initially sends power to the pressure switch 34 the pressure switch 34 initially sends power to a lead l 3 on the blower motor 22 that operates the blower at a low speed the pressure switch will then continue to send that power to the lead l 3 on the blower motor 22 that operates the blower at a low speed unless the pressure sensor 36 detects pressure at or above a threshold pressure indicative of pressure downstream of the blower 22 which is indicative of high back pressure in the exhaust vent line 30 upon detecting pressure at or above the threshold pressure the pressure switch 34 will switch power to a lead l 4 on the blower motor 22 that operates the blower at a high speed upon operating a higher speed the sensed pressure will be even greater this prevents the blower motor 22 from short cycling between speeds assuming a sufficient vacuum is being drawn upstream of the blower the vacuum switch 40 will have been triggered and the controller 38 will activate the igniter and fuel gas valve and will continue to send power to the pressure switch 34 however if after a brief delay the vacuum switch 40 is not triggered by a sufficient drop in pressure upstream of the blower 39 s 22 fan output the controller 38 will not activate the igniter and fuel gas valve not shown of the burner 24 and will discontinue supplying power to the pressure switch 34 it should be understood that the pressure tap for the pressure sensor 36 could be located at any location where the pressure measurement can be correlated to the back pressure and flow output through the exhaust vent line 30 thus the pressure sensed by the pressure sensor 36.