Can only be used to heat homes;
cannot presently be economically
No freezing or corrosion problems. adapted to cooling.
Minor leaks cause little or no
Large air ducts needed.
Fan power is required.
As air is used directly to heat the Large storage space needed for rocks.
house, no temperature losses due to
heat exchangers (devices which
Heat exchangers needed if system
transfer heat from one fluid to
is to be used to heat water.
another), when the system is used
for space hat.
Undetected leaks can degrade
No boiling or pressure problems.
Possible fungus or algae growths
in rock beds. Long term effects
Washing rock can be difficult.
Water or Liquid
Holds and transfers heat well.
Leaking, freezing, and corrosion
can be problems.
Water can be used as storage.
Corrosion inhibitors needed with
Can be used to both heat and
water when using steel or aluminum.
There are liquids which are noncor-
rosive and nonelectrolytic; however,
Compact storage and small con-
they are toxic and some of them are
Compatible with hydronic
A separate collector loop using a
Or, alternatively, a water draining
or inhibited water system, are
Pumps are quieter and more
required to prevent freezing. In
effective than fans.
warm regions, where freezing is
infrequent, electric warmers
or recirculation can be used.
May require excess heat protection.