Providing auxiliary heating in a solar house could have temperatures rising.
Hydronic professionals are sometimes asked to provide an "auxiliary" heating system for a passive solar house. You know the type: A wall full of south-facing glass, intended to soak up lots of free solar Btus; a live-in photon collector, where owners hand out Oakleyg shades while greeting visitors at the door.
Upon examining plans for such a house, the heating contractor quickly spots the insulated concrete slab floor specified for thermal mass, and envisions it filled with tubing. This will be perfect they think. Let the sun warm the house by day, and the boiler by night (and cloudy days).