We installed a new electric hot water heater and only after we put in the new one we are now having problems with the water. While in the shower, the water all of a sudden goes to freezing cold back to hot, then 30 seconds later it goes back to cold and does this the whole length of the shower. What could it be? It never did it until we put in the new water heater!
Thank you

Is this an on demand type heater or the older tank style?

Does it do this only in the shower, or at all fixtures? If only at the shower, it sounds like some sediment broke loose inside the water lines and ended up in your shower cartridge. Is this a single handle shower valve, and what brand is it? Im guessing single handle, and debris are effecting the temp/pressure balancing spool. If so, shut water off to house, remove cartridge and clean thoroughly, or replace and while you have the cartridge out, pulse the water on a off a few times to flush the valve body of any other sediment. Let us know how things work out.
Lee.

It is an older type water heater. It once in a while does it in the bathroom sink but not as bad or as often as it does it in the shower. Everything was fine until we changed the water heater.

It is also one handle and it is a peerless. What is it that we remove and what cartridge? In the handle or where? We don't know where the temp/pressure balancing spool is... Sorry...

Who actually installed the water heater? If you did the installation, did you do any soldering on the cold water line close to where it connect to the water heater?

My husband put it in and he used the quick connects to do it. No soldering at all...

It is an older type water heater.
Could you tell us what you mean by older type.

Do you have a two handle faucet you could use to see if the changing temperature occurs there.

Do you have a hot water recirculation line?