I have a large whirlpool tub which could not be filled with hot water from the conventional electric hot water heater. So I installed a Bosch gas fired " continuous hot" water heater.
The tub fills with hot water without a problem . The showers in the house go cold after a few short minutes. There seems to be a constant supply of hot water to the kitchen sink and other sinks in the house.
The showers are not a longer run than the tub. It does not seem to make sense. If I can get a continuous hot water at one fixture , presumably the remaining fixtures would not perform differently .
The shower diverters are twenty plus years old. Could it be that the old diverters are too inefficient?

Please explain your defination of "shower diverter" Your statement,"The shower diverters are twenty plus years old. Could it be that the old diverters are too inefficient?" don't make much sense since most shower diverters are located in the tub spout and have no effect on water temperature.
Kindly rephrase the question? Are the showers tied into the Bosch tankless heater? Regards, Tom

Are the showers tied into the Bosch tankless heater? Regards, Tom

Good question.

Given the description, I'm about as flummoxed as I can get.

The Bosch heater supplies every fixture in the house. By shower diverter , I mean the mixing valve at each shower

Now we need to know the brand name of the shower valves. Do they have temperature control or anti-skald features. What size Bosch Tankless do you have? Is the regular water heater still in the system? Have you checked the elements if it is? Did it ever furmish all the hot water that you desire or just start the problem? Back toi you. Tom