Abstract: A heat pump system includes an outdoor unit, an indoor unit, a compressor, and outdoor and indoor fans. An outdoor heat exchanger coil is connected to an indoor heat exchanger coil by liquid and vapor tubes. A control system limits the pressure in the vapor tubes during heating mode periods of heat pump operation by using a temperature sensor including a switch operatively connected in the indoor heat exchanger coil for sensing a temperature in the indoor heat exchanger coil. The switch operatively connects in series with a motor of the outdoor fan, such that when the temperature sensor senses a predetermined temperature in the indoor heat exchanger coil, it causes the switch to reduce a speed of the outdoor fan, thereby reducing the temperature in the vapor tubes and indoor heat exchanger coil.