Furnace ignites but blower won't start until after the furnace shuts down


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Old 02-04-11, 11:25 AM
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Furnace ignites but blower won't start until after the furnace shuts down

I have a up-flow gas-fired forced-air Carrier 58GP GS series furnace. Couple weeks ago, I noticed the furnace wouldn't keep up with the temperature I set on the thermostat. I noticed the furnace and the blower will not run at the same time. The furnace will ignite, stays on for a few minutes and shuts down. Then, the blower starts, circulates the air for about 10 minutes and stops. As soon as the blower stops, the furnace ignites again and the cycle repeats. If I keep the fan "on" instead of "auto" on the thermostat, the furnace will ignite and stays on until it reaches the desired temperature. After that, the furnace fires once a while to keep the temperature steady. For some reason, the problem went away after I replaced the battery on the thermostat. Two weeks has passed, it happens again this morning. I changed the battery again and it didn't help this time. Is it a problem on the thermostat or the circuit board on the Furnace. Is it something I can fix by myself or I will need call a HVAC guy? How come things always break down in the midst of a cold winter... I've just put a new garage door opener up last weekend because of a out-of-warranty fault circuit board. Now, I have to deal with the furnace...http://images.ibsrv.net/ibsrv/res/sr...ilies/wall.gif

Thanks,

Keith
 
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Old 02-04-11, 12:13 PM
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The manual for your furnace is available at

http://www.xpedio.carrier.com/idc/gr...8gp,gs-2si.pdf


A couple of problems are possibilities --- the limit switch shutting off the burners or a dirty pilot switch shutting off the burners.

If you have a multimeter see if the limit switch is opening when the burners are shutting off.
 
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Old 02-04-11, 12:28 PM
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SeattlePioneer, thank you for the quick response and the manual...
Yes, I have a multimeter. After a quick look at the manual, I have trouble finding the location of the limit switch. Is it something on the control board? You also mentioned the possibility of a dirty pilot switch. Is the pilot light the same as the pilot switch? What do I do about it?
 
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Old 02-04-11, 12:36 PM
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The limit switch has two wires going to it and is usually located in the center of the furnace above the burners.

When the thermostat is calling for heat, you should have 24 VAC on both sides of the limit switch. If the 24 VAC drops to nothing when the burners shut off, the furnace is overheating.


The pilot switch is an assembly that has the pilot burner as part of it, but check the limit switch first.
 
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Old 02-05-11, 11:25 AM
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Okay, this is what I have found so far. When the thermostat calls for heat, the furnace ignites and the limit switch has zero VAC. About 105 second later, the limit switch jumps to 24.9VAC. Then, the burners shut down and the blower starts to run. At this time, the limit switch stays at 24.9 VAC for about 140 sec while the blower is running. Then it drops back to zero VAC and the blower stops. The thermostat continues to call for heat and the cycle repeats. According to the manual, the blower is supposed to run approximately 75 seconds after the gas valve has been energized. Does it mean my circuit board is toasted? What can I do now?
 
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Old 02-05-11, 04:37 PM
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Originally Posted by KeithL View Post
Okay, this is what I have found so far. When the thermostat calls for heat, the furnace ignites and the limit switch has zero VAC. About 105 second later, the limit switch jumps to 24.9VAC. Then, the burners shut down and the blower starts to run. At this time, the limit switch stays at 24.9 VAC for about 140 sec while the blower is running. Then it drops back to zero VAC and the blower stops. The thermostat continues to call for heat and the cycle repeats. According to the manual, the blower is supposed to run approximately 75 seconds after the gas valve has been energized. Does it mean my circuit board is toasted? What can I do now?
You are testing you limit buy testing both sides.
When the call for heat and burner starts you are not showing and 24 volt because the switch is closed and the 24 volts is going through the switch. And when the switch open then you are showing 24 volts. Chick the specks on you furnace on when the blower is suppose to start. ether the blower is not starting soon enough or your limit switch is weak and opening to soon.
Most blower start within 30 to 60 sec. Paul
 
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Old 02-05-11, 11:21 PM
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According to the manual, the blower should start within 75 second but it didn't. Another 30 second later, the furnace becomes overheated, the limit switch closes the gas valve and activates the blower to cool it down. Once the furnace is cool enough, the bowler stops. However, the thermostat continues to call for heat, the furnace fires and the cycle repeats. Am I correct? If I'm right, how can I fix it? Do I need a new board? Can I buy it or I have to call a pro? I have a Carrier 58GP GS series furnace. I live in Chicago. We are expecting single digits temperature most of next week. Right now, I leave the fan "ON" all the time and the house seems to be warm enough but I don't know how long can I keep the furnace running like this. Thanks!

Keith
 
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Old 02-06-11, 10:36 PM
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Any help would be greatly appreciated...

It turns out even I have turned the fan to "On" instead of "Auto" on the thermostat, the limit switch still jumps to 24 volt a few minutes after the burner has been fired. Help....
 

Last edited by KeithL; 02-06-11 at 11:26 PM.
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Old 02-07-11, 12:34 AM
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You have a talent for making replies that are incomplete and hard to interpret.


What should happen when you turn the thermostat up:

1: A spark is turned on and the pilot light lights.

2: After 30 seconds to a minute the pilot switch on the pilot light heats up enough to turn on the main burners.

3. The timer for the fan starts counting at that point and the fan should turn on after 75 seconds or so.

4. The main burners should remain lit and the fan running until the thermostat is satisfied

5. At that point the burners shut off

6. After another minute or so, the fan shuts off.



Does the fan start at 3 as described above?

Or does the fan remain off until the limit switch opens and the burners shut off?


If the circuit board is defective and fails to turn on the fan at 3, the furnace will overheat and the limit switch will open. When that happens a second circuit turns on the fan, which will run until the limit switch closes again. That sounds like what is happening, but it's hard to tell for sure based on your description.
 
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Old 02-07-11, 09:56 AM
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Question

Hi SeattlePoineer,

The pilot light is always on in my furnace. So, this is what happens:
  1. The thermostat calls for heat, the gas valve opens and the burners are lit.
  2. The fan did NOT run at 75 seconds.
  3. Another 30 seconds later (i.e. 105 seconds since the burners have been lit, still no fan), the limit switch opens. The gas valve is closed, the burners shut off and the fan starts to run.
  4. The fan runs for 140 seconds and stops.
  5. Since the thermostat still calls for heat, the cycle repeats.

Based on this, I assume the circuit board is defective because it fails to turn on the fan at 75 seconds. However, that's not it...

To keep my house warm, I switched the fan to "ON" instead of "Auto" on the thermostat. Initially, the burners are lit whenever the thermostat calls for heat. Since the fan is running all the time, the limit switch stays closed and the burners will remain lit until the thermostat is satisfied. However, it changed on Sunday. This is what happens now:
  1. The fan is running all the time.
  2. The thermostat calls for heat, the gas valve opens and the burners are lit immediately.
  3. After about 2 minutes, the limit switch opens (24 volt found on the wires) and shuts off the burners. The fan continues to run.
  4. About 5 minutes later, I hear a "click", the burners are lit again and the cycle repeats.
  5. The fan never stops.

I set the temperature at 72 degree last night and it's 70 degree when I woke up this morning. The thermostat calls for heat all the time now but the furnace can't keep the house warm.

What does it mean, I have a bad limit switch as well? What can I do now? It's going to be sub-zero in my area tonight and I have a 9 months old in the house.

Where can I buy the limit switch and the circuit board? Since my fan is running all the time, I'm debating if I should disable the limit switch by unplugging one of the wire so the burners can remain lit until the house is warm enough. Any idea?
 
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Old 02-07-11, 10:23 AM
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"It's going to be sub-zero in my area tonight and I have a 9 months old in the house. "

DO NOT DISABLE THE LIMIT SWITCH!!!!! The switch is protecting the heat exchanger from damage (ie destroying itself from the high heat and causing a carbon monoxide leak into the living space). Call a service tech to come out and fix your furnace and make sure the heat exchanger isn't damaged and your system is safe. I would also recommend that you get some carbon monoxide detectors if you haven't done so already. At least one in each bedroom.
 

Last edited by jeggs; 02-07-11 at 10:41 AM.
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Old 02-07-11, 10:29 AM
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The furnace is overheating very likely because of inadequate airflow through the furnace. The most common reasons for this a 1) a plugged filter 2) return air vent covered over 3) too many warm air vents closed 4) plugged air conditioning evaporator coil above the furnace 4) dirty fan wheel


You also need to replace the furnace circuit board.


And yes, don't bypass the limit switch that is a very important safety system.
 
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Old 02-07-11, 10:40 AM
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I'm assuming the filter(s) are clean. Does the air flow seem as strong as it had been? Do you have a.c ? If yes, could the coil be dirty? Have you disconnected the thermostat and jumped the wires to run the furnace without the thermostat? Thermostats can produce some funny things. Is this an old thermostat with a heat anticipator?
 
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Old 02-07-11, 11:07 AM
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Thank you for all the responses...

I changed the filter a few weeks ago. I checked it this weekend and it's clean. All the return vents are located on the ceiling. I didn't check closely but they didn't look very dirty to me from the outside. I checked the registers last night. The air-flow seemed to be normal to me but I don't really remember what's normal. I used to have the second floor duct half closed in the Winter but I opened it all the way last night. I have a programmable thermostat from Honeywell. It's about 6 years old. I think I need to check the vents and registers again. It is unlikely I have a faulty limit switch.

Where can I go about getting a circuit board? Do I have to buy it online or I can get it locally? How much do you think it would cost?
 
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Old 02-07-11, 02:48 PM
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Very likely something is impairing the air flow through the furnace. You've checked the easy stuff.

Do you have air conditioning? If so, you need to check the evaporator coils at the furnace for plugging. That can be a job.

Check the fan wheel for significant debris that would impair it's ability to circulate air.

Try turning the fan wheel by hand to see if it turns freely.


Check the ductwork for damage or collapse. Sometimes ductwork can be lined with insulation, and that can come loose and plug things up.


In short, you need to inspect the entire system for anything that is not correct.
 
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Old 02-08-11, 10:44 AM
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Smile You won't believe how I fixed it....

As everyone suggested, I checked the air flow as soon as I got home last night. All the registers were opened. The return vents were clean. The air filter was replaced two weeks ago. There's no dust on it. I didn't remember what is considered a normal air flow. I put a piece of toilet paper on the return vent, it didn't seem to have very good suction. I took a second look at the air filter, I didn't know what exactly it was but it looked a little odd to me. So, I removed it from the furnace and everything worked. I turned the fan control on the thermostat back to "auto". The fan started to run a minute after the blower was lit. There was no problem on the control board either. The furnace was running normally last night. My house was nice and warm.

I have another spare filter in the garage. I compare it with the one I removed from the furnace. It turns out the paper on that filter is a lot thicker and denser than the spare one. I use a basic True Blue filter. It is pleated paper filter with wire backing which costs about $3 at my local big box store. I've never thought the filter could be defective. My furnace runs normally with the spare filter.

So, my furnace got too hot because of lack of air flow and the limit switch shuts down the gas. We had a blizzard in Chicago last week, it was cold. The furnace needed to work a lot harder and got overheated after a few days. One thing I don't understand is the fan control delay on the control board. It looks like the delay becomes longer when the control board becomes hotter. Eventually, it becomes too long so the limit switch kicks in.

Well, I thank you all for your help. I could never figure this out without you guys.

Thank You!

Keith
 
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Old 02-08-11, 01:28 PM
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Good job. not as easy as it looks huh................lol
 
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Old 02-08-11, 03:42 PM
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I can't really think of a reason why the fan wouldn't have turned on after the burners lit. On the other hand, you can't argue with success!


Glad you got things going.
 
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Old 02-08-11, 04:54 PM
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A new filter will not be too restrictive to cause your limit switch to trip. Putting in a more restrictive pleated filter just brought the problem to the surface. You still have an air flow problem somewhere. You should have the system checked by a professional.
 
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Old 01-25-13, 09:51 AM
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i have kind of the same problem. in my case when the limit switch reaches a certain temperature it shuts my furnace off it also cuts my 24vac supply to the thermostat but the fan never shuts off, eventually the furnace will come on again together with 24vac and it will happen again. the point is that my thermostat needs 24vac all the time to stay charged up. help. thanks
 
 

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