We have a Bradford white water heater that is 11 years old. A few months ago we had a plumber replace the starter and burner when we didn't have hot water. We are now seeing a new problem and are trying to determine if the dip tube also needs to be replaced.

We've begun to notice that when a shower is on, all the hot water goes to the shower, meaning we have no hot water to do dishes in the kitchen. We've also noticed that not all sinks in the house get to a hot temperature.

The issue is not a matter of the water being hot then getting cold. The issue is in some faucets, the water does not get hot enough, or is diverted when other faucets are on. We have tried to adjust the temperature on the water heater to no avail.

Any ideas if this is a dip tube issue or something else? Thanks!!

if you have hot water to even one faucet, it is not a dip tube problem.
do you have a recirculating line?

We do have a recirculating line! We thought it was not working/installed correctly, so we turned it off a while ago. It has a timer.

We also have a second water heater that is a pre-heater coordinating with a geothermal system.

What are your thoughts on what is wrong?

what did you do to "turn off" the recirculating line?
did you shut a valve or two. or did you just shut an electric switch.
my thought is that ... cold water goes into the water heater and hot water comes out. there is no way that some faucets get hot water and others don't unless there is a cross connection. one common cross connection problem is a faulty check valve on a recirculation system. depending on how yours is piped, some portion of your piping could cross connect and some could not.
just a guess...