We just replaced a 40 gallon water heater, power vent. The plumbing company said upgrading to code was needed for the plumbing of this water heater. We replaced it with an almost identical model, eg; same capacity, brand. This problem never happened with old water heater, you could use everything in the house and not effect the shower. I tried replacing the cartridge, this did not fix the issue. I've tested the water heater as far as how many gallons of hot water it puts out before the temp drops 30 degrees. The hot water heater is working as specified. The plumbing company says they are stumped. All I know is, it didn't happen until the water heater was replaced. Coincidence?