1. A stoker-fired boiler is different from an oil, gas or electric unit in that the fire must be maintained by periodic running of the stoker, even though there is no call for heat by the thermostat or operating aquastat. This is accomplished by the timer. A normal setting on the timer is 2 ½ minutes of stoker operation per ½ hour. this is a suggested start and times may have to be varied to suit installation conditions, the quality of the coal being used, and the time of year.
2. An outfire in the stoker is a more prevalent condition during the summer months. It can happen even though you find nothing wrong with the stoker mechaninism. The following (A thru C) are some conditions which can cause an outfire. These are situations that start to develop where there is no apparent explanation. For other situations, refer to the service check list.
A. Feed rate too high for summer operation:
In this situation, where the stoker is maintaining boiler water temperature for domestic hot water and there is little domestic water draw, the boiler water temperature can reach the high limit setting. The high limit control overrides the timer, not allowing the stoker to run and results in an outfire. This occurs on a hot water system and can be corrected by dropping the feed rate, raising the high limit setting, lowering the operating setting on the aquastat to create more of a temperature spread between the operating and high limit setting on the aquastat, and decreasing the differential on the operating control.
The bypass piping, as shown in Fig. 7, is very important in helping to alleviate this condition by reducing stratification of the water in the boiler.
Removing insulation from the boiler will also help.
Note when adjusting the aquastat:
The L8124A or L7224A aquastat, furnished with this unit, has three settings.
1. The High Limit Setting: This shuts the stoker “off” when the water temperature reaches the temperature setting and overrides all other controls.
2. The Operating Setting (Low Limit): This setting maintains the boiler water temperature and shuts off the stoker when the water temperature reaches the setting, but not if there is a call for heat.
3. The Differential Setting: This setting determines when the stoker will be fired, as the boiler water temperature cools. For example: If the operating control is set at 160 degrees, with a 10 degree differential, the stoker will come on at 150 degrees (water temperature) and shut off at 160 degrees (water temperature). The thermostat or timer can override this setting, but cannot override the high limit setting.
B. Loss of draft:
This can occur during hot, humid summer weather, with low fire in the burner. Changing the number of minutes of operation per half-hour, increasing slightly the coal feed and air settings may help. Thoroughly cleaning the boiler, flue pipe and chimney, to remove fly ash buildup will also help. Be sure the fire and ash pit doors are closed tightly. Check that the clean-out lever is fully pushed back towards the boiler and that the clean-out cover plate openings are fully closed. Check that all openings, where outside air could infiltrate the boiler base, are sealed.
C. Too much draft:
This is most apt to occur during cold, windy weather where there is no barometric draft control in the stack. In this situation, the fire continues to burn, even though the stoker is not running. Addition of a barometric control, or proper adjustment of the barometric control, if present, will help. Exhaust fans can also cause this problem by drawing air down the chimney, through the burner and out of the fan housing inlet. This can be corrected by providing adequate outside air intake openings for both the stoker and the exhaust fan or discontinuing the use of the exhaust fan.