Furnace cycling repeatedly


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Old 10-19-09, 11:59 AM
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Furnace cycling repeatedly

I have York 90% efficient gas furnace with electronic ignition and controlled by a digital thermostat. It is working fine, heating the house all right. But a few days ago Iíve noticed that it started cycling odd: after it shuts down and sits for a while, it will come back on, blowing warm air, but will shut off very quickly, maybe after just 15 or 30 seconds. I am fairly familiar with it, replaced a few parts myself. What should I be checking this time?
Thank you for the help.
 
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Old 10-19-09, 05:12 PM
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A setting for when your blower fan comes [U]on, is set too close in temperature to the setting for when the furnace shuts off.

What happens is - the furnace runs it 's cycle, the stat shuts off, the blower continues to run to capture the heat from the still hot heat exchanger, and the the blower finally shuts off. But there is still left over heat in the exchanger, which causes the temp inside the furnace to rebound some again. Then when that happens, for reasons already stated, your blower comes back on one more time to cool down the heat exchanger more. And then it no longer does that because it is finally cool, through and through.

Some furnaces use (a)fan control switch(es) that allow you to set the on and off blower settings to settings of your choice. This is DIY-able if you have this.

On some real old furnaces they used to use simple thermal discs to accomplish this. And these were not adjustable or servicable. And when your problem arose, it was because they got weak, and you had to just replace it.

What parts did you replace?
 
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Old 10-19-09, 05:36 PM
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Probably not in this case, Ecman. A 90% efficient furnace is not going to have a temperature sensitive switch to turn the fan on and off. These typically have timers on circuit boards to control the fan.
 
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Old 10-19-09, 08:40 PM
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furnace cycling

Excellent observation regarding temp. limits settings.
Tonight I did some close observations for over an hour. Below I posted its cycle:
ON OFF
11min 14min
6min 2min
10sec 15min
6min 2min
10sec 18min
5min 2min
10sec quit experiment
My 10 seconds are approximate, maybe a few seconds either way. Also, these 10 seconds were just when I observed the blower coming back ON, not the gas.
From what I see, the blower would come back on for a very short period after the furnace shutting OFF and staying OFF for about 2 minutes every time. It does look like the internal bonnet was not cooling off fast enough, causing the blower to come ON again and finally cooling it off below limit.
I also cleaned the filter and checked the drain passage and some accessible wiring: all O.K.
I do have a temp. limits control switch. It is set to the following:
FAN-ON FAN-OFF CUT-OFF
140 90 190
Then I did another experiment: set stat to way too high (76), forcing the furnace to start. Then I lowered it to below room temp, forcing it to shut off. It shut the main gas and the pilot off, the main blower kept going for about 10-15 sec, then shut off. I would think it should've taken longer to cool off that limit switch to below 90 degree. Or maybe it is getting weak?
I am wondering if I need to try setting FAN-OFF to what? lower or higher? If lower, the blower should keep running after gas shut off for longer, possibly really cooling the enclosure off. If I set it to a little higher, the blower will shut off almost immediately after gas shuts off, but maybe it will not try coming back on. Please, toss the coin for me!
 
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Old 10-19-09, 08:58 PM
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Hello umniy,


Since you report that the air is still warm when the fan turns back on after a delay, mthat suggests you'd want to switch the timer to run the fan longer before it shuts off.

In general, you want the fan to run long enough to scavenge any useful heat out of the furnace but not long enough to start circulating cool air that's perceived as a draft.

It might be helpful to post the make and model of the ignition control module. If we can find the instruction manual for that box we can see in more detail what should be happening.
 
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Old 10-20-09, 05:39 AM
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furnace cycling

Makes sense. I have a manual home (I am at work now) and will post the control model later today. I looked at it last night: they explain the proper operation and what should be happening, but there is no lag time for the blower shut off (nor would I expect it in the manual, since it is dependant on settings and unique physical conditions).
Something else comes to mind: my digital stat is set to 70 degrees. The furnace comes ON when the stat is showing 70. Then it runs for some time, shuts off, comes on again frequently, the whole nine yard. All this time the stat is showing solid 70, no change. Why is the furnace coming on and off? Shouldn't the stat temperature go up and down at least one or two degree, in order to start and stop the furnace? May the stat be a part of a problem? I read about heat anticipator, but I am not sure if that's the case with the digital stats.
Thank you for the help.
 
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Old 10-20-09, 05:44 AM
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Digital thermostats don't have heat anticipators. They have settings whioch allow a specific number of furnace operations per hour. It sounds like you need to reduce the number of cycles per hour.

Do you have the manual for the thermostat? The manual should have a recommended number of cycles per hour for your type o9f furnace and explain how you can check the current setting and change it if need be.


If you don't have the manual, post the make and model of the thermostat here.
 
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Old 10-20-09, 05:50 AM
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Originally Posted by umniy View Post
Something else comes to mind: my digital stat is set to 70 degrees. The furnace comes ON when the stat is showing 70. Then it runs for some time, shuts off, comes on again frequently, the whole nine yard. All this time the stat is showing solid 70, no change. Why is the furnace coming on and off? Shouldn't the stat temperature go up and down at least one or two degree, in order to start and stop the furnace? May the stat be a part of a problem? I read about heat anticipator, but I am not sure if that's the case with the digital stats.
Thank you for the help.
Your stat may have a configuration setting limiting the temperature change to a degree or less.

While the display shows 2 digits, it may be sensing 3. So if room temperature was 69.6 degrees, the display would read 70. Likewise at 70.4, it would read 70.

Delta
 
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Old 10-20-09, 08:19 AM
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Originally Posted by umniy View Post
10sec quit experiment
My 10 seconds are approximate, maybe a few seconds either way. Also, these 10 seconds were just when I observed the blower coming back ON, not the gas.
From what I see, the blower would come back on for a very short period after the furnace shutting OFF and staying OFF for about 2 minutes every time.

Seattle, I stand by my general assessment, regardless if cause is from the older adjustable or disc fan control, or the newer fan timer system. The principle is still the same, regarding a malfunction there, or setting, with blower timer control.

If the stat were the issue and short cycling (as the only cause), the burner also should be coming on in short cycles also - not just the blower.

It's possible that if he also has complete short cycling of the furnace, that he also has a stat setting issue. But I do not see that as the reason for the blower seemingly trying to scavenge heat after each cycle, per his very own quote above.
 
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Old 10-22-09, 07:10 AM
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furnace cycling

You are awesome, guys!
I was running around like a chicken without a head for a couple days, not having time to dig into it.
My furnace circuit board is Honeywell S860D.
The thermostat is Honneywell MagicStat/32. There are two adjustment screws A and B that set System on-time. They give the default settings (A-out one turn, B-in) and that's how I probably set it several years ago and never touched since. They have a troubleshooting guide for too frequent cycles: re-adjust on-time setting, but no details which screw and by how much; probably, trial and error. There is no mentioning about fractional degrees or adjusting its accuracy.
I still need to confirm to myself that when the furnace fires for 10 second, it's just the blower going, not gas.
Could you give me a couple scenarios what to do on the furnace and/or stat side and the logical way of trying, not just shooting in the dark?
Regards to all.
 
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Old 10-22-09, 08:42 AM
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Originally Posted by umniy View Post
.
I still need to confirm to myself that when the furnace fires for 10 second, it's just the blower going, not gas.
If that affects your questions here, I'd check that and post it here before everyone has to list various scenarios, maybe for no reason. Tell us all that it does or does not do, first.
 
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Old 10-23-09, 05:44 PM
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furnace cycling

I finally got to double-checking my furnace cycling. Now I am 100% confirming the following:
-stat calls for heat ON;
-furnace starts normally;
-furnace runs;
-stat calls for heat OFF;
-gas and induced draft blower shut OFF;
-main blower keeps running (this time for about 40 second);
-main blower shuts OFF for about 2 minutes;
-main blower comes back ON for about 20 seconds, but gas does not re-light;
-finally, everything shuts OFF.
So, looks like it is not the stat calling for heat too frequently, otherwise gas would also come ON for the second short time. Then, it must be the control's temp. limits, specifically FAN-OFF, which is set to 90 degree? As you suggested, after initial shut-off, the internal temp. may be rising slightly again, just above that setting, causing the blower to come ON again and lowering the temp.? If that's the case, what do I do? Set the FAN-OFF limit lower, or higher (I'd say lower, say to 80-85 degree)? Any other ideas?
Thanks to all.
 
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Old 10-23-09, 06:17 PM
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But how long does the fire burn, when the stat first comes on, to when the stat shuts off?.

If the stat is too sensitive(various causes), it may not run all that long. So then it has to start up again right away because the flame was not on long enough to warm the house enough. An endless vicious cycle from too short of fire cycles.

The blower issue could be something different. But what is odd here is you already have a 50 degree off-on setting swing! That's huge. Watch that dial to see if rather than a steady smooth climb, that instead it suddenly takes a big jump, from say 110 to 140, and you hear it click on. If that is happening that fan limit switch may have seen it's better days. In the meantime you can increase the swing, to see if that noticably helps.
 
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Old 10-23-09, 08:27 PM
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furnace cycling

ecman51, that was quick!
before I did not know to watch the limits switch turning. O.K., here is the latest:
-set the stat to 1 degree above current room temp;
-it called for heat ON;
-furnace started normally;
-switch steady climbed to 135 (it is not at quite 140, as I stated before);
-main blower started;
-switch pulled back some to about 120 and stayed there;
-ran for 7 minutes;
-stat called OFF;
-gas shut OFF, blower kept running;
-switch steady pulled back to about 100, then kind of jumped to 90 and shut the blower OFF;
-it started climbing back up and in about two minutes reached 135 again and re-started the blower;
-with blower running, the switch kept falling down. At about 110-115 it made a sudden jump down to 90 and shut the blower;
-with blower OFF, it started slowly climbing up again, but only to about 105, and froze there.
Meanwhile, the stat called the heat ON a couple times every 10-12 minutes, which is a different issue.
I set the FAN-OFF limit down from 90 to 80. It basically repeated its cycle, but after gas shut-off it took the blower longer to cool it off. And the dial again jumped the last 20 degree down in one single leap. After blower shut-off, it climbed back up some to 105, but never reached FAN-ON limit and the blower never came ON for the second short time.
Seems like that helped. But the remaining question: is the stat calling ON to frequently at about every 10-15 minutes. I could try adjusting its screws A and/or B, but how? They do not give details in thee manual.
O.K., tomorrow will be another day. Out to bed. Good night, everyone.
 
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Old 10-23-09, 08:56 PM
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I's leave the fan off temperature set at 90 degrees. Too low and you will perceive the furnace producing cold drafts.

Increase the fan on temperature to 150 degrees or even 160 degrees and you will probably solve the problem.
 
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Old 10-24-09, 06:17 AM
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Originally Posted by umniy View Post
. I could try adjusting its screws A and/or B, but how? They do not give details in thee manual.
O.K., tomorrow will be another day. Out to bed. Good night, everyone.
Here is a copy of the manual.

On page 6 is the screw settings. i suggest setting it to "high efficency furnace/hot water" A=out, B=in With this setting, you'll get longer run time. (less stop and go)
 
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Old 10-24-09, 07:55 PM
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furnace cycling

Originally Posted by SeattlePioneer View Post
I's leave the fan off temperature set at 90 degrees. Too low and you will perceive the furnace producing cold drafts.

Increase the fan on temperature to 150 degrees or even 160 degrees and you will probably solve the problem.
Unfortunately, in my limits switch FAN-ON temp maxes out at 135 degree, and that's what it is set to.
 
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Old 10-24-09, 07:58 PM
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Originally Posted by Jay11J View Post
Here is a copy of the manual.

On page 6 is the screw settings. i suggest setting it to "high efficency furnace/hot water" A=out, B=in With this setting, you'll get longer run time. (less stop and go)
Thanks for the reply. I do have a manual and the standard settings diagram. My question is: if I still wanted to set it to less frequent calls, what should I do?
 
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Old 10-24-09, 08:04 PM
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Originally Posted by umniy View Post
Thanks for the reply. I do have a manual and the standard settings diagram. My question is: if I still wanted to set it to less frequent calls, what should I do?
With the setting I suggest is the less calls you are going to get..

That setting is going to get you about 3 cycles per hour, where regular gas heat give you 5, and electric heat gives you 9.

If your furnace cycles often when it's very cold out, then you have a oversized furnace.
 
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Old 10-25-09, 06:44 PM
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thermostat tweaking

Originally Posted by Jay11J View Post
With the setting I suggest is the less calls you are going to get..

That setting is going to get you about 3 cycles per hour, where regular gas heat give you 5, and electric heat gives you 9.

If your furnace cycles often when it's very cold out, then you have a oversized furnace.
I am not sure if it is cycling too often. To figure it out, I'd have to keep non-stop monitoring for a few hours, and that's not easy nor fun (duh!). I do not have any automatic logging capabilities.
But if for some reason I wanted to tweak it slightly for less cycling, is it possible to turn the screw a fraction in or out, assuming it is already set to high efficiency furnace?
 
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Old 10-26-09, 05:57 AM
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Originally Posted by umniy View Post
is it possible to turn the screw a fraction in or out, assuming it is already set to high efficiency furnace?
Nope, it's either "open" or "closed", there is no variable. So just set it to 3cph (HE)
 
 

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