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Phil
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7577
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Gas Furnace TUE060A936K Blower does not stop after gas

Customer Question

Gas Furnace TUE060A936K
Blower does not stop long after gas burning stopped. How to troubleshoot the problem?
Submitted: 11 months ago.
Category: HVAC
Expert:  Phil replied 11 months ago.

Hello,

Make sure that the thermostat fan switch is not set to 'fan ON'... if it is set to Fan Auto then the problem is in fan relay mounted in the printed circuit card most likely.

Next please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs tobe taken from straight on, in very good light.

You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here.,Include a picture of the thermostat wire connections to the printed circuit board in the same area.

If you have an error code showing, tell me what the code read out is on the wiring diagram or a sticker next to the diagram please.

If there is no paper clip-icon, look for the '+attach files' link at left of your send button.

I can mark it up with trouble shooting instructions.

We can go from there.

Customer: replied 11 months ago.
Hello Phil,
Attached are Wiring and Schematic diagrams of my furnace and Error Code Flashes. It's 15 years old. Recently I replaced a Gas Valve, but now have a problem with the Blower (its motor was replaced in May of this year). I turned off the furnace right now (after the Blower ran about an hour following gas burning stop). When I turned it on to see the status flashes, first they flashed fast (Normal call for Heat) but then the Blower started and after unsuccessful Ignitor light up, flashing changed to 4 Flashes (Open High Limit Device).
Expert:  Phil replied 11 months ago.

Hello again

http://screencast.com/t/GQVB16F4U

The printed circuit board has a black plastic fan relay on it, with its contacts welded closed if

it removing the red wire from the 24volt side of the transformer results in the fan still running.

Thanks for letting me work with you. When we are finished,if my answer has been polite, professional, and thorough,please locate the 'rate your expert button' and click one of the ratings .If you still need help let me know!.

Rating will not increase your charges, nor close the question, but is how I earn my living.

Thanks again.

Customer: replied 11 months ago.
English is not my native language and for me it's easier to communicate in writing.I've just removed the red wire from the transformer and the Blower did not start running upon turning furnace On. But when I put the red wire back and turned the furnace On it started to work properly: the induced draft blower came up, ignitor lighted up, gas flowed and got burned. About half minute later the indoor blower started.
I turned off the furnace and waiting for instructions from you on how to continue troubleshooting.
Expert:  Phil replied 11 months ago.

Hello again, if the furnace now works and the blower does not stay on all the time, then by disconnecting the red wire you may have solved the problem with the electronics. It will be best to wait and see at this point.

If the blower continues to stay on all the time, then replace the printed circuit card that I marked with a big red box.

http://www.furnacepartsource.com

Call them on the phone, all they need is the furnace brand and model number in order to get you a new printed circuit card.

Note: I am attempting to learn spanish, and its been difficult , your written english is perfect, no small feat to say the least, pronunciation just takes time and practice with a good voice coach.

Thanks for letting me work with you. When we are finished,if my answer has been polite, professional, and thorough,please locate the 'rate your expert button' and click one of the ratings .If you still need help let me know!.

Rating will not increase your charges, nor close the question, but is how I earn my living.

Thanks again.

Customer: replied 11 months ago.
Phil,
First you suggested to replace the circuit board if disconnecting the transformer wire would not stop the blower and now you're suggesting to replace the board after disconnecting caused the blower to stop running.
Why? What is wrong with the board? And how comes that disconnecting and then again connecting of the transfer wire could solve the problem with the electronics? And how is this problem associated with the "4 flashes" error code?
I don't want to replace the board and see the same problem. So please provide thorough explanations if you have them.
Thank you.
Alex
Expert:  Phil replied 11 months ago.

Hello again, so far i am not trying to fix anything, disconnecting the red wire from the transformer, disables the printed circuit card, and if the fan still runs with the power off to the printed circuit card that proves 100% conclusively that the relay contacts are physically burned (welded together).

That was the reason for that test.

However, if removing the red wire from the transformer resulted (accidentally) in a 'fix', that demonstrates an *electroinics anomally in the micro processor chip on the board, those electronics are sometimes influenced at least temporarily by a removal of power.

I will comment on some of the 'why's later, and to any extent that you wish.

Now.... this above ... is my statement after a measly 51 years in the business fixing these sorts of problems in HVAC systems of all types, from home style furnaces to multi million dollar systems seen in industry and tall buildings.

It does not mean by any stretch of anyones imagination that I know what on earth has gone on inside the atomic scale micro processor chip.... those are my *presumptions after experience with these since they came out in this industry beginning in the mid 1980's.

Here are some of the 'why's however.. The integrated circuit (the 'chip') is composed of electronics sub components, so small that hundreds of them can fit in a short strand of human hair... these are primarily transistors, capacitors, and resistors... the transistors function by changing state when power is applied to its base, or not applied to its base... there can be *residual charges* that do not abate as they should at times through the resistors and capacitors within the chip, holding the transistor in a switched position that was not intended by the designer.

Where do these charges come from? Only the shadow knows for sure, they can come from static charges introduced to the circuits from outside means, such as electrical surges on the incoming power, or from even touching the printed circuit card itself (there are lengthy articles on these issues if you would like me to provide links to them, or you can search them on google).

Click here for google search results on static electric discharge effects on integrated circuits

Diagnosis of these sorts of sometimes transient conditions is not worth the time and effort in a lab and is often not more conclusive than my abbreviated note above.

However, there can also be permanent effects, some of those mentioned in this series of articles, that occurs when a high voltage static charge causes an arc between circuits that destroys one of the 'wires' (only 6 or 8 atoms wide of gold, silver or sometimes copper)

Here are a few photo's.

You can click on any of the photo's that interest you, and get to the original web site. most often university or manufacturer web sites.

Here is a good book on the very broad topic of what causes these problems in the manufacturing process, and makes chips vulnerable to later damage post manufacture, (I spent 10 years involved in chip manufacturing facilities, for intel, IBM, and Texas instruments addressing this range of topic)

http://www.amazon.com/Dusty-Dirty-Plasmas-Noise-Laboratory/dp/1461357403/ref=sr_1_1?s=books&ie=UTF8&qid=1451370238&sr=1-1&keywords=micro+electronics+damage

Let me know if this addresses any of your questions on the topic and if not, what else you might be in question about.

We can go from there, without time limit,

Expert:  Phil replied 11 months ago.

When you have the information that you need and are happy with it, please rate my work, it is how I earn my living these days. Thanks

Customer: replied 11 months ago.
You wrote:
"However, if removing the red wire from the transformer resulted (accidentally) in a 'fix', that demonstrates an *electroinics anomally in the micro processor chip on the board, those electronics are sometimes influenced at least temporarily by a removal of power."
Why removal of power by disconnecting the red transformer wire might cause another result than turn off of the whole furnace?
Why with the blower running sometimes the ignitor would light up and sometimes would not with error code "4 flashes"?
Since yesterday night the furnace and blower worked properly but minutes ago I raised temperature on the thermostat. The furnace started but when the thermostat was satisfied and the furnace stopped burning gas, the indoor blower continued to run well beyond the fan-off period (fixed at 100 sec.). However it stopped in 100 sec. after I switched the thermostat off of heat.
Do you still believe that replacing the circuit board would solve the problem?
Expert:  Phil replied 11 months ago.

Hello again, if you had the time out of your busy to day to read that stack of references I left for you last time, you would know why you are getting these variable symptoms. Even then however only someone with microscopic x ray vision would know the real reason why.

We can do a heat stress test on the printed circuit card however.

Use a hair drier to slowly warm the printed circuit card up to120For a bit warmer, but not over 140F... if there is any change in the error codes or other furnace behavior... the card has a cracked micro circuit that opens and closes as the card expands and contracts... that proves a bad printed circuit card.

No change however does not prove the card is good however... but a change proves the card is definitely bad.

Let me know how that goes.

Customer: replied 11 months ago.
I see your point, but do not understand why you exclude the Thermal Limit Switch as the problem culprit. This switch should open at 260 F and close at 230. The error code "4 flashes" indicates that this switch is open. Why do you see the problem in the circuit board rather than in this switch? Or maybe the problem is with hot air output?
Expert:  Phil replied 11 months ago.

Hello again, it very well could be a bad high limit switch, it is just that in this case the most workable test sequence is the card first, then the high limit and a few other things,

Expert:  Phil replied 11 months ago.

The internet has been down in many aspects today, so I am sending this response in snippets to they do not get erased as i am typing.

Do the heat stress analysis on the printed circuit card, be sure you get to at least 120F, 130F would be better. If we get a change of any sort the printed circuit board is bad. It might also be true that the high limit is bad we can check that next.

Expert:  Phil replied 11 months ago.

To test the limit switch remove both wires from it next time the furnace goes off and sends an open limit switch error, use your multi meter to set on ohms to see if it is open (same read as holding both probes in the open air and not touching)

Expert:  Phil replied 11 months ago.

You can order a new limit switch or printed circuit card from

http://www.furnacepartsource.com

All they need is the furnace brand and model number as a rule,

Let me know how all that checks out, we can keep going without a time limit, hopefully the web will stay working well as it is right now.

Customer: replied 11 months ago.
it very well could be a bad high limit switch, it is just that in this case the most workable test sequence is the card first, then the high limit and a few other things"
What a few other things could be?
Expert:  Phil replied 11 months ago.

If you can just do the tests we can get to the bottom of the problem fast, if I spend too much time writing long missives about the business, that takes us off the fast track so to speak,

Other problems can be an oxidized pin in a molex connector. there are a few dozen of those, an intermittent polarity problem at the furnace power supply, electrical spikces caused by intermittent shorts any of the rotating components, blowers primarily,

and over a few hundred things that can go wrong with the printed circuit card electronics.

On this end it is possible for me to loose my train of thought if we skip around during a diagnosis, its best to stay on a linear path, that is generally quite fast and efficient, diversions obscure all that, and I can even forget where I was going. Here at our very reasonable rates it behoves me to be efficient.

I hope you can understand,

Customer: replied 11 months ago.
I tested the limit switch circuit, it's closed.
Are those molex connectors on the circuit board?
If there is an intermittent polarity problem at the furnace power supply, what should I replace?
Expert:  Phil replied 11 months ago.

Hello, testing various things at random takes days or at least many hours and is generally off on the wrong track, as was the test on the high limit switch... if you tested it with the wires removed from the limit switch ... did you do that? or did you test with the wires connected.

Let me know about that detail it is very important.

All of the white nylon connectors you see are molex connectors, there are many types, you can end up way way off into the tall weeds trying to test those with a meter, because just the act of testing them can correct the flaw... there are other approaches, that work way better.

We were on those paths earlier.

Let me know if you would like me to opt out so that you can work with other of our experts who might want to do this random testing approach.

We can go from there.

Customer: replied 11 months ago.
I don't suggest random testing, but I want a full diagnostic plan to nail the problem. So please readdress me to another expert. Thank you.
Expert:  Phil replied 11 months ago.

There are printed full diagnostic plans for many furnaces..but not all furnaces... these are laid out in block diagram form.. and fill several pages, sometimes 6 or 8 pages, and each block will have several trouble shooting steps... it not only takes hours to go through all conceivable malfunctions... and that does not fit our rate or time budgets here, and in most cases it does not fit our customers instrumentation and skill sets.

Beyond that, such charts fail to check the 95% most common failures first, for a ludicrous example, the customer says their house was hit by lightning and a huge fire ball wiped out the TV and set fire to the refrigerator.... and now the furnace does not work.

In that case we can skip directly to a visual examination of the furnace printed circuit card, if that and the thermostat have been burnt to a crisp, in that case it would be ludicrous to do the 4 hour diagnostic block chart procedures.... procedures that our customers rarely if ever have the instruments or experience to execute.

Instead we ask 'how old is the furnace'... if the customer says its 20 years old, we know two things immediately, the furnace is at the end of its normal life span, and two, its not worth the $1,000 or more it will take to sort out how badly the rest of the furnace has been damaged.

I hope that helps, good luck with getting it repaired.