Hot/Cold shower

ScottG wrote:
It's not the pump, though it will aggravate the symptom. It's the mixing valve in the WH and it's common with that WH.
Your particular WH heats water much hotter than a standard unit. Then it uses a mixing valve (which depends on consistent flow) to blend cold water into the hot water outlet in order to keep those temps in check and give more usable hot water overall. That's why your is referred to as a "16" - it gives you the equivalent of 16 gallons of hot water.
It's that fancy mixing valve that's causing the fluctuations. It probably has hard water deposits/calcium built up in it.
You need to replace the mixing valve.

That is why it's the PUMP. When you have city water you're getting the constant pressure.
Depending on your water pump it sense both flow and or pressure. If it sense flow then it turns on and depending on far open you open the faucet it run till no flow and max pressure. During a shower it should be cycling between it's flow setting but never shutting off while maintaining a constant pressure. The water pump feeds both the cold water supply to all devices. This includes all faucets, toilet and hot water heater. The Hot water heater outlet whether from a mixing valve or a hot water tank feed only the hot water side of each appropriate faucet. You still need to use the cold water faucet with the hot water faucet to set the temperature of you're shower water that you're comfortable with. That mixing valve only lets you hold 10 gallons of water at a higher temperature so that you can get 16 gallons of hot water out of a 10 gallon tank. Regardless the hot water without cold water at the faucet will scold you.