Fan Constantly on/Gas Furnace keeps re-igniting and vice-versa

Reply

  #1  
Old 12-26-07, 10:11 PM
Member
Thread Starter
Join Date: Dec 2007
Posts: 2
Received 0 Votes on 0 Posts
Fan Constantly on/Gas Furnace keeps re-igniting and vice-versa

Hi,

1. In which area you live and ambient temperatures you usually experience.
2. House style and construction details.
3. Make, model and age of equipment related to the problem.
4. Fuel type.
5. Water temperature and pressures of boiler systems.
6. What type of zoning do you have with your boiler system.
7. Thermostat type.
8. Anything else that would be useful. No detail is to small. The more information you provide the easier it becomes for the professionals in this forum to determined the cause and correction to the problem.
1. San Francisco.
2. 1930s Edwardian
3. Don't know the age. Kenmore, model 867-767290. (If you know the age, please let me know).
4. Gas
5. N/A
6. N/A
7. Programmable. I can look at it physically for more details if needed.

Everything was working fine until a couple of weeks ago.

Now, one of two things happens:
1. The fan stays on, and the furnace keeps turning off and then re-lighting every so often (much more often than before).
2. Fan turns off and on about the same rate as the furnace in #1 and the heater stays on constantly.

Also, one vent is getting seemingly less air flow than before, other vents are fine.

Filters are constantly replaced on this thing.

Any help would be appreciated. Thanks.
 
Sponsored Links
  #2  
Old 12-26-07, 10:46 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
Received 0 Votes on 0 Posts
A classic syptom of an overheating furnace is a fan that runs constantly and a burner that turn on and off periodically.

A furnace produces a lot of heat, and a warm air furnace depends upon the flow of air through the furnace to strip away heat and prevent the furnace from overheating. If the air flow is reduced enough, the furnace overheats until a temperature sensitive limit switch shuts the burner off. When the fan cools the furnace off enough, the burner turns back on.

The most common cause of this is a plugged filter which apparently isn't the case in your equipment. Having multiple filters can cause that problem, also too many warm air vents closed off, the return air vent(s) being covered up, collapsed ductwork, dirty furnace fan, overfired furnace, improperly adjusted limit switch ---- lots of things.

That one warm air register isn't supplying heat suggests that a reason for that be investigated.
 
  #3  
Old 12-26-07, 10:55 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
Received 0 Votes on 0 Posts
#2 issue:


Usually, the burner should turn on and run awhile to warm the heat exchanger and prevent a blast of cool air into the house. Then the fan turns on and runs.

When the thermostat is satisfied, the burner shuts off and the fan continues to run until the heat exchanger cools off.


A burner that cycles on/off while the fan runs constantly indicates a problem.

Depending on the type of controls you have this could be things like a glazed flame sensor that needs cleaning, a pilot switch that needs cleaning, a poor electrical connection somewhere, an ignition control module that's failing, a pinched thermostat wire that's not making a good connection or other problems.


Unfortunately, the varied kinds of possible problems aregue for having an experienced repairman check out the furnace. I'll be interested to see what bright ideas others may suggest, but this really needs good troubleshooting methods to identify the problems. Gues 'n by golly methods will only serve if you happen to be quite lucky.


But perhaps you will be.

Good luck.
 
  #4  
Old 12-27-07, 11:40 AM
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 8,629
Received 0 Votes on 0 Posts
Always investigate the most common and simple things first.

Besides a filter and other blockages, and limiting, the other issue in your case is to make sure the stat is not coming on and shutting off.

I can't recall reading anything in the thread about any turning up of the stat during any test to see if the furnace runs longer then, or if this occurs mainly when the stat is left alone.

I have come across scenarios a LOT where the stat is cycling more often than one would think it should, and this needs to be checked out so we can possibly scratch that off the list. See if it is the stat that is shutting off the flame.
 
  #5  
Old 12-27-07, 07:10 PM
Member
Thread Starter
Join Date: Dec 2007
Posts: 2
Received 0 Votes on 0 Posts
Thanks for all the replies!


The most common cause of this is a plugged filter which apparently isn't the case in your equipment. Having multiple filters can cause that problem, also too many warm air vents closed off, the return air vent(s) being covered up, collapsed ductwork, dirty furnace fan, overfired furnace, improperly adjusted limit switch ---- lots of things.
I'll double-check all the vents/ducts.

It seems that the burner cycles on/off pretty regularly and the pilot switch is working just fine.

The thermostat is also not a problem as it's on when desired temperature > current temp and off otherwise.
 
  #6  
Old 12-27-07, 07:16 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,503
Received 0 Votes on 0 Posts
Originally Posted by Palal View Post
Thanks for all the replies!



I'll double-check all the vents/ducts.

It seems that the burner cycles on/off pretty regularly and the pilot switch is working just fine.

The thermostat is also not a problem as it's on when desired temperature > current temp and off otherwise.


As suggested by ecman, it's a good idea to turn the thermostat up to 80 degrees or so. There are reasons why the thermostat might turn the furnace burner on and off frequently in the way you describe. Turning the thermostat up to 80 degrees would eliminate that possibility, and if it causes the furnace to remain on, point to the actual cause of the problem.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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