(Note: I wrote my answer before reading that you don't want to use the Metlund because it has a button that is pushed. Did you know you could also get it with a motion sensor?)
Of the circulation pumps that are "on-demand," the one I am most familiar with is the Metlund Hot Water D'MAND System.
- It is designed for use with any hot water heating system and requires no special plumbing.
- The pump can be operated by a button, but also a remote control or motion sensor.
- I have had one in my house for a few years now and it works very well. I have often specified it for clients. You can install as many of the motion sensors or buttons as you need for convenience.
How the D'MAND system works
The way it works is when the button is pushed, the cold water sitting in the hot water line is sent back to the heater instead of down the drain.
- As the water is going back to the hot water heater, new water is filling the pipe.
- Once the new hot water reaches the pump, a thermal sensor closes the valve and turns the pumps off.
- This actually happens faster than if the water was running down the drain.
The cold water returns to the hot water heater either through the cold water line or through a dedicated return line, depending on how your house is set up.
Combining a tankless water heater with a circulation pump
It is important to note many on-demand hot water heaters are not designed to be used with a circulation pump. The hot water returning to the heater causes increased build-up in the lines and can eventually break the heater.
The on-demand circulation pump is usually placed under the furthest fixture, but it doesn't have to be. Since it shuts off immediately upon sensing the hot water, placing it at the furthest fixture keeps the hot water well away from the heater.
Still, it is important to check with the manufacturer before combining these two excellent energy-saving devices.
If you are purchasing a new on-demand hot water heater, Navien makes some models that can be used with any circulation pump. Also and Eternal make combination hot water heaters that have a very small tank built in, to keep pressure consistent and allow for recirculation.
They are as efficient as tankless hot water heaters.
For more information:
Read "9 Best Practices for Choosing and Installing a Tankless Water Heater" for specific advice about on-demand systems.