I turn on the Central A/C unit with the thermostat. The attic fan unit goes on and the system cools the house. When the system shuts down the fan unit shuts off but the outside compressor keeps running. The only way the outside compressor goes now off is when I flip the 240 switch on the main box. This just started. Is it a thermostat problem? Thanks.

With a non heat pump it is a simple 24 volt AC control circuit. In the attic there is a relay that is actuated via the fan control wire directly from the thermostat. On the thermostat there is an auto/man switch that controls the fan. Manual the fan runs all the time. Auto fan turns on and off with the outside compressor.

If the system is not calling for cool and the fan continues to run the fan relay is sticking. common problem. Sometimes after running for several days the relay will stop sticking if not, replace relay.

If the system is calling for cool and the compressor stops I would still expect the fan to run until thermostat tells the A/C to off. Some compressor units include a safety switch or high-pressure cut-out switch A blocked internal valve bad control board or external fan in outside unit defective no air flow in outside unit.
High pressure cut out not good,

Suggest turning temp max cool and observe. If acceptable cool and the compressor runs without interruption all is good, except for the blower in the attic. Safely check relay in attic blower circuit.

Hope this helps.

faulty thermostat switch

Sounds like a sticking relay. You can check the stat simply by remove the faceplate. If fan still runs with tstat cover off its not the stat. Not sure is your blower has a separate relay or if relay is mounted on control board. This can be bypassed with a new relay if you are willing. Good luck!!

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Hi. The most likly cause of this type problem is the thermostat. Check the thermostat housing is it hot to the touch, is it close to a heat soarse, suchas, a motor or flu pipe for a water heater. If the thermostat has a adjustable heat range setting try setting it to the lowest setting if this shuts the unit off the calibration of the unit may not be set right. If the unit is new this should be covered under warranty.

Sorry to hear about your miss-fortune, you are right it is an wiring problem. Can you turn off the blower with the thermostat (cool-off-heat) or what happens when you switch the fan to (auto or on) When you say the outside unit is "rigged" is it mechanically being jammed or electrically? If its mechanically that should be removed (what a crappy thing to do) other wise it will run continuously as you may already know. That is what it sounds like, you should just be able to shut the entire system down by turning off the switch to the furnace. If it is electrically being held closed, he changed some of the wiring around in the attic unit, you better hope he was lazy and jammed a stick in the contactor in the outside unit or else you may need to learn more about HVAC wiring, none the less I would double check the thermostat wires going to the attic furnace there should be nomenclature on the unit or at least somewhat marked. This was probably disturbed when the new blower was installed, by the way that was your initial problem. Good luck and becarful

The attic fan is not a part of your a/c system, and you should not run it while you are running the a/c. If you are currently using the attic fan, it is possible that someone wired it so that the a/c won't turn on while the attic fan is on. Otherwise if the thermostat is set below room temp, and says it is calling for cool, then the problem is most likely coming from an issue at the outside unit itself. Make sure to check your fuses, at the fuse panel and outside breaker. Hope this helps

FYI.. the only part of the system that might use batteries is the thermostat.

In many mobile homes this scenario is very common, less so in permanent homes, but here is how it works; The heat side of your fan which is the low fan speed is not dependant on the thermostat it's heat activated. Your already aware of this to some degree because you had the limit \fan switch replaced. I've taken that approach to that unit myself. In reality what probably is happening is on hot days your unit is warming up by being hit by sunshine and then it clicks the fan on at 140 degrees. This especially happens if the unit is on the roof or a split system in the attic. Remedies; run your A\C to cool it down, keep your attic cooler by way of an attic exhaust fan(which saves you cash anyway). Or if your not regularly using A\C turn breaker off til you need it. In the end there may be something going on, but odds are I'm dead on. Good Luck

I need to verify you have adaquate air flow, to cool the motor. If you have a window open, and the crawl hole open is there enough air flow into the the attic? It there is enough air flow and it still stops after about an hour, does this unit have a humidistate and a temperature switch on the housing or extension.
If all the switches are working correctly then I could be the motor overloading/heating shutting down and then reseting.
InMrFixIt

your landlord needs a service tech !sounds like you have a few issues.possibilities include control issue,plugged evap coil,plugged blower wheel,possible freon issue.This one's a little too big to handle here.

This unit has a bad Start/Run Capacitor or a bad Fan Motor! Replace the $10 Cap...An then the $90 fan motor if the Cap dont solve it!