Solar Thermal Information

21
The solar collection subsystem is essentially the same as
in previous systems.
The storage tank contains domestic water and absorbs
solar-derived heat through the lower coil.
If the domestic water leaving the tank needs a further
temperature boost, it is routed through the modulating
instantaneous water heater via a 3-way diverter valve,
which is operated by a setpoint temperature controller.
If this controller determines that domestic water leaving
the tank does not need further heating, the diverter
valve routes flow directly to the hot port of an anti-scald
tempering valve. This configuration eliminates the heat
loss associated with passing heated water through the
unfired water heater.
Any instantaneous water heater used in this type
of system must modulate its firing rate to adjust for
preheated incoming water. Verify that any instantaneous
water heater being considered for such an application is
warranted for use with solar combisystems that supply
preheated domestic water.
Upon a call for space heating, a controller determines if
the temperature at the top of the storage tank is sufficient
to supply the water-to-air heat exchanger coil mounted
in the supply plenum of the forced-air distribution
system. This decision can be based on outdoor reset
control. However, designers are cautioned that low-
temperature supply air must be carefully introduced into
heated spaces to avoid drafts. This may require a lower
temperature limit on the reset controller.
A motorized mixing valve monitors the temperature
of the supply air downstream of the plenum coil.
The mixing valve adjusts itself as required to prevent
potentially high-temperature water in the storage tank
from creating overly hot supply air to the building.
If the tank’s temperature is too low to properly supply
the plenum coil, the furnace’s burner (or the compressor
in a forced-air heat pump system), would operate as
normal. The space heating circulator (P1) would be
off. A check valve in this circulator, or mounted as a
separate component, prevents heat in the plenum from
migrating backward into the hydronic system.
ANTIFREEZE-BASED COMBISYSTEM #5:
Another possible system configuration uses a storage
tank with an integral gas-fired heat source, as shown
in figure 4-8. This eliminates the boiler as a separate
component. It also eliminates the diverter valve and the
piping associated with connecting the boiler and diverter
valve to the system.
As with the other antifreeze-based systems, solar energy
is added to the storage tank through the lower heat
exchanger coil.
The modulating/condensing burner and heat exchanger
assembly operate as necessary to maintain the top of
the storage tank at an adequate temperature to supply
domestic hot water whenever required (not necessarily
24/7). At other times, such as night setback periods,
the temperature at the top of the storage tank may be
allowed to drop based on outdoor reset control logic.
The position of the burner/heat exchanger assembly
within the tank encourages temperature stratification
and minimizes heating of the lower portion of the tank to
minimize losses in collector efficiency.
The water in the storage tank is not potable water.
It’s the same water that circulates through the panel
radiators during space heating. This allows the tank to be
constructed of standard steel (rather than stainless steel
or standard steel with glass lining to hold potable water).
Domestic water is instantaneously heated as it is drawn
through the system. A flow switch closes it contacts
whenever cold domestic water is drawn into the external
brazed plated heat exchanger. These contacts turn on a
small circulator (P1), which creates flow through the tank
side of this heat exchanger.
Brazed plate heat exchangers have very small volume
and low thermal mass relative to their heat transfer
surface area. They can begin heating potable water in
one or two seconds after flow is initiated. An anti-scald
mixing valve protects against high water temperatures
being sent to the fixtures. As soon as hot water flow
stops, so does circulator (P1).
Very little domestic hot water is held in the heat
exchanger. This reduces the potential for legionella. The
heat exchanger can also be removed and serviced if ever
necessary.
The heated water at the top of the tank provides thermal
mass for a stable supply of heat to the heat exchanger.