I have shut offs on my shower heads in 2 showers so I can shut off until I rinse off, After installing new water heater when I turn water back on I get cold water for a few seconds it slowly gets warm again. It does this in both showers. Help ?

Is the hot water in the pipes cooling off while shutoff? Was the water heater moved when replaced and now the piping is longer? Possibly there is some other demand on the hot water while you are showering and it is pulling cold through your shower valve and then you get a slug of cool water from the hot lines. Was there other plumbing done at the same time as the water heater replacement?

Is it tankless water heater?

Water heater is in same location only larger, not tankless, water cools if you turn half off only ,off for a few minutes no other demand.I would think it is mixer but it does same on both showers, single controls about 7 yrs. Old .Didn't have problem till installed new heater.Thanks for your time.