Question-and-Answer Resource for the Building Energy Modeling Community
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This is a non answer, saying you can't do it (hope to be proven wrong)...
You might be tempted to use the TemperingValve object. Place your stratified tank and your tempering valve on two parallel branches on the supply side of the plant loop, and you should only have two parallel branches, no bypasses are allowed.
Your WaterHeater:Stratified has a setpoint temperature of 70°C (mind the deadband too), while the supply outlet node of the plant loop has a SPM:Scheduled at 60°C.
The problem is that - as your correctly identified yourself - the cold water will always be introduced at demand side outlet of the plant loop (source: here, looking at source code it's actually the outlet node of the WaterUse:Connections object that gets the update).
I have never tried setting the WaterUse:Connections outlet node name to something else than a node on the same branch as its inlet node, but I'm pretty sure that won't work.
As far as I know, there is no way to divert all cold water from the mains directly into your tank.
I assume your situation is similar to one where your Stratified tank is being heated by a condensing boiler on the source side and you want to capture the lower temperatures at the bottom of the tank to properly account for a lower return temperature to the boiler (I know I do too!), but currently you can't out of the box (sure, if you invest enough time into EMS you might get to something that does the job...). This would make a good user voice request, so I suggest you check if there is one already, if not add it and provide the link here, I for one would surely upvote it.
Note: It goes without saying, but modeling the recirculation loop has an effect only if you use heat dissipation on it (a pipe that isn't adiabatic), and more importantly force the plant loop to circulate a given amount of water at all times and not just the appropriate flow for the load corresponding to the water equipment on the demand side (or you better have a SPM on the supply outlet that is higher than the temperature requested at the water equipment).
