Hi, I just wanted to add my $0.02 to this thread.
I agree with Ladiesman217's basic analysis of the system. The right tempering valve is for low flow, and the left tempering valve is for high flow. This is probably because there is a minimum flow rate for the left/"high flow" valve, below which it will chatter on/off & cause random temperature swings downstream. Adding the low flow tempering valve in a parallel pathway helps solve this problem, because its minimum flow rate for proper operation is much lower.
But, even with the smaller right valve handling the low flow, the "high flow" left valve can still chatter on/off and cause temperature swings (the water from the 2 valves mixes downstream). So the "high flow" leg needs to be completely shut off at low flows. That is what the component labeled "J" addresses. It is a diaphragm type pressure regulator. At some point, someone "tuned" this regulator so that it shuts off the "high flow" pathway unless there is high enough system flow for the bigger valve to work properly. Above that flowrate the regulator starts to open & allow the high flow pathway to operate.
I think what has happened here is that the right/"low flow" tempering valve is just plain worn out. This can happen via cavitation that occurs at high pressure drop through any fluid restriction (such as a valve)-- the water flow literally eats away at the metal. The way this system is designed, the "low flow" tempering valve will see maximum flow quite often & be at higher risk. I bet that when this valve is opened up & examined, some metal components inside will be found to be eroded & no longer able to seal. Which is allowing cold water to just leak straight through.
It is definitely worth continuing to try to fix the valve by cleaning out the scaling. It might just be bound up. But, I do suspect that the valve will ultimately need more than just a cleaning & will need to be replaced. I don't think it's necessary to buy the super-expensive Symmons valve as a direct replacement -- I think something like the $100 Honeywell tempering valves (often referred to in these forums) will work fine. If you are up for a little copper soldering & are familiar with the concepts, such system reconfiguration as this would take maybe a couple hours to install.
Note that if you use a different brand of tempering valve it might require minor re-tuning of the pressure regulator so that the "high flow" pathway turns on/off properly under the new system flow characteristics. The regulator can be adjusted by simply turning the screw protruding from the cone shaped dome near the bottom (the adjustment screw is pointed downward toward the floor). I would guess that the chances are the system will still work *without* adjusting the regulator, so don't go and just start making adjustments if you don't have to -- but I wanted to at least mention that this is a possibility.
Finally, please note that I am not a professional & will defer to anyone with more experience on this issue. Good luck!
