23 of 24 © 2005 D 420 - 06/05
Symptom Possible Causes Corrective Action
Boiler does not fire when there is a
DHW or Setpoint Demand
No voltage present on demand
terminals.
Test voltage across demand terminals
using voltmeter.
Control in Unoccupied and DHW
Unoccupied or Setpoint Unoccupied
set to OFF.
Check which schedule the 420 is
operating on.
Boiler contact is not connected to
boiler thermostat connection.
Trace wires from boiler contact to
boiler thermostat connection. Use
the Test sequence to check the boiler
contact.
Display is not on.
No voltage to control.
Check breaker panel or disconnect.
Check voltage using a voltmeter.
Plugs are not connected between
the 420 to the Zone Manager.
Ensure the plugs are secured to the
Zone Manager pins.
Fuse is blown.
Check fuse on Zone Manager. Check
total VA draw on all thermostats and
zone valves connected to the Zone
Manager. This cannot exceed 40 VA.
Troubleshooting
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