Heat pump system defrost control

An outdoor coil defrost control system (hereinafter "defrost control system") for a reverse cycle refrigeration system (hereinafter "system") for heating and cooling a building wherein said system comprises refrigerant compression means, an indoor coil, an outdoor coil, and refrigerant conduit means connecting said compression means and said coils, said defrost control system comprising:

• outdoor coil temperature sensing means (hereinafter "TODCS") having an output indicative of the temperature of said outdoor coil (hereinafter "TODC");
• means (hereinafter "COM") operatively associated with said compression means and adapted to have an output indicative of the operation of said compression means;
• and
• controller means having operative connections to said TODC and COM so as to receive the outputs thereof, said controller having a timing function which is initiated upon (i) the outdoor coil temperature as sensed by TODC being at or below a preselected value and (ii) said compression means being operated,
• the duration of said timing function being determined on a substantially continuous basis by the magnitude of the outdoor coil temperature as sensed by TODCS, and the duration of the timing function of said controller means being determined by a succession of calculations of points (based on (1) the instantaneous value of TODC and (2) the total of previously calculated points), and the addition of each such calculation to the preceding total, such calculations continuing until the total points is greater than a preselected number of points, and
• said controller means having an operative connection to said system and including means responsive to said point total exceeding said predetermined number and thereupon effective to place said system into an outdoor coil defrost mode of operation.