Wasn't sure if this belonged under water, indoor repairs or what...

I had previously installed a small (2 1/2 gal hot water heater in my mom's house since two of the bathrooms are over 60 ft from the hot water tank. It lasted well past the warrnanty (1 yr) but not long enough for me to consider it a bargain. I thought I would buy a 6 gal unit for only about $8 more but with a 6 year warranty. Unfortunately when I picked the crawl space access doors I picked them with only 12 inch vertical clearance and the 6 gal tank is a bit over 14 inches. I considered tearing one apart and reassembling it under the house but the tank is too big. SOOOO...

I'm installing a small bronze pump with plastic impeller. It takes well under an amp to run and moves 12-18 gal per min. Running the water to get it hot takes a looooong time with flow restricted faucets and showers and is wasteful. The pump will get 'er done in well under a minute. Here is a list of the major parts and pieces and a description of operation. Attachment is a wiring diagram.

The pump is installed so that it draws hot water from a hot water line and discharges it into the cold water line. Check valve keeps you from getting back flow. There are lots of ways to control a setup like this but I like this one.

The user (in either bathroom) pushes and releases a buton. The pump runs till the thermostat feels the hot water. The pump automatically shuts off when hot water arrives. You can't run the pump when the thermostat is satisfied, even if you hold the button down continuously.

Patrs list:
pump
check valve
momentary contact SPST switch (2 ea, one for each bathroom)
SPST thermostat normally closed, open on rise
120VAC relay with SPST normally open contacts
Indicator lights if you want to get fancy

Cost of the parts is approximately equal to the cost of a 6 gal 120VAC water heater. There is more installation labor than the heater would be. By insulating the pipes used for supply and return the energy losses are less than the standby losses of the 6 gal water heater so in the long run the pump will save a bit. It is definitely cheaper to run this pump than to run the well to get water to throw away while waiting for the "hot" to arrive at the bathroom.