Time limit switch


  #1  
Old 10-15-09, 01:14 PM
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Time limit switch

I have an 18 year old Tempstar 93 percent high efficiency furnace, running on propane. I believe I can get all the model numbers if needed.

The blower keeps running on this thing after the burners go out, and after some forum research, I found that tapping on the limit switch it would shut off and back on as needed. I am very much appreciative of the forums here.

The switch is a BC-1546-AA which is now obsolete but with some replicas for replacements, so I went ahead and ordered one ($169.00 + = ouch).

I had some information that there may be some contacts in those things that could be cleaned to crutch this along for a while, waiting for parts, etc. I dismantled it and found a circuit board with transistors, resistors = whatever they are called - I'm not real savvy about that stuff. But when I had it apart some of these things are obviously fried and melted together.

The problem started this year with the initial startup of the furnace, but the blower had been running this summer with the AC and no problems. Aside from the tapping on the switch and getting it going and shutting off, everything else is working well and pretty normal.

The question is: whether the fried parts in the switch would just be the normal wear out of those parts or should I be concerned about something causing that failure? The cost of maintaining this furnace has really only been two surface igniters, filters/cleaning, and blower motor oil for almost eighteen years so there's no whining about a $200.00 investment now. But if there is something else I should check, it would be great to get any information the subject.

Appreciate the help,

Bob
 
  #2  
Old 10-15-09, 04:18 PM
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I would say the only things that could possibly affect those components is if the motor itself was faulty, or the capacitor.

You can try to make sure there is no slight grounding of the motor through it's windings, to the case of the motor, by running an ohms test between the wires and case. No wire should show continuity - not even the neutral wire. In ac current components, the neutral and ground should be separated - unlike in dc current(i.e. how you can test hot to case on a car's radiator fan motor, and it will show dead short on your meter, yet it can be perfectly good).

Also see what kind of resistance you get between the heat run wire and the neutral wire.

Then maybe have the capacitor tested or simply replace ($ cheap) to ease fears.

Make sure the blower spins freely by hand. And with a slight push, can easily stay spinning for several revolutions on it's own.

Keep in mind that if you have a/c and the blower ran fine for a/c, that the a/c windings in the motor might be good, but the heat (slower speed) windings may be a little suspect.
MAY be. But maybe not. I am just mentioning stuff that could cause undo strain on other system components. Sort of like if you try to drive a car on flat tires, you strain the engine. (You see folks, marbobj is a top-flite mechanic, and good at helping people out in our automotive forum)

You say the blower runs on after the furnace shuts off. It is supposed to. The question is - for how long? And if by chance a furnace gets pretty hot, but yet say does not high limit, the blower will run longer, after the fire goes out because it will take longer for the heat exchanger to cool down.

Your tapping on the high limit, if that makes the blower go off, maybe jarred the fan "off" switch(if that was sticking), rather than the high limit. They may be inside the same component.
 
  #3  
Old 10-15-09, 05:00 PM
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Thanks a lot for the response and great information Ec. I"ll check those things out. Probably won't won't need it this winter in IOWA !!! anyway but I thought I'd get it going to keep the wife off my back.

This thing was running for a long time after it should have shut the blower down. In fact, now I'm using the service switch to shut it off after about five minutes of cold air.

You say the motor runs slower with the heat than with the AC. I would have never guessed. You're a pretty smart guy .

Thanks again.
 
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Old 10-15-09, 05:24 PM
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Retaining information you have heard about, is different than being smart.

I would say that a mechanically inclined person has the advantage, because that person usually already knows to question things in order of simplest to hardest, most likely to least likely, what tools make certain jobs possible and easier, etc. I started out tinkering with cars, building up my nerve to tackle previous unknown things - and it grew from there. It has saved me a lot of money over the years - plus it has been a fun and challenging ride.

Today I worked on 2 furnaces. One of which where when I went to go out and bring in my multimeter, the thing started up and ran correctly for the rest of the day. Can't test then.
 
  #5  
Old 10-18-09, 07:10 AM
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Well I got the things on this animal checked out and everything seemed OK. I put in the new limit switch I got over internet and came up with a problem right away.

Every time the burners would light and run for a short time, they would kick off and the blower would kick on. Then the burners would light and the blower would kick off. That cycling was ongoing. At that time I had the time on/off both at the old switch settings of 60/60.

I thought it must be either I had bought a bogus switch or the furnace was heating up too much and turning the burners off and the blower would come on and cool it down quickly. Then all over again. This is 100% amateur at it.

I have changed those to 30/90 and it seems to be working. Keeping my fingers crossed.

Any other suggestions = I'm all ears - winter's on the way .

Thanks a lot.
 

Last edited by marbobj; 10-18-09 at 07:37 AM.
  #6  
Old 10-18-09, 11:32 AM
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I think I know the controls you are talking about. I just was in a Tempstar (9000) furnace (late 80's vintage), and pulled the blower assembly to oil the ports.

We have the aforementioned Tempstars, with the numbers you can set, on them, in an 8-unit. And I have never had issues with them or their settings. I will see what either the furnace came with, or what the furnace guy set them to. Maybe what you did is increase the off time so that heat would not build up any more in the exchanger, after each heating cycle, and get hotter from built up heat exchanger heat and temporarily trip into high limit.
 
 

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