A heating or cooling system, such as an HVAC system, of variable output has a number of control elements and may include a variable speed compressor, a variable speed combustion (induced or forced draft) blower motor; a variable speed circulator blower motor; a variable output gas valve or gas/air premix...http://www.google.fr/patents/US7293718?utm_source=gb-gplus-shareBrevet US7293718 - Variable output heating and cooling control
