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Cobra said.. ^^^ you are taking the piss
Yeah, I hope so, but then again, you never know.
The 1/2" fitting is your PTR valve and should not be modified. The probe coming from them is meant to be so that it measures the tank temperature, which means you can't tee before it, not to mention the safety aspects of mucking around with the PTR placement. You can get PTR valves with water take-offs, but these are not that common, and the ones I have seen are at a higher pressure setting than most tanks need.
I am curious. Where does the solar return connect to, or does it connect elsewhere to the 'outlet' you can see?
If it were me and I were a plumber, I would replace the 3/4" union on the outlet near the wall with a 3/4" compression tee. That way you keep the heat-trap and the replacement tee is not much bigger than the union that is already there.
There is probably a tempering valve there somewhere down the line, so just keep that in mind when adding something else. Or preferably use a good plumber.