York Air Handler: NAH

Re: Heaters Not Working / Fan Runs Continuous

The control board in this units operates the indoor blower and electric heaters.

The control board has the following outputs:

• (1) On-board relay to start/stop the fan motor (line or low voltage).
• (1) On-board relay for line-voltage heater control.
• (3) DC voltage outputs for heat relays.

► A commonly missed reason for the heaters not working is a 4-Trip Limit Lockout.

• Typically, there is one limit (usually 200 deg.) to monitor over-heating of the electric heaters.
• If the limit trips four times during a single heating cycle or if the limit stays tripped for more than 80 seconds, the board will enter a hard lockout which energizes the fan and does not allow the heaters to come on.

► The board has no LED light to indicate that it is in lockout.

► The lockout can only be reset by cycling power to the board.

Checkout:

1. Check Thermostat Inputs

• Set the thermostat to "Em Heat" mode and initiate a call for heat.
• Ensure that the fan motor is operating.
• Check for 24VAC between "W1" and "C", and "W2" and "C".
  • No Power = No Call for Heat ⇒ Recheck the thermostat (check the outdoor unit for a Yorkguard).
  • Power Present → Step 2. Check Heaters

2. Check Heaters

• Place a jumper between "W1" and "W2" to call for full heat.
• Use an Amp Probe to check amperage of each heater.
  • No Heaters On → Step 3. Check Limit
  • Some Heater On → Step 4. Check Heat Relays
  • All Heaters On → Step 5. Check Temperature Rise

3. Check Limit

• The limits breaks the "Common" side of DC voltage to the heat relays.
• Disconnect high voltage power from the air handler.
• Check resistance across the limit switch.
  • No Resistance = Open Limit ⇒ Allow up to 1 minute for limit to cool.
    • Limit Doesn't Close = Bad Limit
  • Resistance = Limit Closed ⇒ Re-apply high voltage power to the air handler, then recheck heaters.
    • Heaters Still Not Working → Step 4. Check Heat Relays
    • Heaters Working → Step 5. Check Temperature Rise

4. Check Heat Relays

• Check the On-Board Heat Relay
  • Check amperage on the wire connected to L2 or H1 on the heater board.
    • No Amperage = Bad Control Board or Bad Heater ⇒ Disconnect power and check resistance across the heater.
      • No Resistance = Bad Heater
• Check the DC Relays
• Identify the controlling relays for the heaters that are not working.
• Check for 22VDC on the relay coil.
  • No DC Voltage = Bad Control Board
  • DC Voltage + No Amperage = Bad Relay or Bad Heater ⇒ Disconnect power and check resistance across the heater.
    • Resistance = Bad Relay
    • No Resistance = Bad Heater

5. Check Temperature Rise

• With all heaters operating, monitor the temperature rise.
• Report temperature rise to the supervisor.