Help selecting a tempering valve

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I have a Rinnai tankless that supplies domestic hot water. When it's particularly cold out it also provides hot water to a heat exchanger in the HVAC ("emergency heat"). Consequently, the controller on the Rinnai allows it to heat to 185 degrees. There is currently an Apollo TV 34-200 thermostatic tempering valve that mixes it down for domestic use.

For the last couple of years the max water temperature we could get was about 97 degrees. After completely rebuilding my Rinnai I realized that the problem was the valve (which I have now confirmed).

The installer of the system sweated in the Apollo valve. I would like to replace it with a Caleffi and convert it over to NPT so I can change stuff, rebuild, etc., later more easily.

Can someone provide direction on a valve? For example, I believe that this one will work:

Any other suggestions? Thanks!
Hey, wait a minute.

This is awkward, but...

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