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Here's the situation, you turn on the hot water faucet in the bathroom that is the farthest from your water heater and have to stand there waiting for 5 minutes to get hot water.  What if you could get hot water in a few seconds instead?

I connected 1/2" copper pipe through a ball valve from the farthest end of my hot water run all the way back to the cold supply on my water heater.  This is about 40 feet of pipe.  I installed a 1/2" swing check valve to prevent cold water from running backward through this line.

My first plan was to use a Grundfos Comfort PM recirculation pump to push the cooled hot water back into the cold supply.  I bought a mechanical temperature switch that would close below 105F and open at 115F.  The plan was to install this switch at the end of the hot run and have it directly switch the 120VAC to the pump.  I installed everything and the pump ran beautifully.  The only issue was that when the pump shutoff, hot water kept flowing.  I noticed the cold return on my heater was very hot long after the pump had shut off.  Turns out that the system naturally thermosiphons around and, unfortunately, my pump could only pump and not stop the thermosiphon.

The next plan was to replace the pump with a solenoid valve.  I had one left over from some previous work and installed it.  I also decided I could do better than a simple mechanical switch (which didn't work well anyway) and installed a Chromalox temperature controller hooked to a three wire Pt100 RTD to measure the temperature.  Now I can easily adjust the desired temperature at the end of the hot run.

I also put the whole contraption on a wifi controlled outlet so I can easily control the timing.  No sense recirculating hot water while I'm gone or asleep!

One big question I had when installing this system was how much it would increase my energy consumption on the water heater.  In the last photo above, you can see the self reported energy consumption for March (dashed line) when I did not have recirculation and April (solid bars) when I did.  I can't really see a discernible difference.