Last spring I installed two on-demand electric hot water heaters
in parallel. They worked wonderfully until this winter's cold temperatures pushed the ground water temperature lower than the rest of the year. We had no problem with hot water from our taps at about 1.5 gallons per minute but the shower just never got hot. The standard for new shower heads is 2.5 gallons per minute. We needed to reduce our GPM consumption by about 1 gallon in order to have hot showers.
I did a bit of research and found Bricor
manufactures a number of sub 2.5 GPM shower heads. I contacted them about making a purchase and they replied with their pricing and told me to measure the pressure of the water at the shower head. I dutifully removed my shower head and threaded on $12 worth of parts from the hardware store to measure the water pressure. With that information and a model picked out I sent them payment via Paypal. They made my shower head specifically for me and my home's water pressure to achieve the desired PSI. A few weeks later I received it and put the new head in place.
The pros of the low flow shower head are that my water consumption is down. The on-demand hot water heater can easily keep up with the lower flow so I'm probably using less electricity
as well. Reducing your water and electricity consumption is definitely nice! The pressure of the water coming out of the head is not noticeably less than my shower head off the hardware store shelf.
The cons of the low flow shower head are the expensive price up front (I'll admit, I probably chose their most expensive model), and a major lag in response from a change in temperature at the knob to the time the desired temperature comes out of the head. It also takes a long time for the hot water to reach the shower head from the heaters but I understand that the water used during that longer period of time is the same as before changing to the lower flow.
A less expensive but equally effective way to reduce water consumption and reduce the GPM rate to allow a heater to keep up with demand is to simply regulate the amount of water coming our of the shower head by not opening the valve the whole way.