furnace blower cycles and shuts off permaturely

Hi, I hope someone has an answer to this.
The blower of the furnace turns off while the burners keep burning. After about 30 seconds, the burners shut off (I presume because of high temp limit, since there is no air blowing), then about 10 seconds after the burners shut off the blower turns ON again, then the burners kick in again and the blower runs for about 2-3 minutes but then it shuts off again while the burners burn, and so on repeteadly. All this while the therm. is calling for heat. It is as if the blower cannot run constantly while the burners are burning. Fan is set to Auto on the therm.
Some more info. When the unit is first turned ON the fan appears to be able to run for a longer time while the burners are burning, but eventually shuts off before the burners go out. As the system runs cycling like this, the blower cycles get shorter, that is the blower cannot stay on for more than 2-3 minutes while the burners are burning.
I realize this is very dangerous as the burners and the core will get very hot since there is no air flowing. The exhaust fan is running all the time while all this cycling is happening. I have called a heating specialist to come see the system but I want to make sure I am well educated so that he does not give me the run around.... Also, what should I expect to be charged in the north east for furnace maintanance service if no new parts are needed ?
The system is a York 80 Diamond, natural gas, FHA. The york 80 comes with an electronic furnace controller White-Rodgers model 50A50-210 (I believe the 50A55 replaces the 50A50), so there is no hi/lo limit controls that I can adjust... I don't know the age of the furnace but the house is 20yrs old. Any help and advice is greatly appreciated.
Thanks!!!! Tony
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This is Turtle.
I think you have a blower motor problem and there is 3 things your looking at. New Motor, Oil the motor, or bad capasitor. After 20 years and you have not been oiling the motor regularly, the bearings can be down on the motor and cause this problem.
Now to price in the North east for just a adjustment of a part service call. You can find out this by just calling back and ask what does a service call cost to come service my furnace. He will tell you up front what the cost is for every hvac service contractors have this cost very easily given out.
TURTLE
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Thanks Turtle. The bearings are on the motor are sealed/lifetime lubed. The motor/blower is direct drive. I hand spun the blower and it felt really smooth and did not have signs of worn bearings (no noises nor play either). The motor also appears to run well as the force of the air coming out of the vents has not changed and appears to be sufficient - not hurricane, but definitely good blowing...
Maybe the CAP has gone, but isn't the CAP only used for starting the motor? give it some juice so that it does not trip the breaker, and then once the motor is going th e cap does not serve a function anymore?
Thanks again for your input Tony
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This is Turtle.
the Capasitor is to help start it but if the capasitor is weak or not working it will let the motor run hot and start cutting in and out as it get hot and them cools off to start again.
Motor after starting will run hot and cut out with out the ade of the capasitor. It need the capasitor to run and start also. Most motor will start with out the capasitor but will not run for long and start cutting in and out. In this application it will not run very long with out the ade of the capasitor and start cutting in and out.
TURTLE
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This is Turtle Again.
I have seen blower motor turn free and still have bad bearing. It could have been worn a good bit and then you oil them up and it turns free but the bearings is letting a armoture come too close to the fields in the motor and cause it to heat up because of not dragging but the motor verses the field possion in the motor. Motor on direct drive have to be perfectly in the center of the windins or it will run hot and start cutting in and out. 5 or 6 thousanths of a inch out of center will or can start make it run hot and still turn free as can be.
If the capsitor is weak or out the motor will act up running or starting in either case.
TURTLE
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Andy suggests:
If the problem is in the motor/capacitor/bearings, perhaps the following test could verify it.:
Run the motor on "fan only" and see if the same problem occurs, i.e. the motor starting and stopping. Make sure the thermostat is in a position where the gas or air condx function is disabled...
Here is another possible , often overlooked possibility.
Dirty air filter...... This would prevent enough air from moving thru the plenum where the high temperature switches are located. They heat up and shut off the gas. I have seen this before. Replacing the dirty filters fixed it....However, the usual sympton is that the gas shuts off and the motor continues to run, which is NOT what you are seeing... Just thought I'd mention it...
If the unit has an integral air conditioning condenser, if it is clogged with dust and dirt, the airflow could be restricted just as with the dirty filter. This is harder to clean, but possible, and fairly easy to check --- tho you have to undo a few screws and use a flashlight...
Just a couple other things to check before selling your car to pay for a house call......
Andy
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Andy, I see you've been charged $80+ for changing a dirty air filter that you were too stupid to remember to do. Not all techs are that way. As I see it, before you get all testy, this nasty comment makes up for your last "one liner". Now, Fire Away. Have a nice day. Bubba
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Bubba, I am sorry if I offended you by saying that I don't want to get the "run around". You may be or not a honest tech, that I don't know, but I've been screwed too many times by dishonest people. So I am just trying to get sufficient knowledge to be able to determine if I am being B.S.'ed or if the technician is honest. Can you blame me?
Tony
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On 22 Dec 2004 11:26:37 -0800, testradav snipped-for-privacy@yahoo.com wrote:

I would say that I can understand your interest in not being taken advantage of. However, being an owner of one of those hvac companies, taking pride in my business and taking care of my customers, your comment can certainly be taken as offensive. Can you blame ME? Bubba
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wrote:

I'm retired and I still find it offensive.
_______________
Gary R. Lloyd CMS HVACR Troubleshooting Books/Software Written by a veteran Service Technician
https://www.merchantamerica.com/tmethod /
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Andy writes:
I'm retired, too. I hardly find anything offensive....
Chill out...... Smell the roses.....
Andy
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Naw, Bubba. Did it myself......
I don't go runnin' to a unionized repair shop every time I have a little problem...
Testy,.... hell, no.... But thanks for telling me what a new filter can cost..... I only pay 50 cents.... Andy
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Thanks for the help again Turtle. I can run the fan in ON position and it keeps running continuously without stopping. As a matter of fact, that is how I have it now until the service tech arrives. I also changed the filter last month.
The diagnostic light on the ECU remains OFF, so the ECU is not noting any problems with its sensors.
Tony
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Is there a thermal overload switch in the motor? If so, it could be sensing too much heat, real or 'imagined', & shutting it down.
--
Mark
Pepperell, MA
  Click to see the full signature.
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This is Turtle.
This still does not prove anything.
#1 first when you turn the thermostat to cont. fan mode it shifts the motor to a higher speed winding and a motor can be burnt up on heating / low or med. speed and still run fine on high speed for cooling.
#2 The heating mode of the furnace relay board or relay system will run low or medium speed to run the heat and high speed for cooling mode. You can have bad bearing on a motor and start cutting out on low or medium speed but run fine on high speed for you have more horse power on high and can over come the drag of the bearings. Direct drive blower motor have different house power on each speed as here : [ High Speed 1/2 H.P. 1075 R.P.M.'s ] [ Medium 1/3 H.P. 975 R.P.M.'s ] [ Low Speed 1/4 H.P. 875 R.P.M. ] .
#3 Most all older blower motors have 3 or 4 speed types have completely different motor built into them. A 4 speed motor has 4 different motors built into it. You can burn up 1 or 2 or three of the speeds and still have 1 or more that will still work fine. So your testing just showed that high speed of your furnace is ok but never looked at heat speed or medium speed as it being good or not.
#4 Now you have high speed running for the tech to see but when he gets there , tell him to check the problem speed of medium or the low speed set for heating.
#5 Now you still don't know what it is but you can use the high speed to heat with till he gets there.
TURTLE
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cycle?
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This is Turtle.
Yes it sure can be done. I do them all the time by putting the heat and cooling on the high speed but have to watch the heat rise of the heat by having the high speed blowing on the heat mode. There is speed selectors on most all gas or electric furnaces and you can select any speed you want for heating or cooling modes. There is only on gas furnaces over 35 years or older before you just have one speed for both and have no selection set up.
Now if the low or medium speed is running the motor hot due to bad bearing. You have about 6 months to 2 years before the high speed will not work either and catch you when it is freezing weather and want heat to not freeze to death. Fix it now when you have time to work on it and not wait till your in trouble and need it NOW type job. Now jobs cost a whole lot more than a job done in routen work done on the furnace. It's like a water pump leaking water on your car and then you just put it off till your going on vacation and get in the middle of no where and need a water pump changed out. Done at a regular tiome it would cost about $150.00 and on the road with a wrecker charges and Billy & Bob auto service could cost you about $500.00+ in some cases. The furnace is the same thing. Fix it now when you have time and service to do it on a timely manner.
TURTLE
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Andy writes:
Turtle knows more about this than I do.
If you are marginally technical, you can set the thing for normal operation, pull one of the clips off the gas valve (so it can't get gas), and see if the motor still cuts out. If it doesn't, it is probably a temp limit problem...
Bearing problems are tricky. I havn't ever run into one. Turtle probably has....
However, the more you can do yourself to figure out the problem, the less likely you are to be at the mercy of incompetent techs who want to replace every thing in your furnace in order to cure the problem.
If it turns out to be a temp problem, you can short the temp sensor. If , shorted with a jumper, it still works( and don't do this for very long) you need to replace a $3 temp sensor, which you maybe can do yourself...
Many repair men do not have the high morals of those who post here... Andy
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This is Turtle.
If you want to see if it is a Temperature switch you can watch these things and tell in just a little while. Run the furnace up to 90F on the thermostat and watch the flames on the furnace. If the High Temp switch / TSH is tripping it out. The gas flames will go out when it trips and the fan will keep going. Then in a little while when it cools back down the flames will come back on. Now the fan will never stop blowing during all this time. You don't have to jump it but just watch it .
TURTLE
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Thanks for the help again Turtle. I can run the fan in ON position and it keeps running continuously without stopping. As a matter of fact, that is how I have it now until the service tech arrives. I also changed the filter last month.
The diagnostic light on the ECU remains OFF, so the ECU is not noting any problems with its sensors.
Tony
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