Buffer - eta SH Installation Manual

A minimum power level is necessary to achieve
clean and complete combustion
The smaller the fire in the boiler, the lower the
temperature will be in the combustion chamber.
At about a third to a quarter of the full load, the
combustion temperature drops below 700°C and
the tar components in the wood gas no longer
burn completely. This drastically reduces the
efficiency (it can drop to below 50%), causes tar
build-up in the boiler's heat exchanger and in the
chimney and also results in unacceptable pollution
due to unburned hydrocarbons.
To maintain a clean-burning wood fire with
maximum fuel efficiency, a minimum power level
must be maintained and the best way to achieve
this is to use a buffer storage tank.
Why a buffer storage tank?
Many old log boilers that do not have buffer
storage tanks are still in use, so why do we need
them with today's wood-burning systems? The
answer to this frequently asked question is as
follows: In the days before heating control systems,
the boiler used the thermal mass of the entire
house as a buffer. If the boiler is replaced, and the
radiators are fitted with new thermostat valves
and/or a weather-dependent mixing system, the
log boiler is operated at too low an output level in
autumn and spring when the heating requirements
are minimal. At low output levels, the temperature
in the combustion chamber drops, but production
of wood gas doesn't. Hard-to-burn components
of the wood gas, such as tar or acetic acid no
longer combust; instead, they condense (create tar
deposits) either in the boiler's heat exchanger or
in the flue. The materials that are not deposited
pollute the environment.
A buffer storage tank is necessary to control this
low-load operation, which inevitably occurs with
a modern, energy-saving heating system. Heat
produced by the boiler for which there is no imme-
diate use within the building is offloaded into a
buffer storage tank and retrieved by the heating
system as needed, for example, when the boiler is
not operating.
As a log wood boiler cannot simply be switched off
once it has been started up and while there is still
wood burning in it, it should be possible to store in
the buffer the energy content of the wood that is
still in the combustion chamber.
SH Montage
to the hot water tank
Flow from boiler
"Buffer top" temperature sensor just
under the connection
The boiler controls its output based
on the reading from the "Buffer
middle" temperature sensor. Enough
to provide sufficient heat to start a
consumer. Not too much to ensure
that the buffer is still able to store
heat after the heating circuits have
been switched off.
"Buffer bottom" temperature
sensor just above connection
Return to boiler
No external timer thermostats
The factory-standard controls include weekly
timers for two heating circuits.
If the heating pumps are controlled by the boiler
control, heat can be drawn from the boiler during
periods when the temperature setting has been
lowered as long as there is wood in the boiler.
This is especially important if you accidentally
put too much wood on in the evening, or if you
intentionally heat up the boiler to its full extent in
anticipation of a cold night.
Timer thermostats will switch off the heating
pumps punctually at the preset time, even if
wood is still burning in the boiler, and even if the
(undersized) buffer is incapable of storing more
heat. The only solution for the boiler is to stop its
air intake. Although this puts the fire out without
overheating the boiler, the wood will continue to
emit gas and the unburned wood gas resulting
from the lack of air will create tar deposits in the
boiler and flue.

Buffer

17
2012-11