Outside air reset for a boiler with a primary-secondary loop

edit

I put an OA reset SPM in the primary loop, and left the secondary loop SPM as constant. However, this gives secondary loop supply water temperatures with more variation than expected. Even with constant setpoints everywhere, the variation is high. I'd prefer to have only one SPM in the secondary loop, at its current location or inside the tank. However, OpenStudio requires two SPMs, and a setpoint schedule in the water heater object. Is there any way to get around this? If not, how do all three of these setpoint schedules interact?

Yes, OpenStudio requires a SPM on the final supply node of each loop as a bare minimum. It looks like your original schedule SPMs are applying "Boiler Setpoint Temperature" for the primary loop and "Water Heater Setpoint Temperature" for the secondary loop. This "Water Heater Setpoint Temperature" should also be applied to the Water Heater object, so that it provides the correct temperature for the secondary loop.

Double-check loop settings (min / max temp, min / max flow, etc.) and loop sizing settings for both primary and secondary loops by clicking on the Supply / Demand Equipment dashline

I found that an OS:Schedule:FixedInterval was needed in the secondary loop (OA reset SPMs can't be applied to WH objects), and that the primary loop needed an SPM:Scheduled at ~>5°F than the highest secondary loop supply water temperature during the year. In reality, my 20°F-delta intermittent-constant-flow boiler return water temperature should track the tank temperature when idle, and yield a 20°F delta when firing. A dual setpoint SPM in the primary loop didn't help. How can I make the return water realistic? If BoilerEfficiencyCurve isn't dependent on return water, is this moot?

The "Efficiency Curve Temperature Evaluation Variable" can be set to either LeavingBoiler or EnteringBoiler. So, return water temperature can be important if you're using the second option.

Maybe I missed it, but what about your return water temperature in this scenario is not realistic? Could you post an image of boiler return water temperatures in your initial question post?