Hot Water Circulating Pumps & Re-circ SystemsHot water circulating pumps and systems. How they work and the pros and cons of several systems and pumps |
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Hot Water Circulating Pumps & Re-circ Systems 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. There are several types that include continuously circulating, timer controlled, temperature controlled, demand type, and combinations of all of these. Hot water circulating pumps add convenience and save water, but some waste energy and money!Hot water circulating systems generally provide convenience and save water but at the same time some types of pumps 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 circulating 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 system, but in many cases a home owner wishes to add the circ system to a plumbing system that already exists. There are systems that can achieve this. Some systems require special plumbing, including the above example. A standard hot water circulating system needs to be plumbed with the fixtures in series. In other words, the piping needs to be looped from fixture to fixture. If there are branch lines they won't have instant hot water. In addition these systems need a dedicated hot water return line from the last fixture back to the water heater and circulating pump. Other types of systems include what I like to call Cold Water Return Systems. These systems utilize the cold water piping as the return line for the pump. Typically the pump is mounted under the sink where the instant hot water is needed. Again there are several types of these systems. Hot Water Demand Systems There are basically two types of system that use the cold water return line to complete the pumping loop. One type is the "hot water demand system". With this type of system you press a button when you want hot water, and the pump comes on pumping the hot water from your water heater to the fixture where the pump is located. When hot water reaches the fixture the pump shuts off. Then you turn on your hot water and it's there nearly instantly without running any water down the drain. This is not a circulating system. You only fill the hot water pipe when you want hot water...just like you would do if you did not have a pump. This means you are not wasting energy by keeping the piping full of warm or hot water. So the only energy you are using is the pumping energy. The most powerful pump on the market draws about 3 amps maximum, so lets use that to calculate the cost of the pumping for a year. These pumps typically run for less than a minute...unless you have an awful big home. Let's assume the pump is needed about 5 times a day which is fairly typical. That works out to about 30 hours of run time a year. With a current draw of 3 amps at 120 volts the pump uses 360 watts per year. 360 watts times 30 hours per year works out to 10.95 kWh per year. At 10 cents per kilowatt hour that amounts to $1.00 per year. One dollar to save up to 15,000 gallons of water per year. Manufacturer's of demand systems include Chilipepper Sales, Act Metlund, and Taco. Temperature controlled circulating pumps "The other type of cold water return system is similar in that a pump mounts under the sink, but the pump does not get activated by pressing a button. One such system is the Autocirc1 system by Laing pumps. With this type of system the pump is controlled by the water temperature at the pump. When the water cools to 85 degrees the pump comes on, and at 95 degrees the pump shuts off. The pumps are small and use little electricity, about 33 watts per hour. I don't know how many hours per day they run so I cannot calculate the amount of energy they use for pumping. Making a wild guess, let's assume they run 5 minutes every hour. At 10 cents per kilowatt hour that would be about $2.40 per year. That sounds reasonable. However, the hot and cold water piping is now being held between 85 and 95 degrees, and with the 70 degree ambient temperature there will be significant heat and thus energy loss from such a system. Another point; even 95 degrees is lower than the 98 degree human body temperature, so the water won't really be hot, it will actually feel cool to the touch. You still have to wait for hot water if hot water is what you want. Furthermore, the cold water line will no longer have cold water in it. It will now have warm water. Manufacturers of the temperature controlled type pumps include Grundfos, Taco, Laing, and RedyTemp. |
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