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> If your goal is saving energy/money, you don’t want a system capable of going from cool to toasty in 20 minutes.

Depends. As explained in a sibling comments, I have some rooms that have combined UFH and radiators, and if the desired temp is more than 1 celsius away from the current temp, then both are driven, otherwise it's just the UFH.

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Indeed “depends” is almost always the answer.

So long as you can get the boiler return water temps low enough, you can operate the boiler in its high efficiency range.

Most dual-temp setups are set for the highest temp and mixed-down to provide the lower temp for under-floor. That’s cheapest in terms of equipment and install but cannot be as efficient as a system that mixes down when both loads call but also lowers flow temp (thereby lowering return temp) when no high-temp rads are calling.

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