Systems and methods for defrost of heat pump systems
The present disclosure relates to a heating, ventilation, and air conditioning (“HVAC”) system include a supply damper, a return damper, and a defrost damper which are operable to control a supply airflow, a return airflow, and a defrost airflow to flow between a supply duct, a return duct, and an indoor heat exchanger without substantially flowing into and substantially cooling an indoor space. A reheat coil may also warm the supply air flow and a defrost return line may be used to bypass a bi-flow expansion device and an indoor heat exchanger.
1. A heating, ventilation, and air conditioning (HVAC) system operable to use a refrigerant to heat or cool an indoor space and comprising:
a compressor, an outdoor heat exchanger, an indoor heat exchanger, a bi-flow expansion device, and a four-way valve operable to flow the refrigerant through the bi-flow expansion device in a first direction in a cooling mode and in a second direction, opposite the first direction, in a heating mode;
a supply duct including a supply damper operable to control a supply airflow through the supply duct from the indoor heat exchanger to the indoor space;
a supply air temperature sensor positioned in the supply airflow;
a return duct including a return damper operable to control a return airflow through the return duct from the indoor space to the indoor heat exchanger;
a defrost duct extending between the supply duct and the return duct between the indoor heat exchanger and the supply and return dampers, the defrost duct including a defrost damper operable to control a defrost airflow through the defrost duct; and
a controller programmed to receive temperature measurements of the supply air from the supply air temperature sensor and selectively position the defrost damper, the supply damper, and the return damper into closed positions, fully open positions, and positions therebetween;
wherein in a defrost mode:
the four-way valve is operable to direct the refrigerant flow in the first direction through the outdoor heat exchanger to defrost the outdoor heat exchanger; and
the controller is operable to position the supply damper, the return damper, and the defrost damper based on the received temperature measurements of the supply air from the supply air temperature sensor to control the supply airflow, the return airflow, and the defrost airflow to flow between the supply duct, the return duct, and the indoor heat exchanger without substantially flowing into and substantially cooling the indoor space.
2. The HVAC system of claim 1 , wherein in the defrost mode, the defrost damper is fully open and the supply damper and return damper are fully closed.
3. The HVAC system of claim 1 , further comprising:
a reheat coil positioned in line with the supply airflow passing across the indoor heat exchanger; and
a three-way valve, wherein when in defrost mode, the three-way valve is operable to flow at least a portion of the refrigerant, which is warm and compressed, from the compressor to the reheat coil so that a supply airflow from the indoor heat exchanger to the indoor space is warmed.
4. The HVAC system of claim 3 , wherein:
the reheat coil is positioned downstream from the supply damper; and
in defrost mode, the defrost damper is fully open, and the supply damper and return damper are partially closed.
5. The HVAC system of claim 3 , wherein
the controller is further programmed to:
selectively operate the reheat coil.
6. The HVAC system of claim 1 , further comprising:
a three-way valve located between the outdoor heat exchanger and the bi-flow expansion device, wherein the three-way valve is operable in the defrost mode to direct the refrigerant flow through a defrost return line and a defrost expansion device to the compressor and thereby bypass the bi-flow expansion device and the indoor heat exchanger;
wherein the supply damper and return damper are closed when the HVAC system is in the defrost mode.
7. The HVAC system of claim 6 , wherein in heating mode the defrost damper is closed and the supply damper and return dampers are at least partially open.
8. A method of defrosting a heating, ventilation, and air conditioning (“HVAC”) system operable to use a refrigerant to heat or cool an indoor space, the method comprising:
compressing the refrigerant with a compressor;
activating a defrost mode and during the defrost mode:
flowing warmed and compressed refrigerant from the compressor through a four-way valve and an outdoor heat exchanger to defrost the outdoor heat exchanger; and
flowing a supply airflow across an indoor heat exchanger while not substantially cooling the indoor space by measuring a temperature of the supply airflow using a supply air temperature sensor positioned in the supply airflow before the supply airflow enters the indoor space and controlling, using a controller and based on the measured temperature of the supply airflow;
a supply damper within a supply duct to control the supply airflow from the indoor heat exchanger to the indoor space;
a return damper within a return duct to control a return airflow from the indoor space to the indoor heat exchanger; and
a defrost damper to control a defrost airflow between the supply duct, the return duct, and the indoor heat exchanger,
wherein controlling the supply damper, the return damper, and the defrost damper controls the flow of the defrost airflow into the indoor space.