Furnace for central air heating, as what for central air cooling

After reviewing the spec sheet for the unit i have found that the LX series has a variable speed blower that will allow the unit to operate under low ambient conditions. Is specifies the system can operate down to 20 degrees. Anything below this and the system will not operate. If you do not have the LX unit and do have the standard, the. The unit is not permitted for low ambient use.

Most central air conditioning systems do not work under low ambient operation and doing so can damage the system. Residential condensers will very rarely come with a crank case heater for the compressor as well as a low ambient control that can modulate the condenser fan speed to maintain proper operation of the system. This is however common practice on heat pumps and ductless units. Most will only allow operation down to 32 degrees. A low ambient kit is normally required to lower the minimum operating temperature of the system.

The issue on running an air conditioner in cold weather is that refrigerant will migrate underneath the oil in the compressor crank case. When the compressor is then energized the refrigerant will rapidly change state as it is agitated and absorbing heat. This will cause the oil to be forced out of the crank case and will cause the compressor to run dry on oil for a period of time until the crank case has refilled with oil in the system. This can cause the compressor to fail. A crank case heater is utilized to prevent this by keeping the compressor pre heated.

So in conclusion if your system is the LX model it will stop operating when the outdoor ambient is below 20 degrees. The fact that the system is advertised for low ambient operation means that it is ok to operate down to 20 degrees. If it is the standard model then you may have damaged the system from running it in low ambient conditions.

Nice chart. This looks like abnormally frequent triggering of the defrost cycle. Typically this will occur if the unit is severely low on charge or when the outdoor thermostatic expansion valve (TEV) has failed and is restricting flow of refrigerant through the outdoor coil so severely that the coil freezes in an abnormally short period of time, triggering frequent defrost cycles. You would have a similar problem if your outdoor coil was exceptionally dirty, but I assume you would have noticed that.

To positively diagnose this, a technician would connect a manifold to the service ports when the unit is heating, and verify that the subcooling of the refrigerant returning from the interior coil is at least 8 C to confirm the unit is not too low on charge. If the charge is adequate, then check the saturated suction temperature (SST) and also the superheat of the vapor in the suction line returning to the compressor. If the SST is much less than 6 C below ambient and the superheat in the line is much more than 11 C, then the outdoor TEV has likely failed in a closed position or become plugged. Depending on the unit this may be a fairly major repair. In a 29+ year old system it may not be worth it versus replacing the unit.

EDIT: Some defrost control boards have a manual setting that forces a defrost cycle at a programmed interval. If you have one of these control boards, you should verify that it has not been set to force a defrost cycle every 30 minutes. Also, someone should confirm that the "freeze-stat" that signals the defrost board when the outdoor coil needs defrosting has not become disconnected or damaged. You can eliminate these by looking at the outdoor coil after the system has been heating for 15 minutes. If it is covered with frost and really does need defrosting in that short a time, then this problem is not merely a control malfunction and is more likely a refrigerant charge / TEV problem.