Puzzled by weird [mis]behavior of AC: what is actually at fault?

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Old 07-09-15, 10:13 PM
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Puzzled by weird [mis]behavior of AC: what is actually at fault?

I have a situation that several people have been puzzled by, so, I hope someone on this forum might have had a similar experience or have some clues anyway.

HVAC System: 1/1-stage cooling/[gas] heating + 3-zones controlled by Honewell MABS EZ-4 zone control panel (+ motorized dampers). This control panel is powered by a separate 24V transformer (independent of the fan/furnace control (relays) board).
The furnace is by HEIL/ICP: model NTC6100KJA1.
All those are in the attic.
The outdoor unit is also by Heil.
The thermostats are RobertShaw ("vintage" SZC250, which are equivalent to 300-201 that are still listed in the catalogs) - standard 4-wire thermostats)

Synopsis: The system gets in the state with the fan blowing forever and the outside unit not running, regardless whether any zone is calling or not.

In more detail:
All is happening during the cooling, with the thermostats in the "cool" mode.
Most of the time, the system is operating just fine.
Occasionally (after a long hot day), several "faulty" states are observed.

Observed [mis]behavior:
1. (state 1) The fan (blower) is running, while no zone is calling. The outdoor unit is not running. [not in a purge mode] Once it happens, the fan keeps blowing forever.
2. (state 2) The fan (blower) is running, a zone is calling, the outside unit is not running. Once it happens, it goes forever.
3. The only way to stop (1) or (2) above is to remove a fuse on the fan/furnace control board, and then reinsert it.
Stopping the blower via the front-cover end-switch has no effect: once it is closed again, the fan continues blowing, and the outside unit is not starting.
Once the system is in the state (2), stopping all zone calls from the thermostats doesn't "reset" the fan-ever-blowing state either.

This (1 and 2) has happened ones during one night, and then 3 times the following night.

4. Once, while standing near the outside unit, we observed the entrance in the state (2) from the properly-working state: While a zone was calling and the unit was working, suddenly, there was a telegraph-type on-off-on-off jitter in the unit's relay (obviously caused by some jitter of the voltage on the control wire, but we didn't have enough time to measure that voltage in the process), and after a few times, the unit switched off (the control voltage was zero after that). The fan continued blowing, and never stopped. The outside unit never came back on.
(I.e. we entered the state (2) described above.)

5. In at least 3 cases, in which the state (2) was observed, the calling zone was the same.
In the remaining 2 cases (out of total 5), I do not have 100% certainty if the state (1) was entered directly from a working state, not via the state (2) with the subsequent change in the set-point on the thermostat that removed the call on a zone. What I mean is that it is quite possible that
the faulty behavior always starts from the state (2), and always happens via the process observed in 4 above.

Additional information:
(a) All components of the outside unit have been checked and appear to be fine (pressure, etc.).
(b) The fan controller/relay board is configured with the dip-switches so that during the normal operation the fan comes on with the delay (30 or 60?) seconds after the the outside unit does.
(c) zone control panel information:
http://customer.honeywell.com/resour...0s/69-1360.pdf
(d) During normal operation, when a zone is calling, and both the fan and outside unit are working, disconnecting the "G" (green) wire that comes from the zone-control panel on the fan board has no affect. I.e. the fan in that case is working because of the signal on the Y(ellow) wire.
(e) During the state (2) the voltage on both Y and G seems to be varying (although this fact is not 100% reliable).

The question is <b>what is the culprit for the problem</b>:
A. zone control board
B. furnace/fan control board
C. thermostat
<b>and why?</b>
In particular, how that explains the following facts:
(i) fan blowing while no calling is in progress
(ii) once states (1),(2) happens, zone calling doesn't start the outside unit
(iii) the problematic state is cleared by removing and reinserting the fuse on the furnace/fan control board (i.e. only cutting the power to that board (and to R)), and not to the zone-control board or thermostat.

The technician who has been troubleshooting this issue is still not 100% sure and consider that it could be one of two components from that list of 3 above.
 
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Old 07-09-15, 10:27 PM
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Welcome to the forums.

Just a quick pass thru as I'm leaving for the night.

When the condensor should be running and it's not..... check the output of the zone board.... Y to C and see if you have 24vac present. If yes.... it sounds like a problem with the condensor. Check outside right at the unit for 24vac.
 
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Old 07-09-15, 10:50 PM
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Thank you!
Yes, that's exactly what the technician suggested, and I am not sure what he measured when it finally happened in his presence.
I did this quickly, in a rush today, when it reoccurred, and as I wrote (it was buried in the middle):

(e) During the state (2) the voltage on both Y and G seems to be varying.

This is Y-to-C and G-to-C.
I checked on the terminals of the furnace fan control board.
 
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Old 07-10-15, 05:23 AM
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(e) During the state (2) the voltage on both Y and G seems to be varying.
Sorry, that was wrong. I double checked that when it happened again. The corrected version is:
(e) During the state (2) the voltage on both Y and G is zero
 
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Old 07-10-15, 10:37 AM
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If the system was already checked by a AC tech, what did he say or suggest ?
 
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Old 07-10-15, 02:11 PM
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He is thinking that it is likely one of the two components from the 3 I listed in the original message here (ABC).
He does not have a clear scenario of what can explain the observed behavior. This is, in large part, because he doesn't have a deep understanding of the details of how the two control boards work inside, and treats them as "black boxes": here are the inputs, and here are outputs. I am not blaming him. But in this situation that type of knowledge/understanding would be helpful for linking to the misbehavior symptoms.

In my opinion, it would be especially helpful to understand:
1. What could be happening with the furnace fan control board that could prevent the zone control board from "posting" the call on Y?
2. If it is the zone control board that keeps the fan running, - would cutting the power to the "R" wire (that's what removing fuse does), and then restoring that power clear that "stuck" state?
3. Could a thermostat, if faulty, cause a jitter on the output to "Y" of the zone control board (despite the logic elements in between the two)?
 
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Old 07-10-15, 04:10 PM
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Sounds like a control board issue, you will have to use a multi-meter trace the 24 volt from the T-stat to control board(s). If you see 24V between Y and C, the outside condenser should run, if you see 24V between G and C, the inside blower should run. If you see 0 between Y and C, (also 0 between G and C), they should stop. If they don't stop, then find out where the 24V come from, IF 24V is not suppose to be there, why didn't the unit cut the 24V off?. That AC tech should be able to figure this out, You can try to trace it yourself, or call another AC tech. And the voltage should not be varying anywhere.
 
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Old 07-10-15, 07:06 PM
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1. What could be happening with the furnace fan control board that could prevent the zone control board from "posting" the call on Y?
It's the other way around..... the zone control board tells the furnace board what to do.

2. If it is the zone control board that keeps the fan running, - would cutting the power to the "R" wire (that's what removing fuse does), and then restoring that power clear that "stuck" state?
Instead of cutting power to the R terminal. Try disconnecting the G terminal to the furnace.

3. Could a thermostat, if faulty, cause a jitter on the output to "Y" of the zone control board (despite the logic elements in between the two)?
Yes.... the thermostat can cause the Y output to jitter.


I went back and reread your first post. You said when the green wire was removed the inside blower remained running since the Y terminal was active. Most systems require the green wire to activate the blower. After the G terminal is no longer powered..... the delay turns the blower off.
When you disconnect the Y connection..... the A/C and blower both stop ?
 
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Old 07-10-15, 08:25 PM
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PJmax
1. What could be happening with the furnace fan control board that could prevent the zone control board from "posting" the call on Y?
It's the other way around..... the zone control board tells the furnace board what to do.
That's under the normal operation. My question is how, if faulty, the furnace board can prevent ("neutralize") the call from the zone board.

2. If it is the zone control board that keeps the fan running, - would cutting the power to the "R" wire (that's what removing fuse does), and then restoring that power clear that "stuck" state?
Instead of cutting power to the R terminal. Try disconnecting the G terminal to the furnace.
That was tried. No effect.
Since there is no voltage on G at that point, disconnecting it doesn't have any effect.

I know that removing fuse (and waiting for 30-60 seconds after that) resets that "stuck" state. Since it seems that the only way how fuse removal can affect the zone board is through the "R"ed wire, I wonder if the absence of voltage on "R" can do anything to any elements on the zone board? I suspect the answer is "no", and the only "role" of the voltage on "R" is to be passed to "Y" or "G" to activate those.

So, based on these two facts, it seems to me that it is not the zone board that is responsible for these two behaviors (ever-running fan and that it can be reset by the fuse removal).


3. Could a thermostat, if faulty, cause a jitter on the output to "Y" of the zone control board (despite the logic elements in between the two)?
Yes.... the thermostat can cause the Y output to jitter.
So, there is no element in the zone board that would smooth out jitter if it is coming from the thermostats? I would have expected that there is some filtering effect at the input of the op-amps of the zone board. And I see a bunch of capacitors on the zone board near each thermostat input, so, I assume those should have the filtering effect.
My thought is here: if we assume that only one element is faulty, and the rest are functioning properly, - just a faulty thermostat couldn't produce jitter on the output of the zone board, - as that jitter would be filtered out within the zone board.

So, based on these conclusions, I am guessing that neither the zone board nor a thermostat alone can produce the observed symptoms.
So, it must be either the fan control board (But how can it create jitter on "Y"?) or yet something else (what?).

I went back and reread your first post. You said when the green wire was removed the inside blower remained running since the Y terminal was active. Most systems require the green wire to activate the blower. After the G terminal is no longer powered..... the delay turns the blower off.
When you disconnect the Y connection..... the A/C and blower both stop ?
The way I read the manual (and the way the A/C tech explained to me) is as follows: for heating, yes, G is required for the fan. For cooling, "G" is not required (and that is selectable by one of the bin switches). If configured that way, voltage on "Y" activates the circuit on the fan control board that, in turn, after one of the chosen delays (by bin-switches), turns the fan on.
So, it is possible that if the relay on the furnace fan board gets stuck, that would explain why the fan keeps running even when the zone is not calling.

What I cannot explain is why if the zone ultimately starts calling, that has no effect on the outside unit. I.e. why there is no voltage on "Y".
Actually, the only way I can envision that is if one the relay is stuck, the side effect of that is that there is no voltage on "R", and thus, nothing happens on "Y", even if the zone board internally connects those, trying to set the voltage on "Y".
It would've been nice to have the circuit diagram of that board... Judging from the diodes and the capacitors near the input of the fan relay (which is DC), - I assume those constitute a bridge rectifier. I wonder if one of the diodes being leaky would explain what we observe (the stuck relay - is likely, jitter on "Y" is possible (due to the reverse current), but no voltage on "R"?).
 
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Old 07-10-15, 08:32 PM
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clocert

Sounds like a control board issue, you will have to use a multi-meter trace the 24 volt from the T-stat to control board(s). If you see 24V between Y and C, the outside condenser should run, if you see 24V between G and C, the inside blower should run. If you see 0 between Y and C, (also 0 between G and C), they should stop. If they don't stop, then find out where the 24V come from, IF 24V is not suppose to be there, why didn't the unit cut the 24V off?. That AC tech should be able to figure this out, You can try to trace it yourself, or call another AC tech. And the voltage should not be varying anywhere.
Which control board? Furnace fan or zone control board?
Please read my thought in my response to PJmax.

When the fan is stuck running,
V of Y-C = 0
V of G-C = 0.
 
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Old 07-10-15, 09:41 PM
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Additional discovery

OK, I just had this all happening again, and was able to do careful measurements of the signal voltages.

1. When the fan is in the "forever-running" state, there is no voltage on "R".
As a result, there is no voltage on "Y" and "G", even though the zone board is calling. I can completely reboot the zone control board, and the fan feels no effect, it just continues running.

2. When the system is in its "proper" state, all voltages ("R", and, when calling, "Y" and "G") are there as expected.

3. I found that removing-waiting-reinserting fuse procedure doesn't always provide the effect (anymore?).

4. When the furnace switches into the "ever-running-fan" state, I found that the exhaust fan is also running.
(Checked: No heat mode from the zone board, none of the zones is calling for heat, emergency heat is also "off", and this is a 1/1-stage system.)
And I hope no gas/ignition gets turned on, i.e. it is only the fans that are going crazy.

So, I am 95% convinced it is the furnace fan control board that is faulty.

Any thoughts why that might not be the case?
 

Last edited by St-Ranger; 07-10-15 at 10:00 PM.
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Old 07-10-15, 10:28 PM
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When the fan is in the "forever-running" state, there is no voltage on "R".
R is your 24v source.

Many furnaces are set up so that if a safety sensor or stat opens...... the 24vac supply is disconnected and the blower fan(s) run until the problem is fixed. Usually the problem happens during the heating season.

Page 39 in the following link is the full wiring diagram.
http://dms.hvacpartners.com/docs/101...2_ICP_3048.pdf

Check the yellow and blue from the transformer and make sure the 24vac to the board is present. Then you'll need to check the safety switches and sensors to see if one is open. It could be just a corroded or loose connection too. The exact sensors your furnace uses is on the schematic on the blower door. The sensors are probably wired in red wire. (shown as orange)

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Old 07-10-15, 10:30 PM
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I also think the problem is with the furnace control board. but I do not understand why R has no voltage. (R should always have voltage unless the transformer stop working. What type of furnace control board or transformer do you have? ). Also If G has no voltage, the blower fan should not run. I am a little confused here.
 
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Old 07-10-15, 11:16 PM
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PJmax

When the fan is in the "forever-running" state, there is no voltage on "R".
R is your 24v source.

Many furnaces are set up so that if a safety sensor or stat opens...... the 24vac supply is disconnected and the blower fan(s) run until the problem is fixed. Usually the problem happens during the heating season.

Page 39 in the following link is the full wiring diagram.
http://dms.hvacpartners.com/docs/101...2_ICP_3048.pdf

Check the yellow and blue from the transformer and make sure the 24vac to the board is present. Then you'll need to check the safety switches and sensors to see if one is open. It could be just a corroded or loose connection too. The exact sensors your furnace uses is on the schematic on the blower door. The sensors are probably wired in red wire. (shown as orange)
That's exactly the manual and the page I've been looking at all Friday long.
I even had that page printed for convenience. If you look at the RHS of that page ("ladder diagram"), then it is clear that only the interlock switch cuts off the power to the transformer, but not any other switch.
The interlock switch is the one for the front panel, and yes, I have to "secure" it while testing with that panel removed.

If I am reading these two diagrams correctly, all other switches affect only the gas supply and ignition, without cutting off anything else.

Besides, it looks like all those switches are on the heater (furnace combustion chamber) side: pressure switch, high temperature switch. I actually had an incident with this system some 5 years ago, when in the middle of the winter, after a power outage, the high-temperature switch was stuck. As a result, the fan kept blowing, while no heat was produced. The tech first reset the switch, and then later, replaced it.

So, this time, my first inclination was to check if there was a similar sensor on the cooling side. So far, I was not able to find any.
There is a possibility to have a remote temperature sensor ("ZoneMAX") on the zone control board, which prevents coils from freezing over, but is not present in my system.

clocert

I also think the problem is with the furnace control board. but I do not understand why R has no voltage. (R should always have voltage unless the transformer stop working. What type of furnace control board or transformer do you have? ). Also If G has no voltage, the blower fan should not run. I am a little confused here.
When you are in cooling mode, you don't have to have "G" (if the the furnace fan board is configured that way). It is sufficient to get a signal on "Y", and the furnace control board energizes the fan with the preset delay. (See my previous message describing this.) If I am guessing correctly, this essentially means, that if the board goes "crazy", it has ability to energize the fan even without any zone board connected.
The furnace control board and transformer came with that Heil/IPC NTC6.... furnace (see the original post in the thread).
 
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Old 07-10-15, 11:41 PM
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No need to highlight and repost my text... it just makes the thread longer.

Look at the highlighted picture.... the transformer secondary is run thru the orange loop. If any of those sensors is defective..... the 24v is interrupted.

If you were to look at the back of the control board. You'll see that one of the transformer secondary lines, I believe yellow, goes to that 6 pin plug. It then goes thru all the safety switches, comes back into the 6 pin plug and then goes to the R terminal.
 
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Old 07-11-15, 09:50 PM
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Re: excessive quoting: Oops... sorry, I forgot to trim the quote to just two lines. That's what happens when you are tired and exhausted from 100F+ heat in the attic, and are trying to post to the forum after 1am.

You might be right about the sensor-switches... But I am not sure where they would be installed.
The diagram on the furnace front panel is exactly the same as in the manual you referred. So, it is not clear which switches are installed. I know that there is a switch for overheating. So far, I was not able to locate it. (And I don't know if that would be called "main limit", "aux limit" or ..? - from the names on the diagram you posted. Do you know?)

But the next time I have time to sweat in the attic, I will try to find the switches, using the diagram on p.40.

Today, I had a chance to do the following experiment: I disconnected all three wires (G,Y,W), except for R, - that go from the zone control board to the furnace, while the furnace was in the "fan-stuck" state. Upon closing the interlock switch, the furnace continued running both fans.
So, I am concluding that neither the zone board nor thermostats are at fault here. Rather, it is either one of the switches or the furnace fan control board.
 

Last edited by St-Ranger; 07-11-15 at 10:08 PM.
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Old 07-12-15, 11:18 AM
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Look at the six pin plug. Do you see two red wires going to that plug ? That would be the safety loop. Check the two red wires at the plug.... if the power is on.... there should be 0v between them or 24vac between each wire and ground.
 
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Old 07-13-15, 09:53 PM
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Thank you for your suggestions!

Today, another, more senior tech came out and on his second attempt was able to catch the "faulty" state. During that, he jumped out the three switches (limit switch and two rollout switch), and the both fans continued running.
So, he is convinced it is the furnace controller board. He is going to order one tomorrow morning, and install it once it arrives.

One thing that he observed was that the voltage on "R", as provided by the board was fluctuating between 28 V and 24 V, and he claimed that once it came down as low as 22 V for a quick moment. While that was within the acceptable range, by itself that might be a sign for what is faulty.

In any case, - thanks a lot for the suggestions!
 

Last edited by St-Ranger; 07-13-15 at 10:29 PM.
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