Ac unit won’t turn on.

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Old 07-16-18, 09:12 PM
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Ac unit won’t turn on.

the ac unit model number is acs030a2c1. 30k btu and 2.5 Ton.

The furnace is a comfortmaker. Model number nug9075fgb1.

i replaced the furnace circuit board “original was a Honeywell st9120c4016” this past January when it went bad during the winter. I had a spare board “Honeywell st9120u1011” that I replaced it with. It was a used board prior to being installed. It has worked great up until today. Today my thermostats were calling for air, the furnace fan was running but the ac won’t turn on. I have 24 volts from the transformer to my zoning system board. I have 24 volts from my zone board to my furnace circuit board but it doesn’t have 24 volts to the ac unit outside. The Red and white wires on he furnace board from the thermostat has 24 volt power, the red/yellow and white connection on the furnace board from the ac unit outside doesn’t have anything going out. I can push the contractor in on the ac and the unit comes on and stays on until I release the pressure switch.

I had one more of those used Honeywell boards laying around so I hooked it up to the furnace and still no 24 volt power going to ac condenser unit. Am I just unlucky with the fact that two used boards have gone bad. Last board was stickered 2008. Is there something I’m overlooking. For the mean time I have the contractors jimmied outside to make contact and I’m just watching the thermostat to make sure to get it shut down before the coils would freeze to keep the house below 80. Is it a board issue? Anyways to check other than what I have done?

thanks
jason
 
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Old 07-17-18, 04:54 AM
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To rule out the thermostat as the problem, take a jumper wire and temporarily connect the R terminal to the Y terminal on the thermostat. If the outside unit turns on, the thermostat is bad.

If the thermostat isn't the problem, check to see if you have a condensate switch. Some systems have a float switch for the condensate. If the condensate drain gets plugged, the float rises and turns off the 24VAC going to the outside unit so that the system won't develop any more condensate and overflow.
 
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Old 07-17-18, 07:54 AM
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Thanks

ok thanks I’ll try that tonight. I know the thermostats are good because the zoning circuit board shows that both furnaces are calling for cool and it has opened both dampeners. I have the power coming out of the zone board going to the furnace board but from there the red and yellow connection and the white connection going to outside ac has no power. There’s the typical only red and white wire going to the ac.


My other question is with the contactor switch. If I jump the wires from the 24 volt power source and apply directly to the control wires going to the ac contactor theoritcaly that should activate that contactor switch correct to make the connection and turn the unit on? The ac condensing unit has 220 at the unit I know that because I have checked. I jumped the wires on the furnace control board so 24 volts went outside on the control wires and it still never activated so I’m thinking that my contactor went bad and that may have caused the board to blow or am i crazy?

thanks a ton.
jason
 
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Old 07-17-18, 09:35 AM
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If you have 24VAC across the contactor coil (usually the side terminals), then you're right, the contactor is bad. The contactor is a relay, the 24VAC from the thermostat causes the contacts to pull down activating the relay and sending 240VAC to the compressor and condenser fan.

You can turn off the power to the outside unit (there should be an electrical disconnect box nearby) when you check for the 24VAC. While you don't have to turn off the power to the outside unit, it just makes working around the contactor safer, since there is no high voltage present. Since the 24VAC comes from the house, there's no need for the outside unit to be powered.
 
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Old 07-17-18, 10:28 AM
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If the contactor is bad could that of blown the board up? I just don’t know why the board in the furnace has power going to it from the thermostats/zoning board but it doesn’t have it going out to the ac condenser. It’s A Honeywell ST9120C4016 board and the status light is a slow blink on and off
 
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Old 07-17-18, 12:30 PM
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There are a couple of reasons (besides the board being bad) why you wouldn't have the 24VAC present at the contactor:

1) If your system has a float switch in the condensate drain system and the drain pipe is plugged, the float will rise, shutting off the 24VAC to the outside unit.

2) Many outside units have a low pressure switch in the 24VAC line going to the contactor. The purpose is to prevent the compressor from running if the system pressure is too low (very low on refrigerant), thus preventing possible damage to the compressor.

I would suggest that you start by checking if you have 24VAC present across the contactor coil when the thermostat is calling for cooling. If you don't have 24VAC across the contactor coil, check for the voltage where the low voltage cable comes into the outside unit from the house. There should be wire nuts connecting the cable from the house to the internal wiring in the outside unit. If you have 24VAC where it comes into the outside unit but not at the contactor, you have one of 3 possible problems: low pressure switch has tripped, high pressure switch has tripped, time delay relay is bad.

If you don't have 24VAC coming into the outside unit (at wire nuts), try temporarily jumpering the R and Y terminals on the thermostat.
 
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