The last function is the drain. You can see in the photo that there is a stop valve in the blue line which connects to the single drain pipe. This pipe also provides the fresh tank drain and the pressure relief drain. The pressure relief valve will screw into the same fitting where the pressure guage is now (you can see it sitting on top of the pressure guage).
I tested the pressure rise due to water heating; don't forget that I have an accumulator, which helps buffer any water expansion. With the system at the nominal pump pressure of 45 psi and water around 65 degrees, I turned the water heater on and set the temperature at the mid-point. System pressure rose to 65 psi when the heater turned off. I then set the temperature at max. The pressure rose to 90 psi. PEX can handle 400 psi, but I don't want the system to exceed 100 under any circumstances. 90 psi is too close, so I think I will install the pressure relief valve. If it functions, it only drains a couple of ounces to relieve pressure due to water expansion.
Done!
But there's lots more to do on the furnishings and axles!