Return Riser And Safety Devices - eta SH Installation Manual

Return riser and safety devices

Return riser
Wood contains water. If the temperature in the
boiler is too low, steam condenses from the flue
gas onto the heat exchanger surfaces. Corrosion
and a leaky heat exchanger are the result. To
prevent this, the water temperature at the boiler
inlet must be at least 60°C. Since the return
temperatures are usually lower, a return riser is
needed - preferably with a mixer that provides
controlled, heated flow to the boiler return.
The return riser mixing valve also controls the
buffer charging output. To reduce the output, the
return temperature is raised over 60°C to reduce
the spread compared to the boiler's setpoint
temperature. With the spread the output that can
be consumed from the boiler is limited.
Safety valve against overpressure
A safety valve with 3 bar opening pressure must be
installed on the boiler. No shut-off valve may be
installed between the boiler and the safety valve.
If solar or other heat sources provide energy to the
buffer storage tank via a heat exchanger, a safety
valve (3 bar maximum) is also required on the
buffer storage tank. Normally an expansion tank
that is too small or defective, or blocked heating
lines, are the cause for activation of the safety
valve.
The safety valve must be on top of the boiler or
in the flow in order to also discharge heat in an
emergency. Only this way can it discharge heat by
blowing out hot water and steam.
The discharge must be directed to the sewer via
an easily visible, open flow path (siphon funnel)
so malfunctions and, above all, a non-closing
valve can be recognised. If no sewer is available,
the discharge must be directed into the ground in
a pipe so nobody is endangered by hot water or
steam.
16
Thermal emergency cooling valve against
overheating
The safety heat exchanger built into the boiler
must be connected by the heating technician to
the house's cold water supply via a thermal outlet
valve (opening temperature 95 °C) to protect the
boiler against overheating if the pump fails. The
minimum pressure in the cold water line must be
2 bar.
Connect the supply line to the lower connector of
the safety heat exchanger; the upper connector is
for outlet to the sewer. To prevent the supply line
from being shut off accidently, remove the levers
from shut-off valves or the hand wheels from
valves and hang them there with a piece of wire.
The discharge must be have an easily visible, open
flow path so malfunctions can be recognised.
Direct the discharged water to the sewer via a
siphon funnel or at least with a pipe into the
ground so that nobody can be scalded if the valve
is activated.
Even for cold water coming from a domestic well
with its own pump, a thermal emergency cooling
valve must be installed on the boiler. With a
generously dimensioned air vessel, enough water
for cooling will come even if there is a power
failure. If the electricity supply is very uncertain,
a dedicated air vessel for the thermal emergency
cooling valve is required.
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