Is the other Desuperheater line to the geo unit coming off the buffer tank? Is the desuperheater line you have labeled the supply or the return?

I would swap the cold water to house and the desuperheater positions and put a check in between them.

The desuperheater line depicted on the diagram is the supply to the desuperheater. It is supposed to provide cool water to the desuperheater.

The copper pipe that is the depicted supply to desuperheater is warm to the touch, so it must be drawing water from the buffer tank. That's not good.

I agree that a check valve is needed. Wouldn't it be better to put it after the cold water supply line? That way, neither of the two lines before it could draw from the buffer tank.

Water is circulated when ever the unit is running...I think it runs if you are heating or cooling...so when the tank starts getting warm you will have warm water at the inlet to the house cold water supply the way it is plumbed.

That is why I think you need the cold water before the desuperheater and a check between the two as I stated above.

A check in the cold will stop convection currents but when you are pulling cold water for the house and the unit is running you are going to get water that is the temp of the tank to your fixtures with your current setup. It was plumbed wrong from the start in my opinion...

Thanks so much for your additional reply, newbostonconst. May I ask a follow-on question? If I switch the cold water and desuperheater supply line and put a check valve between them, won't the desuperheater supply line continue to have convection currents when the unit is running? (that supply line is warm to the touch when the unit is cooling). Would it make sense to add the check valve after both supply lines?

Again, thanks for the replies. I have the plumber coming out next week.

One check is enough. convection currents are fine. Geo desuperheater is supposed to heat the water while heating or cooling. Desuperheater pump will overcome convection currents. If you are worried about the warm pipes then insulate them.

Good luck...