Desuperheater - making hot water

The GSHP desuperheater has a pump that pulls water from the holding tank, heats it put by 5 degrees and then dumps this hotter water back in to the holding tank.  It'll keep doing this as long as it has heat to dump and the water from the holding tank is below a set temperature.  The desuperheater can be set to heat the water to either 125 degrees or 150 degrees. 150 degrees is obviously way too hot to have coming out of the tap so we cut the temperature down with a mixing valve that adds cold water. The nice thing about the 150 degree water is that it serves as a nice heat energy storage tank.  We use less hot water from the 150 degree tank than we would from 125 degree tank. So we can use a smaller holding tank as well. The hotter the input water to the ASHP, the less work it has to do.

The hot output of the holding tank feeds the cold input of my air-source heat pump hot water heater. The hot output of the ASHP goes to the house. We use the mixing valve to make sure the water we put in to the ASHP water heater is not too hot. That's the grey knob in the center of the picture above.

There are two aspects that confuse me about the mixing valve: One is that because the GSHP runs intermittently we won't get a full holding tank of water at 150 degrees. So I cannot easily configure the mixing valve knob setting.  The second issue I have is about the mixing valve itself and how it works.  I had two mental pictures for how it worked. The simple picture was that the knob sets the ratio of hot and cold water that get dumped to its output. The more complex picture is that the knob sets the ratio and that the valve somehow senses the temperature and actively varies the ratio between the hot and cold inputs to set the output temperature.  I think the reality is somewhere in between. I need to call the manufacturer of the mixing valve to learn more.

Basically, I think there is some sort of temperature sensing element in the mixing valve and it attempts to vary the ratios between the hot and cold inputs over a small range.  But if the hot water is too cold, it reaches a limit and mixes in more cold water than it should. For example, if we set the mixing valve for 150 degree input water from the holding tank then it'll probably work fine if the holding tank is at 140. However it will also likely mix in too much cold water if the holding tank is only at 100 degrees.

My plumber was very smart in installing a temperature sensor on the connection between the holding tank and the ASHP water heater. I hadn't thought of that. But now I'm watching it to try to figure out if the mixing valve is set properly.  Due to heat leakage from the ASHP, sometimes we see the heat convecting out of the cold input and raising the temperature observed by the thermometer.