Furnace blower cycles on and off

______________________________________________________________________ Hello! I am a homeowner who is not too knowledgable regarding furnaces. I have a Coleman Manufactured Housing Systems furnace (1971A404) with a Sears Programmable Weekender (153-0589) thermostat (it seems to be a relabelled Honeywell programmable thermostat). When the thermostat requests heat the burner lights. After a short time the blower kicks on. It runs for a short period of time (usually between 50 and 60 seconds), then turns off. The burner, however, does not turn off. After a short while, the blower turns back on. Over and over and over… All during this time, the thermostat shows that the current temperature is below the requested temperature: the thermostat should be telling the furnace to run continuously all during this time. What could be causing the furnace to stop and restart the blower over and over? Is there something I can check? Thank you very much for any assistance you might be able to provide me. If you need more information, please ask, or feel free to contact me directly at: t_mass ey at i_name .com (without spaces or underscores). I would appreciate any help you could give me. Tim Massey Replies: Xenos Webmaster “Re(1):Furnace blower cycles on and off” , posted Fri 24 Oct 22:08 ______________________________________________________________________ Inside you furnace there is what is referred to as a fan limit switch. The fan settings on this switch need to be changed and it sounds as though its now out of calibration. Normally there are 3 settings and standard settings are 200,150,90. 200 is the limit 150 is fan on and 90 is fan off all of these are in degrees F. Hope this helps. Xenos. The best way to escape a problem is to solve it. hardbop58 “Re(2):Furnace blower cycles on and off” , posted Fri 7 Nov 12:17 ______________________________________________________________________ I have encountered a similar problem. The fan limit switch in my furnace is a round disc with three adjustable set points. The entire disc rotates as temperature changes indicating that it is controlled of of a thermocouple. I think the problem is that I have a big differential between my night-time and morning temperature settings on my programmable tempertature controller. I turn down to 56F at night and the at 5:30 am it changes to 68F. My theory is that when the fan first kicks on, the cold air pulled in from the return ducts takes the temp below the ‘off’ set point on the fan limit switch after only about 20-30 seconds. The short cycling continues until there is some equilibration in temperature between the return and the house temp. Am I on the right track? The probelm is that I have seen no major difference in the cycling when moving the low-off setting up and down. By now I have screwed around so much that I can’t remember the original set points (I broke the rule of not writing everything down). Any suggestions for a troubleshooting strategy. Thanks. Hardbopper Xenos Webmaster “Re(3):Furnace blower cycles on and off” , posted Fri 7 Nov 17:15 ______________________________________________________________________ hardbop58 I would say you on the right track. Normal settings would be 200,150,90. Try raising the Middle setting to 160 and the low one at 80. Xenos. The best way to escape a problem is to solve it. hardbop58 “Re(4):Furnace blower cycles on and off” , posted Tue 25 Nov 20:03 ______________________________________________________________________ Thanks Xenos, I have fiddled with those settings but the problem continues. I’ll go back to the 200/160/90 setting and see what happens. Do you think that the fact that I’m turned down to 56F at night is part of the problem? When the Tstat tells the furnace to ramp up to 68F it pulls a bunch of 55F air across that switch when the fan kicks on. In other words have I created this problem by having too low a night time setting? I did not have this problem at my last house, but I have noidea if that furnace had a similar fan limit switch. Thanks again for the response. Hardbopper Hardbopper tmassey “Re(2):Furnace blower cycles on and off” , posted Sat 25 Oct 12:12 ______________________________________________________________________ Thank you very much for your answer! There is a limit switch. It is *not* popping out. But you’re saying that, even though it’s not tripping, it’s still failing? I will see if I can purchase a replacement. That is certainly a simple fix if it is the problem! Thank you very much for your help. This is an outstanding resource and I appreciate the effort you have put into this. Tim Massey Xenos Webmaster “Re(3):Furnace blower cycles on and off” , posted Sat 25 Oct 13:17 ______________________________________________________________________ Hold on a sec. The problem may or may not be the limit. Some furnaces use a single fan limit switch for both the limit and controlling the fan. Others use separate controls. On this device do you see fan settings ? and what manufacture is it? Also could you describe it? Xenos. The best way to escape a problem is to solve it. tmassey “Re(4):Furnace blower cycles on and off” , posted Sat 25 Oct 22:11 ______________________________________________________________________ I may have been assuming too much. I have gone back and studied the wiring diagram and the physical devices and here’s what I’ve found out: On the wiring diagram are three switches: lower limit, fan switch and aux limit. Two of them (the lower limit and fan switch) are inside the Control Box. One of them (the aux limit) is not. It is the aux limit that is bolted to the left side of the blower, on the bottom rear. Neither of the limit switches seem to be in the least adjustable. They are black plastic bodies. The aux limit seems to have a round pop-out part that is currently pressed in, but it does not seem to be adjustable. The lower limit and fan switch are just black plastic bodies. According to the block wiring diagram, the lower limit and fan switch are in series with the blower motor on the primary side of the transformer. The gas valve, aux limit and thermostat are in series on the secondary side of the transformer. Again, none of these switches seem to be adjustable. Only the aux limit seems to be anything more than a completely sealed switch, and due to its position, I can’t really see it. There is a sticker on the blower that references it. It says, in part: Furnace is equipped with a manual reset limit switch on rear left side of blower. If burner does not function, turn system switch to off and push the reset button in center of limit switch. Turn system switch to on and check for burner ignition. Of course, the burner *does* function, so this sticker doesn’t really apply per se. However, it made me think that that switch was the fan limit switch. The wiring diagram calls it the aux limit, though. To reiterate, the problem is the blower turns on for about a minute, turns off for a minute (or a little longer), turns on for a minute, etc., even though the thermostat is calling for heat the entire time. I have called a local heating and and cooling company to come and do a basic furnace maintenance call. I asked if they would be testing the limit switches and they said yes. It was $70 for this. I’ve lived here for only a year: it’s probably a good idea to do this no matter what. But I would like to know what I’m talking about even if professionals are going to come and take a look at it. I’m always wary of getting taken advantage of. The company that I called is popular with the home-improvement radio talk show hosts. I hope that means that they’re honest, but I have a hard time trusting complete strangers… Anyway, thank you very much for reading over all of this. You certainly seem like a very knowledgable person: I greatly appreciate you using your time to help ignorant people such as myself! Tim Massey Xenos Webmaster “Re(5):Furnace blower cycles on and off” , posted Sat 25 Oct 23:21 ______________________________________________________________________ First off its not the limit that we are concerned with, its just that some manufactures make a single fan and limit control. Obviously your problem is that this fan control is turning on too soon and as soon as the fan starts it cools off ( of course the motor could have a problem). Normally it shouldnt start till the furnace hits 150F then when the fan starts the temperature will lower to about 110. If there are no temperature settings on this control I would say t will require replacement. Xenos. The best way to escape a problem is to solve it. tmassey “Re(6):Furnace blower cycles on and off” , posted Tue 25 Nov 17:08 ______________________________________________________________________ I wanted to follow up with this and let you know what has happened. The furnace repair guy came. I asked for a general checkup on my furnace. However, he only came out to “repair” it. I explained to him what was going on. He too felt it was the limit switch. However, again, it’s not adjustable. He said that the blower was blowing too hard: that it was not the original blower, and that at its current setting (medium-fast) it was blowing too fast. He changed it to low: still happened. He then cleaned the orifice of the burner. That seemed to help: at the time, the furnace seemed to run better. However, since then, it still does it. Not all the time, but sometimes all night, and other times not at all. So he turned a solid problem into an intermittent one! I paid $70 for the priviledge, too. :) Anyway, I’m not sure where to go with this from here. Should I just preventatively replace the switch? It can’t cost that much, and it should be very easy for me to do even by myself, if I can figure out where to get the part. Otherwise, I’m kind of out of ideas… It’s hard to tell if the heat *is* hot enough, but the switch is wrong, or maybe the switch is working correctly, and it just plain is not hot enough… I’m toying with the idea of getting a themometer or thermocouple that will let me know what the temperature actually is. I was really hoping the guy that came to look at the furnace would do that kind of stuff. I asked if they would give the furnace a thorough examination, and they said they would. He didn’t really do *any* kind of PM exam: it was just an attempt to fix my problem. I *would* like to fix the problem, but I’d also like to know that the rest of the furnace is OK as well… Thank you very much for any help you might be able to give me. Either way, I wanted to update this article and let others know what has transpired. Hopefully, it will be helpful to someone! Tim Massey Xenos Webmaster “Re(7):Furnace blower cycles on and off” , posted Tue 25 Nov 18:45 ______________________________________________________________________ If it’s non adjustable and you have your furnace professionally looked at I would replace the switch. I am not sure where to buy coleman parts other than from a dealer. Xenos. The best way to escape a problem is to solve it. tmassey “Solved! Was:Furnace blower cycles on and off” , posted Tue 3 Feb 15:43 ______________________________________________________________________ Hello! Sorry for the delay. I thought I had already posted a reply outlining what I did, but I don’t see it, so I’ll (re)write this one… I started by shorting out the fan limit switch. The switch is designed to prevent the blower from turning on unless the temperature of the air was at a certain point, and turn the blower off if the temperature dropped too low while running. With the switch shorted out, the fan would start to run as soon as the burner lit, and would stay running the entire time. So obviously, the switch was involved in the problem. Either the switch was bad, or the furnace was not putting out enough heat to keep the switch closed, even with the blower at low. At this point, I decided to replace the fan switch. It’s a Therm-o-disc part, designed to turn on at 110 degrees, and off at 90 degrees. It’s a very simple bi-metal switch that is not adjustable. The part was less than $9 at Grainger, and they had it in stock, so I had very little to lose even if it wasn’t the problem. However, it seems to have solved my problem. The furnace blower still cycles two or three times during heating, but the blower still runs for decently long periods of time: several minutes, as opposed to less than 60 seconds. All-in-all, not bad: $9 to fix it myself! I’m pretty sure that this is the part that I purchased: http://www.grainger.com/Grainger/productdetail.jsp?xi=xi&ItemId=161163 2210&ccitem= I don’t have the part number in front of me (it’s installed in my furnace! :) ), but the specs seem right: close at 110, open at 90. As you can see, it’s a very simple part, with no adjustment of any kind. I hope this information is useful to someone! Tim Massey

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