When house cools furnace blower shuts off but ac unit fan does not shut off

Check the float switch.

Fan should shut off when temp cools down so it looking like what you said.

You most likely have a bad fan limit / cycling switch. If you kill the power to the furnace see if you can trace the wires from the motor to where the connect to the switch. It looks like a round button with tabs sticking out of it for the wires to connect. Mark down where the wires were connected and take it to a HVAC business or furnace supply house. Be sure to have the model off the furnace with you so you can get the correct parts.

Likely have a faulty blower motor module.

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Maybe turn selector switch to off.see if every thing shuts off.if yes. turn FAN selector switch from (auto) to (on) mode.does the outside unit come on or does just the furnace blower only run ? the inside fan should be the ONLY thing on.if not, reverse the (G) & (Y) wires inside the new thermo.

Depending on the age of the furnace you may be able to adjust the cool down. On some furnaces there is a silver box usually made by honeywell. This is your fan limit control. If you take the cover off you will see a dial with different temperature settings with 3 pins in a grove. The first one is fan on with temperature rise this turns the blower on when the furnace starts and warms up. The second is the fan off after furnace shuts down, this would be the one you want to adjust to a higher temperature to keep the blower from blowing cold air. The last pin is the high limit if the furnace gets to this temperature it will shut the furnace off but keep the blower running. Make sure that the thermostat fan option is set to auto and not on.

You will likely need to install an isolation relay in the W circuit from the thermostat. Some thermostats rob a small current through the W circuit to power the thermostat. This small current can be enough to start the fan circuit timer. You can test this by disconnecting W at the furnace and allowing the timer to shut down. If the fan shuts off with the W disconnected you can install a simple relay.

The relay coil will be connected to Common on one side and the W from the thermostat to the other. Then you will run a wire from R on the furnace to the relay and from the N.O. Contacts to the W terminal on the furnace. This will kill that signal that is starting the timer and the W can steal it's power through the coil back to common.

Yes. If the high limit is tripped, the furnace will bring on the fan continuously until the limit resets. Often, they will not reset if tripped repeatedly. If the furnace is too large for your application, you probably are not able to move enough air to keep the heat exchanger cool enough.

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

Try cycling the switch at the furnace to reset the blower board. If this doesn't work remove the green wire from the blower board(computer board) to eliminate a shorted t-stat or shorted wire between the furance and t-stat. If blower remains on, then blower board is shorted. I've seen all these problems.