27
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;
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 automatic reset type thermostat fitted to the delivery auger tube, 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. 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. This control feature will still be active even if the woodburner selector switch is in the "OFF" position.
The stoker is allowed to re-start once the temperature of the delivery auger tube falls below 60
o
C. The indicator light will return to
green (stoker running), but the error message will still be retained on the display, until cleared by the operator. This is accessed via the
"manual/status" menu as detailed on page 28. However it is important that the cause of the burnback is established. The condition of
the burnback thermostat itself should also be checked to ensure it has not overheated and become damaged. If this is the case, the
thermostat should be replaced.
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 device
. 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
fully extinguishes the burning pellets. We recommend that the burnback thermostat is always replaced after the water dump has been
activated.
Please check the water level in the water dump container 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 please contact Hoval Service.
Please see Appendix C, for further details on the above.
Power interruption.
In the event of a power interruption. It is possible (as pumps maybe off) that the independent heat exchanger
cooling coil, 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. 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 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
