HVAC Overloads and Safety Switches Don’t Just Fail
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A float switch is wired into the thermostat wiring to kill power to the system if the drain system begins overflowing. The switch opens to create an incomplete circuit, similar to how an open drawbridge stops traffic. Float switches are normally closed, and they open when they are filled with enough water.
When a float switch opens, the first thing you should do is check to see if there is water inside; you want to see WHY the switch is open, not just replace the switch. If there is NO water in the open float switch, then you’ll want to check for a path with an ohmmeter. If there is no path, then it’s a failed switch. However, if a switch is failed open, it’s likely that it has been opened and closed many times in the past.
Thermal limits can sometimes be called overloads as well. Those safeties open when a component overheats, such as a surge protector. Modern surge protectors use normally closed thermal limits that open when the surge protector gets too hot. High-limit and rollout switches on furnaces as well as thermal overloads on compressors and heat strips are thermal limit switches. Fuses work similarly, but they break completely instead of merely opening like a switch. Again, make sure you figure out why thermal switches opened instead of just replacing them.
In furnace systems, poor airflow can cause the high-limit switch to open. Dirty coils, filters, and blowers are potential causes of thermal switches opening. A compressor could go out on thermal overload due to a failed fan motor. However, sometimes, thermal switches reset themselves quickly due to electric overheating on the windings. If you replace the capacitor and correct the locked rotor condition, then you won’t have the thermal overload anymore.
Compressors also get too hot when there’s insufficient or way too much refrigerant moving through the compressor. When the compression ratio is high, a compressor will run hotter than it should (especially due to low suction pressure and high heat pressure). Systems with long runtimes and excessive superheat can also cause compressors to overheat. All sorts of restrictions can also cause overheating. In those cases, the compressor should be cooled with water from a hose.
To tell if a thermal switch has been reset, put your meter on the ohm scale with the ringer on and put the leads on your contactor across the two legs. Generally, the condenser fan motor will have high enough resistance to prevent your meter from ringing, but the compressor has low enough resistance to make the meter ring.
Low and high-pressure switches are also safeties that look similar on schematics. The high-pressure switch opens when the pressure increases (the line is above the right circle on the schematic), and the low-pressure switch opens when the pressure decreases (the line is below the right circle on the schematic).
If a pressure switch is stuck open, check the conditions that may have made it open (such as undercharge). Jump out the low-pressure switch and see why it went out on low pressure. Do the same to the high-pressure switch if it has failed (possibly due to failed condenser fan motor in cool mode, failed blower in heat mode, poor airflow in heat mode, or overcharge).
In all cases where you encounter open switches, the most important thing you can do is find out WHY those switches opened.
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