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Trane XL80 model#TUD100B948AO short-cyling-5 repair visits-


golgi

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Have Trane XL80 model#TUD100B948AO.Initial problem started several years ago.Would turn up thermostat,furnace would start,run correctly,but never continuously until the call for heat was satisfied.Would have to turn thermostat way down,wait 5 to 10 minutes,turn it back up to restart furnace.Worsened during colder weather. lService man came out,recommended replacing vent motor assembly.I did not.Second serviceman  said,golly I(meaning ME,a 50 year old housewife!) should try and clean out the pressure switch tube.Lived with the problem for a year or so and had serviceman come last week.Couldn’t identify problem,but said his company standards reccomend I replace hot surface igniter,although he states it’s fine.Problem worse,he returns,red light on Ignition control module now glowing red-I order part online and replaced.(mySELF! I am woman,hear me roar)NOW furnace short cycles until call for heat is met-we’re talking 1-2 minutes,burners shut down ,repeat.Same serviceman returns,says need new vent motor.Leaves a message on my phone early the next morning,saying”,NO,nevermind,I’m sure it is the fan limit switch,about a $12 part.”

Won’t return my calls.

    Ok

My fan limit switch is a Honeywell L 4064A2881.I have SCOURED the net and CANNOT find it under $100.00.

The new module will not last long under these short cycling circumstances and I am ready to strangle someone.

Oh,I also cleaned the flame sensor with fine steel wool –it’s fine.

Please help.

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Why not just replace the flame sensor? You cannot tell if it is good or bad by looking at it or checking it with a meter either.  Cleaning it won't always work. Also, the burner needs a good ground wire. Make sure you have no corrosion on any of the connections. If the module does not get ignition confimation then the furnace will short cycle. The fan limit does 2 things, bring on the fan when the heat exchanger is warm and if the fan motor fails it shuts down the furnace, replacing it won't solve the problem of short cycling.

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NO, no, no, no!

Please tell what limit settings are set at

Should be as follows: off,on, off

First tab will shut fan off at a predetermined temp (usually 75-100 deg)

Second tab will turn blower on when exchanger reaches predetermined setting (usually 110-150 degrees)

Third tab will be the limit setting (200 degrees max) If heat exchanger reaches the predetermined degree the valve will close (circuit will open) until exchanger drops below limit setting.

Now if these are set appropriately, your static pressure will need to be in question.

static pressure = density x gravity x height

Using a dual manometer will make this a simple task.

To sum this up, If your return air system is undersized, or clogged, you will not have enough airflow to keep that exchanger cooled down enough to keep your limit from tripping.

Can't tell you how many times I've seen people cramming two filters in their frame, or not changing the filters, or using too thick a filter that can cause this!

NOTE! Flame sensor would not allow valve to operate more than 1-2 minutes, so that will not be an issue here. And yes, you can check them w/ a meter (Meter must have a micro amp setting).

But would like to put money on the settings being off on fan/limit ;)

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OK,the first tab is set at 100deg.Second tab:125 deg.,third tab 200deg.

 Walked through the fan limit switch's operation cycle with fellow from Keith specialty.He said it sounded like it was performing as it should and he wouldn't replace it unless I was sure.Something is cutting the power to the unit-gas valve.

Filter is Brand new last week.and repairman checked all the ducts-even drilled hole in one.

Knew it wasn't flame sensor-they rarely need replacing-just cleaning.

Do those settings sound okay for the limit switch?

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Those sound O.K., check and make sure none of your rubber hoses from draft inducer to vacuum switch are cracked or split.

it is somewhat MIND BLOWING that these so called techs [seems you've had several out there], did not solve this with first visit!!!!

Bypass your vacuum switch for one cycle by jumping it out [CAUTION, only one cycle! several disasters could result & probably will result from leaving a safety (vacuum switch) jumped out!!:poison:]

If system completes cycle, then problem exists in either vacuum switch, draft inducer, or a possible stack issue. DISCONNECT JUMPER!!!

What you need to do is troubleshoot this system. The process of troubleshooting is to eliminate all variables!

There are two types of people in this field: techs & part changers!

Let's try to be techs here!

Please report back with results from above test!

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Could you be VERY specific with instructions on "jumping out" vacuum switch? To be clear:that would be the switch connected to the draft inducer motor with the plastic tube?

 I want to make sure I am completely comfortable doing that.

Since draft inducer assembly has come up several times as possible culprit,can you tell me why ? I don't understand how it can produce enough draw to flip switch countless times to initiate cycle -(especially Now,during this short cycling)and still be defective.Does it have to maintain that pull during entire cycle to keep switch open,until call for heat is satisfied?

As an aside,we removed the whole motor and cleaned out vacuum tube from the back of the panel  to the front.All clear.

I apologize for my lack of HVAC terminology,doin' the best I can here!I really want to thank you for your patience and help!...and BOY! do I hear ya- '"There are two types of people in this field: techs & part changers!"'

I haven't Really listed all the parts I was told to replace...at a cost that could have purchased a new furnace and a half!

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Sounds to me like a clogged exhaust vent somewhere.  Birds nest, plugged filter, plugged Acoil, etc.

If the inducer is closing the pressure switch and therefore allowing the ignitor to glow but not allowing it to continue beyond a few minutes, that's where I would start.

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A pressure switch is not that much money. I would replace it before jumping it.

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[user=4554]applianceman18007260692[/user] wrote:

A pressure switch is not that much money. I would replace it before jumping it.

And we find the pres. switch to not be the problem, we now swap parts until we mysteriously come across right one?:shock: Why not just force her into buying a new unit?

Jump it out, if pres switch is indeed the culprit THEN you swap it out!!! Takes two seconds to jump it out.

In actuality, instead of jumping out the switch you should attach a manometer to the draft inducer port and take a reading in inches of W.C. (u-tube or electronic will work fine). Take your reading and compare it to the rating on switch. If your reading does not match you should start looking at inducer side.(I mentioned it being a possible stack issue, and as [user=34365]ROBBYRIG[/user] pointed out you may want to check for a clog in the flue/stack. or check your ohm readings on motor itself)

Continue your process of elimination and move to the next diagnostic.

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Yep,and this pressure switch is $75,add that to the flame sensor you told me to replace and the multiple service call costs,plus the $100 ignition control module and yeah it DOES cost THAT MUCH!

 Sorry,temper got the better of me.

Topps,I only live outside of Cleveland..it's not THAT far away......lol! Aren't you just dyin' to come look at my furnace?

 I am not equipped with a manometer and have no training other than reading everything I can get my hands on.I called my parts guy that I have been doing business with for over 20 years-LOTS of business as I will repair just about anything,and he reccommended a fellow who has been in the business 45 years.I really feel sorry for him when he shows up as I will be forcing printouts of your suggestions,plus tech alerts plus a complete rundown of EVERYTHING.

Have you ever heard this info on the White-rogers gas valve?Is it something I should consider? Looks EXACTLY like my gas valve.Found this on the internet:

The White Rodgers 36-E valve has had problems with loose solder connections under the plastic cover (that is why I had the lamp connected). The solution is to take off the cover and resolder the joints. This has been a problem on the valves used for Direct Spark/HSI and the ones used for Intermittent pilot. A valve that is used for intermittent pilot will have three wires designated PV/MV(Common), PV(Pilot-Valve) and MV(Main-Valve), a valve for Direct Spark/HSI/Standing pilot will usually only have two but possibly three depending on the brand of the controller. Test for this problem by whacking the gas pipe with a screwdriver and if the flames go out while the valve stays powered then you have this problem. Other symptoms include the valve working for periods of time and then stopping. In both case power to valve will remain for a few seconds until controller drops out.

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Golgi, just jump the pres switch out! I only suggested the manometer as a point proven.

Jump the switch out and report back with the results. We should then be able to walk you through the rest of the process.:D

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Topps,I do not know how to "jump out".I would have to be walked very carefully through the procedure,and I'm not sure I would be comfortable without a diagram.This would probably be more than you are willing to get into,and I don't blame you.

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