Solar Thermal Information

35
latter present few options if a leak, corrosion or scaling
ever develop to the point where the coil can no longer
function.
The approach used in figure 5-13 eliminates the need for
this internal heat exchanger.
Solar collection is handled by a closed-loop, slightly
pressurized drainback subsystem. The storage tank is a
simple, carbon steel vessel with no internal components
or special detailing.
The boiler operates as necessary to maintain the top of
the storage tank at a suitable minimum temperature for
domestic hot water delivery. When a hot water fixture
opens, a flow switch detects the flow of cold domestic
water and turns on the small circulators (P3). This moves
heated water from the top of the storage tank through
one side of a stainless steel brazed plate heat exchanger.
Cold domestic water flows through the other side of this
heat exchanger.
Because the plate heat exchanger has very little water
content relative to its plate area, it warms and transfers
heat within a second or two after the flow of hot
water begins. Thus, domestic hot water is produced
“instantaneously” upon demand. The thermal mass of hot
water in the storage tank allows for stable DHW delivery.
This approach also minimizes the quantity of domestic
hot water held in the system at any time reducing the
potential for legionella.
Space heating is provided from a manifold station
serving panel radiators (sized for operation at low supply
water temperature). Each panel radiator is controlled by
its own thermostatic radiator valve. A variable-speed
pressure-regulated circulator (P2) modulates its speed
as necessary to maintain a constant differential pressure
across the manifolds as zone circuits open and close.
A HydroLink serves to hydraulically isolate the space
heating distribution system and two other potential
heating zones (served from the bottom connections of
the HydroLink) from the heat source circulator (P1).
This system provides the benefits of a single storage
tank, elimination of heat exchangers between the solar
subsystem and space heating circuits, buffering for a
highly zoned distribution system, instantaneous domestic
water heating and an easily serviced external heat
exchanger for domestic water heating.
Figure 5-14 shows the original system schematic at the
top, and its operating mode while supplying domestic hot
PRV
sight
glass
static
water
level
lift head
hot
storage / drainback tank
air space
air return
tube
(P3b)
(P3a)
TRV
TRVTRV
outdoor
temperature
sensor
thermostatic radiator valves
on each panel radiator
pressure-regulated
circulator
(P2)
manifold distribution
system using PEX or
PEX-AL-PEX tubing
panel radiator
AB
A
B
mod/con boiler
HyroLink
cold
check
valve
stainless steel
brazed plate
heat exchanger
flow 
switch
(P3)
(P1)
heat source
circulator
other heating loads
to / from
other
panel 
radiators
figure 5-13