the surroundings heat exchanger envelops an enclosure &# 39 ; s insulation so that it exchanges part of its heat load directly through the enclosure , instead of indirectly with the surroundings which would then exchange an equal amount of heat with the enclosure . this reduces the temperature differentials required to drive heat a transfer because little heat remains to be transferred by the indirect path . this is augmented , in some cases , either by exchanging heat with media other than gas , comprising conductive solids or liquids , natural or forced convective liquid systems , or phase change systems comprising thermal storage or combination refrigeration / heat - pumping systems or combining the heat supplier of a refrigeration system with the heat absorber of a heat pumping system ; or by avoiding unnecessarily high temperatures when pumping heat into hot water systems by either or both of the following means : regulating temperatures at lower set points or zoning heat pumps according to user need , which improves efficiency significantly , when heat is provided by heat pump , as it does not when heat is provided by electric resistance heating or combustion of fossil fuel .