I recently received a call from a customer complaining of having no mixing valve on her tankless coil. What I found was two boilers in this home with one having a mixing valve and the other piped into that same mixing valve. Someone apparently removed the valve on the second boiler because the stubs were still there. I installed a new mixing valve but wasn't getting any hot water out of it. I figured out why they did this. The tankless needed to be replaced but they took the cheap way out and tied into the other tankless. The tankless coil is too far gone to steam and the customer doesn't have the money to change it out right now. This service call brings me to my question, I've seen this several times and don't know why it does this. When I check the water temp it starts out by spiking up to 150-160 then drops down to what I set it at, usually 125 degrees. Can anyone tell me why it does this and how could it be prevented? Thanks!