Thermo Pride Gas/Propane Furnace Premiere Series 2-Stage CHX3 - Manual (Page 5)

All installations and services must be performed by qualified service personnel.
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: The CHX3/CDX3 furnace models are sealed combustion design, which does not
require an air shutter adjustment (air shutters are not used) for proper flame characteristics.
Burner box access cover must always be secured with all screws in place and tightened before
operating furnace.
: Personal injury or property damage could result from repair or service of this furnace
by anyone other than a qualified heating contractor. Only the homeowner/user routine
maintenance described in the Users Information Manual may be performed by the user.