33
Hoval STU Wood Pellet Boiler
Operating Instructions
Important Notes for the Boiler Attendant
Check the level of fuel in the bunker on a regular basis - Espe-
cially as the weather gets colder. Trim fuel if required. Check with
fuel supplier and establish likely delivery lead time.
Start Up/Ignition
With a cold 'Boiler-Out' condition there are two possible options
for lighting the boiler, either by manual intoduction of an ignition
source to a primed or filled retort, or, as an option, an automated
priming system followed by electric ignition. If fitted the Electric
Ignition system is initiated by an additional switch KINDLE/
IGNITION fitted on the Stoker Control Panel.
Having fired the Boiler and placed it into operation the boiler would
normally kindle or sit in 'stand-by' when not required or when the
connected load has been satisfied. Hoval recommend that it can
be held in this kindle condition for up to a maximun of 72 hours.
If it is anticipated that the connected available load would lead
to any boiler kindling in excess of this period then it should be
switched off.
More detailed description of both methods of Ignition as Follows;
Electric Ignition (Option)
With this option the Wood Pellet Fuel within the retort can be
brought to ignition using an electric hot air jet to initiate the com-
bustion process. The source of the hot air is a 3kW electric hot air
gun mounted within the plenum of the stoker, which operates for a
short controlled period and directs hot air into the filled retort onto
the wood pellet fuel until combustion is established.
To set in operation
1.The Operator MUST ensure that the correct shut down and
switch off sequence has been followed (this is described below).
Provided that the boiler has previously been shut down correctly
there will be little or no pellet fuel within the retort or the Delivery
Auger of the stoker. It is therefore
IMPORTANT
that the Operator
CHECKS the retort visually prior to starting the Electric Ignition
sequence to confirm that the retort is
CLEAR OF ANY FUEL
. This
may involve opening the boiler door briefly to check the condtion
of the hearth and retorts.
The above process is necessary to avoid overfilling or over feed-
ing the retort prior to the introduction of an ignition source.
Ensure that all Isolating switches associated with the stoker are
'ON' and the stoker Control panel has power to it.
2. On the Stoker Control panel set the KINDLE/IGNITION switch
to the igntion position. The Wood Burner Switch (OFF/AUTO) on
the Stoker Control panel should be set at Auto.
Day to Day Operation
The STU Wood Pellet boiler has been designed to enable
the effective controlled combustion of wood pellet fuel without
excessive manual input. The stoker will automatically adjust the
firing range to match the load with damper control on both the
forced and induced draught fans. Continuous O
2
monitoring of the
flue gases ensures close control of combustion along with clean
and efficient operation. It will respond quickly to changes in load
and the standard stoker control package includes an airless kindle
(or boiler 'stand-by') feature when heat is no longer required or the
control thermostat is satisfied.
In kindle mode a small ignition source is held within the retort of
the stoker and this can be used to bring the boiler back to full fire
within a few minutes without any outside or manual attention. In
kindle mode fuel usage is negligible and there is no heat to dis-
sipate. The STU boiler can usually be held in this condition for up
to 72 hours.
In common with most solid fuel boilers optimum combustion condi-
tions are achieved when the boiler is allowed to fire continuously
over extended periods and therefore Hoval recommend that an
appropriately sized STU wood pellet boiler is selected to meet the
likely required load.
The stoker control is designed such that the modulating fuel feed
and the pre and post run of both the FD and ID fans ensures
smokeless operation.
Fuel Supply to Stoker
Always ensure there is sufficient wood pellet fuel covering
the pick-up auger or available at the outlet of the pellet stor-
age silo.
Where fitted, the slot of the pick-up or bunker worm must ALWAYS
be covered with adequate supply of fuel; it should be checked
routinely during operation. Failure to keep an adequate supply of
fuel over the pick-up will result in the boiler going out. The bunker
must be secure and should prevent any foreign material and/or
water entering the stoker system.
IMPORTANT: The pick-up screws should be isolated before
entry to bunker and correct health and safety procedures fol-
lowed when entering any confined space.
Provision should be made to prevent any dust or fuel escaping the
bunker-particularly during the delivery, the bunker should be fitted
with a ventilation grille and filter, this is to allow a discharge path
for the conveying air used during delivery.
The filter will require
cleaning after each fuel delivery;
this can be done using either
a brush or vacuum. (Spare filters are available).
Boiler should be
switched OFF during a pellet delivery.
Sequencing of boilers under BMS control.
When a bank of boilers are sequenced to operate under the dictates of a BMS control system, it is very
important that on-line boilers are "properly" matched to the available heating load at any one time.
This will avoid too frequent cycling of the stokers as they stop and start (introducing cold pre & post purge
air into the boiler each time this happens).
Over time this can result in the boilers suffering from thermal shock and premature failure of the pressure
parts.