Will I get instant hot water if I install a tankless water heater?

Tankless water heaters will heat up water as soon as it's turned on, however, the cold water between the tank and your taps still has to travel through the pipes in the house to get to the fixture. You will notice a few seconds of cold water coming out of the fixture before the hot water arrives at the tap.

Also, a tankless water heater needs a minimum flow to get the unit to operate. If you just turn on the tap slightly you may find it doesn't turn on. You need at least 66 gallons per minute to have the unit heat up the water flowing through it.