Shower water temperature fluctuation to extreme

by Bob
(Upstate New York)

We recently installed a tankless electric water heater in our ranch home and are really dissatisfied. The shower water, which is close to the heater is hot then cold and cannot be regulated. It is frustrating and my people are hesitant to shower. This could be a problem!!! I paid about $1,300 for heater and installation all to get away from domestic hot water from the oil fired furnace running all year. I would love to know what, if anything, can be done about this.