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Problem after change to new thermostat - weird problem with outside condensers

Problem after change to new thermostat - weird problem with outside condensers

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  #1  
Old 08-10-12, 02:20 PM
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Problem after change to new thermostat - weird problem with outside condensers

Hi everyone,

Before my old thermostat was broken, I often had my SYSTEM on thermostat set to COOL and FAN set to ON. So that fan can run continuously even the cooling turned off when set temperature is reached. Everything worked fine for years until 2 weeks ago the system would not turn off even when cooling reached at the set temperature. In order to have the system cooling stop running, I must have the thermostat set to OFF.

Last week I replaced the old thermostat with a new one, same brand and model, robertshaw 9500. Follow exact wire connections on old thermostat and connect them to new one. Now I have a new problem. The problem is when I set FAN to ON to use fan only in a cool weather night; COOLING system set to OFF, the condenser units would kick on running and producing cold air with just only FAN set to ON .. The system behave as if I have the thermostat set to COOL. Other than this strange problem, the system works normal as far as set a temperature at 78 degree and turn system to COOL and set fan to AUTO. System runs and stops at 78 temperature.

FYI, I did not touch any wires at the control board at the air handler unit where there are wires/connections. I suspect there is a defect contactor or fuse in the system? Anyone has any idea where should I look before calling in the pro. I suspect it's an easy fix as far as replace defect part. I just need where to start looking and run some test.

Thank you all for your help.

Chris
 
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  #2  
Old 08-10-12, 04:32 PM
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Make sure U have all the switches on the base in the correct positions.

It has temperature differential settings of 1 to 3; I'd use 2 (or 3) as 2 is 1-degree F of differential.

A setting of 1 is only .5-F; 3 is 1.5-F differential; a setting of 3 or 1.5-F differential will provide longer run cycles & increase SEER when weather is less than summer design.

Check to make sure the TH base wired correctly; it appears it's wired so when the fan switch is on, it sends control voltage to the condenser contactor coil.

If you continue to have problems call Robertshaw tech-support at: 1-800-445-8299.
 
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Old 08-11-12, 11:14 AM
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There isnt a jumper wire at the thermostat is there? Like between y and g on the t stat?
 
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Old 08-11-12, 04:14 PM
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ender2272 - no there is no jumper bet. y and g on the thermostat.

HVAC RETIRED - thanks for your suggestion. I'm going to open up air handler unit and check those wires again.

Would a thunderstorm strike on the condenser unit outside and short a wire? Is that possible? Or any of the fuse on control panel where those wires from outside condenser units connected with wires going to thermostat be the problem if fuse is broken, defected? What part can be defected/broken that would make the system behave this way besides incorrect wire connections? (but i didn't touch any of those wire connections). Strange problem isnt it?

anyone with any ideas, please kindly put your input here ..

thank you all for your time! Cheers! Chris
 
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Old 08-11-12, 10:18 PM
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I got my AC working like before. Everything works!

So I opened up the control box at the air handler unit and saw the yellow wire from outside condenser unit coming in to connection at control box has its sleeve cracked open and leave the copper wire with no protection. I cut off the damaged part of that wire and do the reconnection as it should be. That solved the problem. Now I can have just the fan on without AC kick on running on a cool nite. AC cooling also works fine as well.

I have another question, please.

At first after finished reconnect the yellow wire, I set the FAN to ON at the thermostat to test the fan to see if the fan working, and all I got was a loud click coming from the control box (I think it came from the overload relay) and the fan didn't run. I then set the FAN to AUTO, and set system to COOL. Again, all I got was the same click and COOLING not working, no fan working either. I got scared and immediate turn system off at the thermostat after these 2 test.

Next, what I did was set the FAN to ON .. without being panic cause at this moment fan does not run. I went to the control box and press the reset button on the overload relay. Voila! Fan run after the relay reset. I turned system/fan on and off couple times to test after and every function on system work just fine.

My question is why was that the system didn't turn on after I reconnect the wire? Why is it I have to press reset button on the relay to have the system works again? I'd like to know what reason behind this .. any input is very much appreciated.

I'm glad I was lucky to take a risk of fixing this myself. But then again I had a strong feeling that it's not a difficult fix which I need to call a pro because cooling was working ok; it's just that the fan would not run by itself. I learned new things after this ... I'm very happy and trully appreciated everyone input, help and time.

Thank you all! Chris
 
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