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Both air and liquid space heating systems require a heat delivery network to
transfer heat from storage to the building.  Most of the buildings in the
United States are heated by circulation of warm air through the building.
The air is usually heated in a central location and ducted to the individual
rooms.  This method is used particularly in residential buildings.
Hydronic heating is another common heat distribution method.  In hydronic
heating systems hot water or steam is circulated through pipes to
"convectors" located in the individual rooms of a building.  Modern hot water
convectors are comprised of one or more finned tubes located on the wall near
the floor.  These baseboard heaters deliver heat to the room mainly by
convection as air moves through the fins.
A less common heating system consists of lengths of tubing embedded in the
floors, walls, or ceilings of the living space.  Warm water is supplied to
the tubes by a boiler and the heat is transferred to the room by convection
2.5.1 Heat distribution for liquid-type solar systems.  The temperature
requirements of a hydronic heating system are dependent on the amount of heat
exchanger surface.  Most baseboard heaters have comparatively small surface
areas, so they require higher temperatures, typically about 180 deg, F.  If
larger heat transfer areas are available as in older or modified hot water
systems, temperatures of 120 deg. F may be sufficient.  Temperatures of 100
deg. F are adequate for the system which uses entire floors, walls, and
ceilings as radiator surfaces (Lof, 1977).
During the winter, typical liquid-type solar systems are seldom operated at
delivery temperatures above 150 deg. F.  Thus it is evident that the use of
solar heated water in standard baseboard heaters is impractical.  Only
modified baseboard heaters of adequate size or radiant panels are suitable
for use in hydronic systems which use solar heated water.
One of the most economical means of auxiliary heat supply and heat distribu-
tion for liquid-type solar systems involves the use of a warm air system.  A
typical system is illustrated in Figure 2-16.  In this system the warm air
furnace is located downstream from a liquid-to-air heat exchanger which is
supplied with solar-heated water.  The furnace can then serve to boost air
temperature when insufficient heat is available from the solar heated water,
or it can meet the full heat load if no heat is available in solar storage.
Auxiliary heat can be supplied by a gas, oil, or electric furnace, or by the
condenser of an air-to-air heat pump.
Another method of heat distribution involves the use of a water-to-air heat
pump which draws heat from the solar storage tank and pumps it to a condenser
coil which is placed in a central air duct.  The advantage of this system is
that it can effectively use heat from solar storage at temperatures down to
45 deg. F, thus more of the stored heat is available.  Also, average storage
temperatures are lower, resulting in significantly increased collector
efficiency.  Heat pumps are discussed in Section 2.5.3.

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