 12-07-08, 02:24 PM
M
Member
Join Date: Jan 2008
Location: Tennessee
Posts: 438
Upvotes: 0
Received 0 Upvotes on 0 Posts
Furnace help

For one thing it does not sound like a 40 year old furnace in 1968 they had standing pilots and you either had fire or you didn't. It sounds to me like flame recognition problems in a furnace with some sort of electronic ignition. First you would want to check all the wiring connections, especially the ground wires. If this is a 40 year old furnace and the fire is going in and out the only thing it could be is a gas valve failing but I have never seen one do as you have described.
 
  #3  
Old 12-07-08, 02:40 PM
S
Member
Thread Starter
Join Date: Dec 2008
Location: midwest
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks Mike,
I forgot to mention that this furnace does have a standing pilot (that is ALWAYS on). A helpful home center tip was to replace the flame sensor but another one thought it was not the problem because the pilot is alway on...
Hence my confusion and seeking your insights - shantosh
 
  #4  
Old 12-07-08, 02:50 PM
M
Member
Join Date: Jan 2008
Location: Tennessee
Posts: 438
Upvotes: 0
Received 0 Upvotes on 0 Posts
Furnace trouble

Standing pilot and flame goes on and off check limit if it is a rectangular silver box mounted on the firewall behind the gas valve and your indoor fan is running properly the limit may be going bad. They are a bi metal device that reacts to tempreature change. If you make sure you have a clean filter and your fan is running ok so that you are sure your air flow is right you will have to replace the limit. You could try to adjust it but that may create a dangerous situation. Put a thermometer in one of your supply vents if your supply air temp is 140 degrees or less the limit should not be tripping. If it is not the limit looki for a loose connection going to the gas valve.
 
  #5  
Old 12-07-08, 06:26 PM
M
Member
Join Date: Dec 2004
Location: San Jose,Ca
Posts: 1,277
Upvotes: 0
Received 3 Upvotes on 3 Posts
40 yr old Bryant had a flame switch type pilot if memory serves. I would check the flame switch to make sure it is clean and of the proper size. A flame switch is a safety that incorporates a bi-metal switch. If the pilot flame is weak, the flame does not impinge on the bi-metal properly and the switch may not close or open during burner operation.
Clean the pilot. And you may need to replace, but go there later.

Also the thermostat may be the culprit. The heat anticipator may need adjusting if it has one.
 
 

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