York Air Handlers - Some Aux Heaters Not Working

York Air Handler: MP, MX, MV, AHP, AHX, AVY

Re: Some Auxiliary Heaters Not Working

The 4HK heater kits for these air handlers consist of In-line beakers, a control board, snap-disc limits, and DC voltage relays. The control board has two heat stage inputs, "W1" and "W2".

• In most instances, "W1" and "W2" are wired together to energize all heater on a call for Aux Heat or Defrost (see wiring diagram for outdoor unit model)

Cases where the customer reports cold air in Defrost, or the system is unable to maintain temperature in cold conditions - it is likely that some (or all) of the heater banks are not working.

• MP, MX, AHP, AHX Air Handlers use Board P#:
• MV, AHV Air Handlers use Board P#:

Possible Causes (From Most to Least Likely)

1. Bad Air Handler Breaker
2. No Power from Panel Box
3. Mis-Wired Low Voltage
4. Bad Relay(s)
5. Bad Control Board
6. Bad Heater(s)

How to Test: (Assuming indoor fan is operating)

1. Check Power & Heater Amperage
   1. Check Air Handler Breakers
      • Set meter to 600 VAC
      • Check for 200 - 230V outlet power at each air handler breaker
        • No or low power - Check inlet power
          • No inlet power = check breaker box
          • Inlet power but no outlet power = Bad Air Handler Breaker
   2. Check for 24V power from "W1" to "C" and "W2" to "C".
      • No power = Check for 24V power from "R" to "C"
        • Power on "R" but no power on "W1" or W2" = No call for heaters
   3. Check the air handler control board's LED light.
      • No Light = No power or Bad Board
      • Green Light = Normal Operation
      • Flashing Light = Fault (See Air Handler - Flashing Code 3)
        • With both "W1" and "W2" powered and no fault indication, all heaters should be on.
   4. Use an Amp Probe to check amperage on each heater (approx. 18 amps per heater).
      • A heater with no amperage would indicate a mechanical failure.
2. Check Board, Relays & Heaters
   1. Check the DC Voltage input on each relay
      • The relays are energized by 22V DC from the control board, so be sure to set meter on DC Voltage when testing coil voltage.
        • "Purple" to "Brown
        • "Yellow" to "Brown"
        • "Gray" to "Brown"
          • If it is too difficult (or dangerous) to access the heat relays, an alternative would be to test voltage at the molex plug of the air handler control board.
          • No DC Voltage to the relays would indicate a Bad Control Board
   2. Check Line Voltage Power through the relays
      • Be sure to set meter to 600V AC.
      • Check from Relay inlet power to opposite side of breaker.
      • Check from Relay outlet power to opposite side of breaker. 
        • 200-230V in but not out = Bad Relay
        • 200-230V in and out but no heater amperage = Bad Heater