I'm not a HVAC expert but a 5 ton unit sounds pretty large for a 3000sqft house. You can probably approach this in one of two ways. One is probably what the previous owner did and just throw more cooling capacity at the problem (two units, larger units, etc.). Alternatively, you can start looking at ways to get more out of your existing unit. Some things that might help:
- Improve attic insulation
- Improve attic airflow
- If your windows are old, consider replacing them with more efficient windows
- Find and plug sources of air leaks - doors, windows, electrical outlets and other exterior openings
- Plant trees and shrubs to help shade the house
- Install ceiling fans to help circulate the air
- Add insulation to the exterior of the house (requires replacing siding usually)
- Seal all duct work with proper HVAC tape to prevent leaks
- Insulate duct work
- Replace your air filter
- Install an air exchanger
The benefit of trying some of the above is that they also help when heating.
If you want anything close to an accurate answer. You're going to have to tell us a bit more about the wiring between the thermostat and the furnace, or include some photos of the wiring. And include make, and model numbers for all the equipment involved.
Most furnaces manage the blower themselves during a heat call, i.e. the thermostat simply energizes the W wire. Some also manage it during a cool call, but not all do. So when calling for cooling, the thermostat usually energizes both Y for cooling, and G to turn on the blower.
Since you say the blower doesn't come on for cool calls, or when the fan switch is moved to the ON position. That points to a problem with the "fan circuit". Between the thermostat and the furnace, there should be a wire (usually green) connected to the G terminals of both devices. When this wire is energized, it tells the furnace to start the blower. Some furnaces have an on/off delay, so the blower may not come on immediately. On some units, this delay is adjustable.
Since you haven't told us the make and model of any of the equipment, there's no way to be specific about what happens in your furnace. Some furnace control boards have terminals for HEAT, COOL, and FAN. While others may only have HEAT, and/or FAN. The motor speed wires are connected to these terminals, to allow the furnace to run the blower in different speeds depending on the situation.
The first thing I would do, is test for voltage between the grounded ("neutral") and the COOL terminal on the board, during a cool call. If you measure about 120 volts (line voltage), that means the board is energizing the terminal to turn on the blower. This means there's a problem between the board and the motor, or with the motor itself. If the terminal is not energized, then you'll have to start moving "back" through the circuit.
If the furnace is signalling for the blower to turn on, but the blower is not starting. You could try connecting a different speed wire to the terminal. Since the blower works for heat, you might try putting the wire that's on the HEAT terminal on the COOL terminal. If the blower fires up, you know it's a problem with the motor. WARNING: Don't run the system this way for long, you're just checking to see if the blower starts.
If the furnace is not signalling for the blower to turn on (no power to COOL terminal), then you'll want to start by making sure the thermostat call is reaching the furnace. Disconnect the wire connected to the Y terminal, and set the thermostat so it's calling for cool. Measure voltage between C (or the common side of the transformer), and the wire that was connected to Y. You should get about 24 volts AC. If you don't there's a problem with the thermostat, or the wire between the thermostat and furnace. If you do, then the problem likely lies within the control board.
Without more details about the system, it's impossible to offer any more specific advice. It's possible that there's an open limit, which is preventing the blower from turning on. However, without a schematic, there's no way to know if there are any limits that might be involved.
Best Answer
I would suggest listening to your unit when it is running , not just the fan but the compressor also. Possibly make a recording I am talking about the outside unit.
When the unit is not cooling try listening and recording again, The problem could be as easy to fix as an intermittent contactor (a contactor is a high power relay that turns on the compressor) some units have different relays for the fan and compressor so listening is important, the compressor not humming away may indicate it is “sticky” or possibly the starting cap is getting close to failure.
A cap going bad normally causes dimmed lights and hard starting it’s hard to explain the sound but it can take 10-15 seconds when normally it is up to speed in 3-5 seconds. When the start capacitor fails it may blow the fuse or circuit breaker. Or it may over heat the motor and the mother’s internal thermal overload turns it off until it cools.
If I was called for a intermittent cooling issue I usually check the cap they are a cheap fix and look at the contactor and see if it has been throwing excessive arcs. Other things like bad valves in the compressor can be the problem but if your technician put a gauge set on it that would be obvious. One of the last things of the top of my head would be the thermal expansion valve on the evaporator (inside coils) these can stick closed and prevent refrigerant from flowing so the outside unit starts pumps up to shutdown pressure then waits for a cycle restart timer 3-7 minutes on most systems. Edit: You might ask your tech if he measured the cap or inspected the contactor. (Both are in the outside unit). At this time your ears are thallus you’re best troubleshooting tool. Intermittent problems are hard to find because everything is running when we get there.