Water Source Heatpump

Water Source Heatpump (WSHP) is a decentralized HVAC system with terminal units at or near the occupied space. The WSHP system is used in many building types including hotels, multi-family residential, offices and school. The terminal units are also called watersource heat pumps.  A watersource heatpump has an enclosure, fan and motor, drain pan and a refrigeration circuit that can operate in both heating and cooling mode.  In cooling mode, the refrigeration circuit removes heat from room air and rejects it into a common water loop.  In heating, the reverse happens and heat is added from the common water loop into the room air. WSHPs come in a wide range of configurations including wall, horizontal ceiling concealed, Vertical concealed, large capacity, stacked and rooftop models.

A closed loop WSHP system includes a small central mechanical system with a boiler (for supplemental heat), some form of heat ejection (closed circuit evaporative cooler, cooling tower+ heat exchanger or dry cooler) and circulating water pumps. The circulating pumps move water to the water heat exchanger in all the heatpumps. The heatpump is connected to a 2 pipe uninsulated water loop that runs throughout the building. Once of the great energy efficiency advantages of a WSHP system is its ability to move heat from one part of the building to another. For example, heat gain on the sunny south side of the building can be moved to the shaded north side.

The system can also be connected to a ground loop and is referred to as Ground Source Heatpumps (GSHPs). For GSHPs system, the heat rejection and addition equipment is replaced with a horizontal ground loop (smaller systems) vertical bore ground loop, open water loop (typically residential only) and closed water loop. GSHPs systems reject heat into the ground in the summer months and remove the heat in the winter months making it one of the most energy efficient systems available. The GSHP system is all electric so it is a strong candidate for a zero carbon project (Note: a small electric boiler would make a WSHP a zero carbon system). Due to the temperature ranges, GSHP systems  often have insulated piping and anti-freeze.

WSHPs have drain pans so consideration for condensate piping must be given. Providing enough pitch for good drainage can be a challenge.  In those cases, small condensate pumps with overflow safety switches can be used.

Temperature control is achieved by a zone thermostat or sensor per unit which reverses the refrigerant circuit for heating and cooling. Some models have either 2 stage capacity control or inverter compressors for modulating capacity. Dehumidification models are available with hot gas reheat coils.

Since the WSHP/GSHP system is decentralized, a Dedicated Outdoor Air (DOAS) system is required to deliver outdoor air to maintain acceptable indoor air quality. It is good practice and often is a code requirement that the ventilation unit have some form of air to air energy recovery. The Ventilation unit can be integrated into the watersource heatpump system by using a water to water heatpump. This allows heat in the building to be used to heat the ventilation air improving building energy performance and lowering the carbon footprint.