My crawl space under the kitchen has both hot and cold domestic water pipes and hot water heat pipes. The hot water heat pipes are hung on the bottom of the floor joists and most have pipe insulation. The section where the kick heater's hoses are attached is not insulated. Its the same area where the hot and cold water pipes are, but they are buried under fiberglass insulation.
One thought is to add some 2x6 wood to make the floor joists 2x16 only where the kick heater connections and fill it with fiberglass and a foam board on the bottom.
The water into the kick heater is often not hot enough to turn on the fan in the kick heater.