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D.E. Anderson
D.E. Anderson, Industrial Refrigeration Service Manager
Category: HVAC
Satisfied Customers: 184
Experience:  15 years experience. Design, install, service of residential & commercial hvac/r equip.
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What is the test plug next to the DOM pot on the control

Customer Question

What is the test plug next to the DOM pot on the control board for a Marvair AVP 36
Submitted: 9 months ago.
Category: HVAC
Expert:  D.E. Anderson replied 9 months ago.
Hi, I'm going to see if I can help you out. Are you sure the model is an avp 36 and not an AVPA36? Thanks
Customer: replied 9 months ago.
Builders plate is obliterated. Can you tell me the difference?
Expert:  D.E. Anderson replied 9 months ago.
AVPA is a 410a model. Is this a heatpump or a AC? I would assume if the data tag is that deteriorated that it is old enough it is most likely a r22 model. And the board you are referring to, is this the defrost control inside the control panel where the contactor and capacitors are located?
Expert:  D.E. Anderson replied 9 months ago.
If it is the board inside the control panel, then the test terminals are for the purpose of testing the defrost cycling during heating
Expert:  D.E. Anderson replied 9 months ago.
What is the issue that you are experiencing with the machine?
Customer: replied 9 months ago.
It is R22. It is inside the electrical control. It has two "pots" to adjust delays, with the test jumper next to the DOM. What I'm getting from the tech is that the unit is shutting off prior to satisfying temp. The tech said he does not see the red led blinking.
Customer: replied 9 months ago.
These units are set at 72 degrees. The one unit is shutting off at 86.
Expert:  D.E. Anderson replied 9 months ago.
Ok, give me just a moment
Expert:  D.E. Anderson replied 9 months ago.
You have a green LED but no red led blinking, correct?
Customer: replied 9 months ago.
My tech says yes. Green is on. No blinking at all from the red. But the unit sounds like it has tripped a LP or HP twice, as the compressor is off, and so it the cond. fan. So my question about the test jumper "YES NO" on yes.
Evap fan is still running in the enclosure.
Pressures are 280 and 85.
Expert:  D.E. Anderson replied 9 months ago.
If the compressor is off and you have a head pressure of 280 and a suction of 85, it sounds over charged and would have appeared to have either shut off on high pressure control or on internal overload. I have a correction to make about the test jumper, it shortens the delays for the delay on make timer, anti-short cycle timer to ten seconds, and the low pressure switch bypass becomes two minutes
Expert:  D.E. Anderson replied 9 months ago.
Ask your technician to equalize those pressures through his manifold, then close his manifold valves, reset power so that it resets the system, jump it into test mode and observe the pressures on start up. This way we can observe these pressures for ourselves and follow the clues the refrigeration circuit provides for us through pressure readings
Expert:  D.E. Anderson replied 9 months ago.
I find those pressures while it off to be very odd. It should be trying to equalize
Customer: replied 9 months ago.
I'm afraid I have put both of us into the weeds with this question. Let me rephrase. Unit ran continuously. When I inspected the site, I found that both of the cond. coils were plugged with dirt and moss and mold. I had the tech clean both completely. Nothing else. Now the one unit is shutting down prior to satisfying temp. I need to know the reason why the jumper on the one unit has been left on test. We inherited this job. What are the consequences of this jumper being placed on YES vs. NO.
Expert:  D.E. Anderson replied 9 months ago.
The jumper on test simply reduces the delay on make timer and the anti short cycle timer to ten seconds. It also reduces the LPS bypass to two minutes. The S jumper being set to yes VS no, will allow or not allow electric heat and compressor to operate at the same time in the event there is a call to W2 at the same time there is a call to Y.
Expert:  D.E. Anderson replied 9 months ago.
This is also assuming your model has a board new enough to have a S Jumper
Expert:  D.E. Anderson replied 9 months ago.
My experience shows a jumper closest to the DOM pot below it with settings for defrost and also Test. The test simply reduces the delay times and anti short cycle time.
Expert:  D.E. Anderson replied 9 months ago.
If its not a heatpump there shouldn't be defrost times. If its just test YES or NO, then you are just shortening the delay times.
Expert:  D.E. Anderson replied 9 months ago.
This would not prevent system operation. In the event of a short cycle it would actually cause the equipment to operate sooner than it would if it were set to no. This could be negative for the life of the compressor.
Customer: replied 9 months ago.
So if the jumper were on YES, and the system was otherwise ok, it could NOT cause a shutdown?
Expert:  D.E. Anderson replied 9 months ago.
No it should not, it actually should facilitate a faster start up time once the machine was turned back on.
Expert:  D.E. Anderson replied 9 months ago.
But a failed delay timer circuit on the PC board would.
Customer: replied 9 months ago.
The failed delay timer circuit. What would this symptom look like?
Expert:  D.E. Anderson replied 9 months ago.
Well you aren't timing out to start up. Also, according to the documentation I pulled from marvair's website, the red LED should be illuminated all the time during normal operation. I would say assuming this information is up to date with your board or vice versa, then the lack of any illumination of the red LED would be an indicator of a bad board. Can you give me the part number off of the board for me to verify the LED function information I pulled from their website?
Expert:  D.E. Anderson replied 9 months ago.
But even with the LED lit, you could still have a defective timer circuit which you would not be able to verify without understanding the intricacies of the board design electronically. Your primary symptom is a lack of start up. With the power off, your technician can ohm out those pressure switches to check for continuity therefore, verifying they are closed. With that information, you will know its definitely not locked out.
Expert:  D.E. Anderson replied 9 months ago.
At least not due to a pressure switch.
Customer: replied 9 months ago.
I don't have that info. I'll have to go visit the site again. I can get back to you later in the week. My thoughts are that the machine in question had its condenser cleaned, and amp draw went from 16 amps to 10.5. Sister unit went from 18 amps to 11. Inside temps in enclosure dropped a few degrees. Then the unit in question just stopped. So the first unit has to carry the load. Memory serves that the unit in question has the test jumper in question on Yes. Other unit No. And the adjacent Pot on the unit in question is set at a different position than the other unit.
Customer: replied 9 months ago.
This is a telecomm enclosure. Little traffic, few door openings. The economizers are closed/defeated. Don't see the potential for any latent heat problems/ frosting of evap coils.
Expert:  D.E. Anderson replied 9 months ago.
It sounds like the board is bad. As if the timer has stuck open. Which would make sense with your circumstances. Do you remember if the board had the red LED lit up constantly? If not, then you should change the board. The board is not a required part for operation and it can be wired around that board to function. The board is a safety mechanism and also operates the electric heaters, but they could easily be wired through a contactor. But for the purposes of achieving immediate cooling it could rather easily be wired to run. It is an AC and not a heatpump, correct?
Expert:  D.E. Anderson replied 9 months ago.
But to answer your initial question, there is no effect on whether it operates or not by changing the jumper to yes or no. It simply reduces delay.
Customer: replied 9 months ago.
Ok. I think the circumstances are that the lack of cooling was addressed: dirty coil was cleaned. As a result of that, the machine is locking out. So the cleaning changed conditions. I think that when the units called for cool, they could not achieve set point and cycle off. Now this unit is cycling off, or running lowing suction and tripping the low press safety. So do you think that the board could have been faulty during the dirty coil phase but because of conditions allowed the unit to keep running, and now that the lack of cooling due to dirty condenser coil has been corrected, the board is now allowed to malfunction?
Expert:  D.E. Anderson replied 9 months ago.
I think it would be coincidental that the board failed at this time. I would say there is some potential that your theory could be accurate and allowed the board to malfunction when a fault occurred. But looking at those pressures you told me earlier, I would not think it tripped a low pressure switch. Usually the low pressure swith is more like a loss of charge switch that will open around 10-20 PSI which means it would have still allowed the machine to operate to the point of a freeze up.
Customer: replied 9 months ago.
The lps is 30 to 40 psi , I think. Those pressures I gave you were running pressures. Ambient 82 degrees. Enclosure was running about 86.
Expert:  D.E. Anderson replied 9 months ago.
84 psi would give you a 50 degree coil. Enclosure at 86 I don't think that the pressure would have dropped that low. 280 would give you a condenser around 127 degrees
Expert:  D.E. Anderson replied 9 months ago.
But if a fault did occur I could see where it would have caused a problem in the board if the board was near failure. But we would have no way of verifying that
Customer: replied 9 months ago.
Tech has no numbers for cond. coil temp or evap. Temp diff between return and supply was around 20 degrees. I'll try to get out there with him later this week.
Expert:  D.E. Anderson replied 9 months ago.
20 degrees is an excellent delta T There is no issue there.
Expert:  D.E. Anderson replied 9 months ago.
I really believe your board is simply defective. It would not be uncommon for an older unit
Customer: replied 9 months ago.
Ok. I'll have the tech work on it this week. Thanks.
Expert:  D.E. Anderson replied 9 months ago.
Ok thanks

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