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There are a number of types of recirculation systems for residential plumbing. There are the traditional full-time circulating systems that continuously pump a small flow of hot water through the piping from the heater to the fixtures and back to the inlet of the heater.
Full-time recirculation pumps are very wasteful of energy. Your water heater will have to work much harder to supply the heat needed to replace the heat being radiated from the piping system. In years past when energy was cheap and plentiful these systems were very popular, however, in today's energy conscious environment they are becoming rare in residential plumbing systems.
There are ways to reduce the energy loss such as putting the pump on a timer to reduce the number of hours it runs. The downside to this is that if you want hot water when the system is off, you will have a long wait and you will run a lot of water down the drain. And the system is still continuously losing heat energy while operating.
Temperature control of the pump can further reduce the energy loss from the system, but not very much unless you turn the temperature down substantially, which kind of defeats the purpose of having a recirculating system to begin with.
Another problem with recirculation systems is that the water flow is very low, usually not enough to turn on a tankless water heater. This is unfortunate, since tankless heaters require even more time than a storage type heater to provide heated water to the fixtures.
Several manufacturers like RedyTemp, Grundfos, Laing, and Watts are now offering what I like to call luke-warm recirculating pumps. These pumps are again, low flow hot water circulating pumps, but controlled by the water temperature.
These pumps and in some cases valves are placed under the sink at the fixture furthest from the heater. The sense the water temperature in the piping and turn on around 85 degrees F, and off at 90 degrees F. The turn on and off temperatures may vary, and some models offer the ability to adjust the temperature.
The problems are the same as using a traditional circulating system, large energy losses from circulating warm water through the piping. It's less wasteful than the higher temperature circ systems, but still uses far more energy than a normal residential plumbing system. Since this system tends to raise the temperature in the cold water piping quite a bit since it is in the recirculating loop.
Many people do not like luke-warm cold water, and they end up running water down the drain purging the warm water out of the lines to get cold water.
The most energy efficient type of recirculating system is the hot water demand system. The demand system pump only operates when you desire hot water. When you push the start button, or activate it with some other means such as a motion detector, the pump comes on and speeds the heated water to your fixture, sending the cooled off water from the previous use back to the heater through the cold water piping.
The pump shuts off when it detects the arrival of hot water. At that point you can turn on the faucet and you have instant hot water. No more hot water was used than if you had not used a pump, except for the energy to run the pump itself. The pumping energy for a typical use is microscopic. These systems typically use about $1.00 per year in electricity to operate.
Several companies offer hot water demand systems including TACO pump, Metlund D'mand Systems, and the RedyTemp can be operated in a demand mode as well. Most of these systems use standard old fashioned low power low flow recirculation pumps. Metlund and RedyTemp use Taco pumps.
These manufacturers tend to provide miss-leading information about the pumps inferring that they can pump 14 and even 28 gallons per minute. That would be true if there was no pipe hooked to the outlet of the pump, but there is, and that is why many of them are not recommended for tankless heaters; they don't pump even the 1/2" to ¾ gallons per minute needed to turn on the heater.
Chilipepper Sales offers the CP6000, by far the most powerful hot water demand pump on the market, and it can typically pump about 3 gallons per minute in most homes. It can turn on any tankless water heater on the market including all Bosch models, all Noritz models, all Takagi models, and all Rinnai models
The CP6000 also has the lowest price of any demand pump.