Need help with a Gremlin! Please!

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Need help with a Gremlin! Please!

Postby john57classic » Sun Dec 29, 2013 3:59 pm

Simple explanation of the state I am observing. Thermostat clicks on to call for heat and immediately clicks off like it's satisfied. While in the "state" the main blower won't run when the switch is moved from auto to on. Initially when in this state the thermostat briefly displayed NO AC between the two clicks.
Now the Gremlin part! sorry for long post.
This state first occurred the w/e before T'sgiving. I check the breaker and then called and talked to the on call tech and he talked me into waiting for the regular work week to save some money. OK fine the downstairs heater was still running and it wasn't going to get that cold. Tech comes Tues. and goes upstairs to troubleshoot gets the unit running by bypassing something I guess and watches it cycle, tells me he believes the control board needs to be replaced, shows me what he says are some hotspots and as he is replacing he tugs on a wire in the main harness and the fan cuts out so he says we should probably replace the harness as well but he is not sure what the availability would be. Takes my $75 , tells me the board will be $650 and his boss will call later about the harness. Boss calls $525 for the harness 7-10 days delivery or we could put in a new furnace for $3200. The thing is the heater has been running for the last 6 hours. I get nervous replacing two things to fix one problem so I take it under advisement.
Heater runs until Saturday and then the state returns, click, click etc. I shut it off and figure I'll call for repair Monday. Sunday on a whim I turn it back on and it starts running again! Now for some reason I get the idea to swap out my thermostats, I'm sure the tech would have suggested replacing if he thought that was the problem but the upstairs and downstairs thermostats are identical so I swap them out power up and both units start running. I cross my figures and hope the d'stairs unit starts acting up so I'm only out a couple hundred bucks for a T'stat instead of $1200 that I'm looking at.
Now both units keep running and running and even though I did not see anything wrong I begin to think maybe there was a bad connection on the upstairs T'stat. Nope, early Christmas morning as I'm waking up, click-click, click-click, crap tell everyone that they will have to bundle up when we go to bed tonight. Next day I get the idea to go up and jiggle the wiring harness just to see if maybe I can get the furnace to run. Well on my way up to the attic I switch the T'stat to on and .... you guessed it, the unit starts running and has been running since! What gives???
I've worked most of my life in large manufacturing plants, around digitally controlled boilers, furnaces, refrigeration units and lots of other stuff and never run up against anything like this.
Any help would be greatly appreciated the thought of laying out $1200 and not knowing whether the thing will be fixed is giving me a knot in my stomach
Thanks and happy Holidays
john57classic
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- Need help with a Gremlin! Please!

Postby Freon » Sun Dec 29, 2013 11:23 pm

Please post the age, make and model of the furnace, the control board and thermostat.

Turn off the power to the furnace.

First step is carefully check all wiring at the furnace and the thermostat. You are looking for any bad connection or broken wire. Sometimes the thermostat wire can fatigue and break yet the strong insulation hides that fact giving you intermittent operation.

Check the grounding of the furnace through the 120 volt AC power connection. The bare copper ground from the circuit breaker should be firmly attached to the furnace frame, usually at the power connection box.

If it is easily accessible, examine the control board for cold solder joints. Wiggle components and make sure they are still securely soldered to the PC board.

Next time this event happens, go to the furnace and measure the AC voltage at the furnace control board between W and the common (C). You should see 24 volts AC. If you do not see 24 volts AC then measure the voltage between R and common (C). Again you should see 24 volts AC.

If you see no voltage at the W terminal and 24 volts at the R terminal, then very CAREFULLY jumper the R and W terminals, thereby calling for heat. Does the furnace light?

Are there any LED lights on the control board showing error codes?

An intermittent problem is the most difficult to diagnose. I hate throwing parts at a problem too so be patient.
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- Need help with a Gremlin! Please!

Postby john57classic » Tue Dec 31, 2013 4:39 pm

Freon, thanks for the feedback. The unit is a 15 year old York unit have to find the model number. The thermostat is a Honeywell Chromotherm III. It uses 5 wires and contains a tag that says must be connected to common to function. I saw this when swapping the thermostats.
I think the testing and jumping you suggest is what the tech did when he was here and got the furnace to fire. I believe at some point he was also not finding 24V where he should at least at first but then.... I am very curious about the NO AC that the T'stat flashed. Haven't found anything on the internet on that.Any chance the 120 - 24 V transformer could be an issue? My experience with them is when they go, they go but all my experience is with industrial, high voltage transformers.
Thanks again
john57classic
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- Need help with a Gremlin! Please!

Postby Freon » Wed Jan 01, 2014 12:11 pm

Before we go to the transformer let's make sure the correct voltages are getting to the correct terminals. After the test I mentioned before you will know if the transformer is bad (no 24 volts AC between R and C) or if the thermostat side (thermostat or wiring between furnace and 'stat) is the problem (no voltage between W and C but 24 volts between R and C).

Lets us know how the tests go.
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- Need help with a Gremlin! Please!

Postby john57classic » Sat Jan 11, 2014 9:32 pm

So the gremlin finally came back earlier this week, I waited until today to see if he wanted to leave. Same situation as previously described. T'stat calls for heat and immediately clicks off.
Got the tools and went up to the attic, shut off the furnace power,removed the panel and started checking things Freon suggested. All the connections on the T'stat wires look good and there does not appear to be any break or even kinks in the wiring. Furnace ground is rock solid to the frame. Turned the power back on and got the voltage meter and checked the W to common NO VOLTAGE ! but I remembered the T'stat was in the off position, I yelled down to the wife to switch it to heat and when she did I immediately saw 24V and ...... You guessed it the furnace fired up. I cycled the furnace a few times and it kept coming on???
Any thoughts are appreciated.
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- Need help with a Gremlin! Please!

Postby Freon » Sun Jan 12, 2014 12:01 pm

You said, "T'stat calls for heat and immediately clicks off." When you say "clicks off", are you referring to the furnace stopping its heating cycle or are you saying the thermostat is no longer calling for heat?

The thing you need to do is wait for the next time you call for heat and the gremlin strikes. Then, leaving everything as it is, go measure the voltage between W and C at the furnace. If you find zero volta AC then you know there's a thermostat issue, assuming the "clicks off" refers to the furnace.

If, however, the "clicks off" refers to the thermostat stopping its call for heat, then we need to investigate the thermostat.
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- Need help with a Gremlin! Please!

Postby john57classic » Sun Jan 12, 2014 4:47 pm

I'll try to clarify onT'stat. When working normally the unit will make a click when turning on to call for heat and a yellow light will come on indicating the system is running, then when satisfied there will be a similar click and the light goes out indicating it is back off. When the gremlin is around it clicks the light comes on but there is immediately another click and the light goes out, nothing on the furnace ever starts that I can tell.
But now I'm apparently in a different condition. It's warm here so there has been no need for heat but last night I noticed the T'stat blinking a lo battery message. Not sure why it has batteries maybe to keep programming if power is lost. Anyway I changed the batteries and left it to mess with today. This morning I put the T'stat in heat mode and it looks like it normally does i.e. click and yellow light comes on only thing is nothing on the furnace starts. I go up up and check and sure enough no voltage between W and the common and there is 24 V between R and common. I have the wife cycle the T'stat off and back to heat to see if the control board light indicates anything but there is no blinking.
The thing is if the problem is with the T'stat I'm really confused because the first thing I did when all this started was to swap the upstairs and downstairs T'stats and the problem stayed with the upstairs unit.
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- Need help with a Gremlin! Please!

Postby Freon » Sun Jan 12, 2014 10:05 pm

Make and model of the thermostat?

As the thermostat is currently installed, is there a wire connection between the C (common) on the thermostat and the C(common) at the furnace?
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- Need help with a Gremlin! Please!

Postby john57classic » Sun Jan 12, 2014 10:23 pm

Honeywell Chronotherm III and yes there is a common connection between the T'stat and the control board. Things continue to evolve right now the T'stat is displaying -AC which I'm still researching but this is what it displays when the power to the furnace is shut off, which it is not. Since I saw 24V across R and common I am starting to think it has to be the control board unless a break suddenly developed in a wire that has been in place for 15 years.
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- Need help with a Gremlin! Please!

Postby Freon » Mon Jan 13, 2014 3:44 pm

You should always see 24 volts across R and C. You're measuring the transformer secondary windings.

The question is, when there's a call for heat and the furnace does its gremlin dance, is there 24 volts across W and C at the furnace? If there is no voltage between W and C AND the thermostat is showing Heating, then you have a wiring problem. With the furnace turned off, I would carefully check the control wiring both at the thermostat and furnace. Sometimes there can be a fatigue break but the strong insulation does not break giving you intermittent connections.

I assume you did replace the t'stat batteries.
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