York Air Handler: MP, AHP

Re: Won't Stop Heating/Runs Continuous

The auxiliary heaters are controlled by a computer board and DC voltage relays.

Checkout:

1. Check Outdoor Unit & Air Handler

• Outdoor
• Check to see if the outdoor unit is operating.
  • Outdoor Running → Step 3. Check Thermostat
  • Not Running + System Heating ⇒ Continue
• Air Handler
• Do not turn off power to the air handler or you will lose diagnostic codes.
• Remove the blower access panel.
  • Be cautious not let the panel insulation suck in.
• Check the control board for any active faults.
  • Fault Code + Heating ⇒ Report to Supervisor

2. Check Low Voltage

• Check for 24V power from "W1" to "C" and "W2" to "C".
  • Power Present → Step 3. Check Thermostat
  • No Power → Step 4. Check Heaters

3. Check Thermostat

• Check to see if the thermostat is call for heat.
  • Calling for Heat ⇒ Adjust the setpoint and see if heat goes off.
  • No Call for Heat + Heat Running ⇒ Continue
• Remove the thermostat from the sub-base.
• Re-check 24VAC power on "W1" and "W2" at the air handler.
  • Power Goes Away = Bad Thermostat
  • Power Still Present ⇒ Remove the White wire from the sub-base & Re-check voltage.
    • Power Goes Away = Bad Sub-base
    • Power Still Present ⇒ Remove "W1" and "W2" from the air handler.
      • Heaters Go Off = Check for Shorted Control Wiring
      • Heater Stay On → Step 4. Check Heaters

4. Check Heaters

• Identify any operating using an Amp Probe to check amperage on each heater (approx. 18 amps per heater).
• Check voltage across any operating heater.
  • ​​​​​​​115VAC Present = Heater Shorted to Ground
  • 230VAC Present → Step 5. Check Board & Relays

5. Check Board & Relays

• Trace any operating heater back to its controlling relay.
• Check for 22VDC on the coil of the relay.
  • ​​​​​​​No 24VAC + No 22VDC + Heater On = Bad Relay
  • No 24VAC + 22VDC = Bad Heater Control Board