Desuperheater provides 'free' hot water
By PAUL SCRIVENS
Originally Published: January 30, 2015 6 a.m.
Question: Could you explain what a hot water desuperheater is? What does it do in a domestic hot water system?Answer: Desuperheat is the opposite of superheat as described in superheated steam (steam that has been heated beyond the boiling point). Desuperheating is reversing that process and lowering temperature back toward a liquid by giving up heat.Desuperheaters are an established technology, with a 15- to 20-year history in residential applications. They are also called heat recovery units or heat transfer or heat exchange units. A domestic refrigerant desuperheater is a heat exchange unit that takes heat from a heat pump's (or central air conditioning unit's) high-pressure refrigerant line and uses it to heat water for domestic use. It is a supplementary heat source, as it only works when the air conditioner or heat pump is operating. Desuperheaters can typically provide about 5-8 gallons of hot water per hour, per ton of refrigeration capacity. For a typical 3-ton central air conditioning unit, a desuperheater could provide a full tank of hot water every 3 hours. Most discussions concerning desuperheaters and hot-water generation focus on air conditioners and warm climates in which the air conditioner is in a cooling mode. The refrigerant cycle produces a lot of heat that has to be dispersed by the condenser and fan outside; a desuperheater takes advantage of the heat excess and exchanges it for free hot water. A heat pump's advantage over an air conditioner is that it can heat and cool by reversing its compressor configuration. However, in the heating mode, the heat is used to heat the home, and the desuperheater steals some of that heat. In the refrigeration cycle, refrigerant is pumped from the compressor at high pressure and high temperatures and circulates it through the desuperheater refrigerant coil; it loses about 20°F during the transfer. Water is pumped at 2gpm from the main water tank through the desuperheater water coil and back, adding heat to the water with each pass through the heat exchanger. It works just like a hydronic heating system. This heat exchange results in increased power efficiency and SEER (seasonal energy efficiency ratio) of about 10 percent for the refrigeration system, while at the same time producing "free" hot water.In Prescott, the cooling season is short, so the heating season provides most of the free hot water. By using a heat pump for hot water, you will greatly offset your current water costs, as it provides enough heat to your hot water tank to prevent its own heating source from operating. Adding a desuperheater will not diminish the efficiency of your heat pump, nor will it cause your heat pump to work harder.Since desuperheater installation requires the use of refrigerant recovery equipment, it is important to ensure the contractor is licensed. Depending on local codes, a plumber may also be required to connect the water side. While most central air conditioner manufacturers honor their warranties following installation of a desuperheater, certain requirements and conditions may be imposed. Consult with the air conditioner dealer prior to installation.In order to operate safely, a desuperheater should come with the following safety devices: a low-temperature refrigerant (>125F) activation switch, high-temperature water output (An ideal and efficient system could be a direct-vent natural gas dual-fuel central HVAC system and a direct-vent natural gas hot water tank with a desuperheater connecting the two units. The heat pump would run most of the time during the heating and cooling season utilizing the desuperheater for hot water generation. When the HVAC gas backup furnace operates at low outside temperatures, the gas hot water unit also operates, and when the HVAC system is off in the shoulder seasons, the hot water tank works as a normal stand-alone gas-fired hot water tank. This combination provides one of the most economical operating costs for heating, cooling and hot water generation.Next time: Passive/active and photovoltaic solar.For more information, visit www.greenhomeenergyadvisors.com.