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Hoval STU Wood Pellet Boiler
Space Requirements
Where ever possible space should be left around the boiler to enable all parts to be examined and the cubicle casings to be fitted or
removed easily allowing for stoker maintenance. Note access requirements in front of boiler for cleaning etc and where necessary auger
withdrawal from tubes.
Additional points
When pipework is being connected to the boiler care must be taken not to damage the casings, which are not designed to be stood on.
Design of pipework runs and the use of expansion joints as required. External weight or thrust from system pipework attached to the
boiler should be avoided by careful design of pipework runs and the use of expansion joints as required.
If the High Limit is illuminated the limit stat will need to be manually reset. Allow the boiler to cool down then unscrew the protective
cover over the reset button. Press the button in and replace the cover. The boiler will automatically resume operation.
Additionally the stoker control includes a number of other
safety features
, these include;
Mains isolator on control panel facia.
The isolator electrically isolates the stoker and pickup auger(s). A lock-off facility is provided.
The operator should employ a safe system of working before carrying out maintenace or servicing of the equipment.
Thermal safety heat exchange device.
Within the waterway of the boiler there are a number of small independent heat exchanger
coils. If the boiler water temperature exceeds 95°C the thermostatic valve opens and allows cold mains water to pass through the coils
to quickly reduce the boiler water temperature. The cooling water exit coil should be directed to drain via a tundish.
Burn Back Protection.
There is a burn back manual reset type thermostat fitted to the delivery auger set to 60
o
C. If the temperature
of the tube exceeds 60°C then the stoker control isolates and stops the supply of combustion air through the FD fan and ceases the
delivery of fuel to the transfer box. This is done by stopping any pickup auger that might be running. At the same time the transfer box
burnback protection fire damper will also motor close. The ID fan is allowed to run on and a timer within the control panel allows the
delivery auger to operate sending partially burning pellets back into the retort within the boiler. During this process a red light will show
on the LRP controller indicating a fault along with an error message on the display. The "burnback initiated" volt free contact will also be
activated. This control feature will still be active even if the woodburner selector switch is in the "off" position.
The Stoker will not be allowed to re-start until the burn-back protection thermostat is manually reset. However it is important that the
cause of the burnback has been established and rectified before allowing the stoker to re-start. The condition of the burnback thermo-
stat itself should also be checked to ensure it has not overheated and become damaged. If this is the case, the thermostat should be
replaced with a manual reset type, range 60 - 90°C.
Note: In the event of a continuing back burn condition along the delivery auger, due to a mains failure, a delivery auger fault, or the ID
fan continually running, then there is a separate thermostat fitted to the auger tube connected to a
water dump
. This is pre-set at 95
o
C
and ensures safe stoker operation in the event that burn back protection, as described above, cannot take place ie, the water extin-
guishes the burning pellets. We recommend that the burnback thermostat is always replaced after the water dump has been activated.
Although the water level in the water dump container is fitted with a level monitoring float switch, we still recommend that the
water level is physically checked on a weekly basis.
This should be within 50mm of the top of the container. Always replace the cap
after checking and / or topping up
.
In the event that the water dump has been activated, the low water level alarm (red) light will be illuminated on the control
panel facia and the stoker will cease to operate. Please contact Hoval Service if this situation arises.
Power interruption.
In the event of a power interruption, the motorised burnback protection damper will automatically close (fail safe).
It is possible (as pumps maybe off) that the independent heat exchanger cooling coil device, as well as the high limit thermostat may be
activated. The cooling coil thermostatic valve will reset itself as the water temperature within the boiler falls, but the high limit thermostat
will need to be manually reset to allow the stoker to restart when power is re-established. The fire damper will also open automatically
when the stoker is required to operate. A firebed burn-back will not occur unless the power interruption is prolonged (greater than one
day's duration). If this happens the water dump device may also be activated to extinguish the burning pellets.
Oxygen High.
Should the fire go out, the oxygen sensor in the flue reads 20.9%. If this condition does not change for 30 minutes then
the stoker is switched off and no more wood pellets will be fed into the boiler. A red light will show on the LRP and an error message will
be displayed.
Oxygen Low.
If the oxygen sensor reads less than 3% for a 10 minute period then the stoker will shut down, again a red light will show
and an error message will be displayed.
Voltage/Phase relay.
Should the stoker control sense a reduced voltage supply or if the phases are altered then the LRP will signal an
error and display a message accordingly.
Control Details and Safety Features
