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Glossary of Terms
Temperature Mixing Valve (TMV)
A valve installed between the solar water
heating system and the fixtures to
automatically mix the hot water with cold
water to achieve a safe outlet temperature of
50
o
C. A mixing valve must stop all hot water
flow if there is a loss of either cold or hot
water supply.
Closed Loop
A system that, typically, has an anti-freeze
heat transfer fluid circulating through a
closed, pressurized solar collector piping
loop. This “freeze resistant” fluid is
separated from the mains pressure water by
a heat exchanger and typically should also
contain a rust inhibitor. Closed loop systems
are used in areas where freezing conditions
are common
.
Drain-back
A system that uses potable water or heat
transfer fluid in the solar collector loop, but
the fluid drains back down into a tank when
the pump turns off, thus preventing
overheating or freeze related issues.
Expansion Vessel
A metal tank with an internal rubber liner or
bladder which is pressurised with air on one
side of the bladder and accepts fluid from
the closed loop on the other side. Because
air can be compressed, the expansion tank
can accept the increase in fluid volume that
results when the temperature of the system
increases. This prevents dumping of fluid
from the pressure relief valve.
Solar Flow Line
The plumbing line supplying heated fluid
FROM the Collector TO the Storage Cylinder
or heat exchanger.
Solar Return Line
The plumbing line supplying heated fluid
FROM the Storage Cylinder TO the Collector.
Insolation
Solar radiation level, expressed in
kWh/m2/day. Peak solar radiation is about
1000 W/m2.
Pressure Relief Valve
Pressure relief valve is normally incorporated
into the pump station of closed loop or
direct flow systems. It opens if a set
maximum pressure limit is reached, thus
preventing damage to the system
components.
Pressure & Temperature Relief
Valve (PTRV)
The Pressure & temperature relief valve
combines a pressure relief valve with a
temperature sensitive core which will open
to dump hot water if it reaches very high
temperatures (typically 90°C or higher). PRTV
are common on hot water storage cylinders
to providing a safety mechanism for the
release of pressure and heat if overheats for
any reason. The pressure relief and
temperature relief settings of the PTRV must
comply with local building regulations.
