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Hot water circulating systems are hot water systems that circulate hot water through the hot water piping so that obtaining hot water is nearly instantaneous. They are often referred to as re-circ, recirc, or recirculating pumps.
This area of the Chilipepper web site provides an explanation of how the various types of hot water circulating systems work complete with illustrations of hot water circulating systems and other basic plumbing layouts.
Hot water circulating systems generally provide convenience and save water but at the same time can waste large amounts of energy and cost the home owner a great deal of money. As an example lets see how much energy is lost from a typical recirculating system. For our example we will calculate the amount of energy lost from a 3/4 inch copper pipe 50 feet long representing the piping through a home, and a 1/2 inch copper pipe 50 feet long representing the return line back to the heater.
Let the ambient temperature be 70 degrees F, a hot water temperature of 140 degrees, a 75 watt circulating pump running full time, and lets assume the piping is insulated with 1/2 thick insulation. There are 8,760 hours in a year. For the 3/4 inch pipe the heat lost from the pipe is 12.4 Btu/ft-hr., and for the 1/2 inch pipe the loss is 9.9 Btu/ft.-hr. Doing the math we find that the losses for this system total to 9,767,400 Btu/year. If the pipe is not insulated the losses come to a whopping 37,492,800 Btu/year. Figuring the energy required to run the pump gives us 657kWhr/year. Gas and electric rates vary widely, but assuming 50 cents a therm (100,000 Btu) and 10 cents/kWh, it would cost $114.53 per year to operate. With the bare pipe version it would cost $253.16 per year to operate. Maximum efficiency is obtained when the plumbing is designed for the circulating system, but in many cases a home owner wishes to add the recirc system to a plumbing system that already exists. The following pages describe the various circulating systems and pumps in detail, how they operate, and the pros and cons associated with them. Traditional Circulating Systems