Why does one of your 50 gal hot water heaters run when you water the lawn on a new construction home?

Someone may have cross connected a line and your drawing hot water through a cold water line.

There could also be a problem with a solenoid valve on a washing machine that would also account for by passing.

Some people have a shower head that they shut off instead of using the shower body thus again causing by passing.

If you are indeed getting hot waterinto a cold water line this could be quite serious as the newer hot water heaters are not hot enough to kill bacteria and in actuality are an incubator for all kinds of bacteria to grow like Legionnaire's etc.

To kill bacterial growth you need at least 140 F degrees and today because of the many law suits the water temperature has been lowered to the dangerously low temperature of 125 Deg F.

Have a licensed plumber check the piping and do not forget to ask to see the license as it is for your protection DO NOT accept any excuses as to why the person checking this system is not a licensed holder.