Parts covered on both boilers and furnaces:
Circuit Board
Flue Connector Box
Fuses
Gas Valve
Igniter
Induced Draft Fan Motor
Limit Control
Main Burner
Manifold
Pilot Burner
Pressure Control
Regulator
Relay
Sensors
Thermocouple
Thermostat (Standard)
Transformer
Vent

Parts covered on boilers only:
Aquastat
Circulator
Coupler
Gas Regulator
Low Water Cut-Off
Pressure Gauge
Relief Valve
Sight Glass

Parts covered on forced-air furnaces only:
Belts
Blower Shaft & Bearings
Blower Motor
Blower Wheel
Fan Control
Pulleys