This article is by no means a troubleshooting guide for hvac. Hvac service work is inherently dangerous and should be left to professionals.
One of the trades that is extremely dependent on multitesters and other testing equipment is heating and air conditioning. Hvac systems are dependent on a series of switches, relays and circuit boards in the newer systems. In residential systems these switches and relays are controlled with low voltage of 24 volts supplied by a step down transformer. When a component of an hvac system doesn’t come on, multitesters can be used to determine where the problem is.
If a relay to a particular device is faulty then a multitester will show if the relay isn’t passing power to it. If we call the side of the relay that takes the high voltage and holds it until the relay is engaged the “line” side of the relay, then we will call the other side the “load” side. If the relay is supposed to be engaged and passing voltage through, then a multitester will reveal that the voltage is at the line side of the relay and not at the load side. We still don’t know for sure that the relay is bad until we check to see if the low voltage is present to the coil of the relay which engages the contacts.
An easy way to test a relay’s coil is to remove one of the low voltage wires and set the multitester to “ohms”. A good coil will show continuity. If the coil is good then the high voltage contacts should show continuity when low voltage is applied to the coil. If low voltage doesn’t seem to be present in the system at all then the step down transformer can be tested to determine whether the transformer is good. A good transformer that has the proper voltage at the primary side will have 24volts at the secondary side.
Capacitors, compressors and fuses can also be tested with a multitester. A “case ground” compressor can be diagnosed by testing for continuity between the electrical terminals and the case of the compressor. Capacitors can be tested with ohms by checking to see if the capacitor is passing a charge back and forth between terminals.
If a condensing unit doesn’t come on the service man will generally check to make sure the thermostat is “calling” meaning it is set properly for the unit to run. He will also check to make sure the air handler’s fan is blowing. If the interior fan is not running as well as the condensing unit, then the transformer may be suspect. If the interior fan is blowing then he will check the fuses or breakers at the box by the condensing unit. If they are good he will use his multitester to make sure 220 volts is present. If the high voltage is there, he will check to make sure the contactor is engaged. If the contactor is engaged and 220 volts is present at the load side of the contactor, then the capacitor will be suspect.