Thermostat shows correct temperature but the boiler does not start when there is a heat demand.

Safety first: Turn off power to both boiler and thermostat at the main breaker. Verify absence of voltage before touching any wires.

Steps:

• Check wiring diagram: Compare the actual wiring to the Danfoss Ally and boiler manuals. Ensure the thermostat’s relay output is wired to the correct boiler control terminals.
• Confirm control type: Verify whether the boiler expects a dry contact or a powered signal and configure/wire the Ally thermostat accordingly.
• Test relay: With power restored and covers in place, call for heat and listen for a relay click. If no click, the thermostat relay may be faulty.
• Bypass test (advanced): Briefly bridge the boiler’s control terminals with a suitable insulated jumper (if safe and allowed by boiler manual). If the boiler fires, the issue is in the thermostat or wiring.
• Correct or replace: Fix wiring errors or replace the Ally thermostat if the relay is defective.