Heat Recovery Water Heating

What is Heat Recovery Water Heating
Heat recovery water heating is the heating of domestic hot water with the waste heat from the air conditioning system. This heat is rejected from the air conditioner's condenser to the atmosphere. By recovering this wasted heat and utilizing it to heat water, it is possible to substantially reduce water-heating costs.

How much Hot Water can be Recovered?
An air conditioning system with a water-cooled condenser, while producing a ton of refrigeration or 12,000 BTU per hour of cooling capacity rejects approximately 15,000 BTU per hour of heat to the atmosphere. An air conditioning system with an air-cooled condenser rejects about 16,00040-17,000 BTU per hour of heat for each ton of capacity. 3,00040-5,000 BTU per hour per ton is relatively easy to recover with a very nominal investment in equipment. This is virtually free heat.

A residence with a three-ton air conditioner could expect from 15-to-25 gallons of hot water raised from 70 to-140 F every hour the air conditioner is running. With a five ton air conditioner a, possible 25-to-40 gallons could be recovered. This hot water would be available during the summer months when the air conditioner is running fairly consistently. As the outdoor temperatures decrease the air conditioner will run less thereby making less hot wafer available. With heat pump, hot water from the heat recovery system is available during the winter as well as the summer months.

Even greater savings can be realized in some type of commercial establishments where the hot water requirements are heavy and there is a high internal heat gain from lights and people in the public area causing the air conditioner to run even during the winter months. Some restaurants, motels and Laundromats have been able to obtain 100% of their hot water during the cooling season from the heat recovery system. The following figures show the gallons of hot wafer that are attainable with a 70 F temperature rise.

Size of Air Conditioning System tons	Gallons of Hot Water per Hour	Gallons of Hot Water per Day
3	15-to-25	180-to-300
5	25-to-40	300-to-480
10	50-to-80	600-to-960
20	100-to-160	1200-to-1800
30	150-to-240	1800-to-2880

It can be seen that during periods when the air conditioning system is operating, the waste heat can produce sizeable amounts of hot water. Additional savings are realized from reduced costs of operating the air conditioning system because of the improved condensing action of the heat exchanger.

How is the Waste Heat Reclaimed?
A heat exchanger is installed in the hot gas line between the compressor and the condenser of the air conditioner. Water from the cold water supply to water heater is circulated through the heat exchanger by a small circulating pump back into the bottom of the tank. A circulating loop between the exchanger and the water heater with counter-flow through the water heater exists when there is no hot water demand.

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