Stoker boiler operating and temperature controls
The stoker boiler operating and temperature controls operate together to produce a safe and consistent hot water heat.
The L8124A triple aquastat is the heart of the system:
It maintains a constant boiler temperature when the boiler is at rest, turns off the circulator to preserve the function of the domestic water coil, and shuts down the stoker when the water temperature goes above a safe level.
The L8124A aquastat has three settings:
1. The low setting determines the temperature that the boiler will maintain. In a system with cast iron radiators, this setting is usually 150 degrees. In a system with copper radiators, this setting is usually 160 degrees. During the Summer, if only domestic hot water is desired, the low limit setting can be turned down to 130-140 degrees. The feed rate and air settings may also have to be adjusted.
2. The high limit setting is usually 40-50 degrees higher than the low limit setting. This is a safety setting only. The boiler does not try to attain this setting’s temperature. The reason for the spread in temperatures is because, even though the stoker has shut down, the hot coals in the pot can raise the boiler water temperature another 30 degrees, possibly 40 degrees in the Summer. If the high limit is not set high enough, the timer will not start the stoker motor and the fire will go out in the pot.
3. The differential setting’s purpose is to stop the circulator when the boiler’s water temperature has dropped to a point where there will be a lack of domestic hot water from the coil. On a copper baseboard system, this setting is 10. On a cast iron radiator system, this setting is 15. On a system with no domestic water coil, this setting can be at 25.
For example: If the aquastat low limit setting is 160, the high limit setting is 210, the differential setting is 10, the boiler is resting at 165 degrees and there is a call for heat, the circulator and will start circulating hot water through the radiators and returning cool water to the boiler. The stoker will also start and try to maintain the boiler water at 160 degrees. The cooler return water will cause the boiler water temperature to fall to 150 degrees, which is the point where the 10 degree differential setting (pre-programmed) causes the circulator to stop, and allows the stoker to raise the water temperature to at least the low limit of 160 or continue to run as long as the thermostat is not satisfied. (At this point, there is also a high enough water temperature in the boiler so that the water coil will provide enough domestic hot water. If the boiler water is too cool to provide enough domestic hot water, the aquastat’s low and high limit setting can be increased.) Once the boiler water reaches 160 degrees, the circulator will start again and the circulator/stoker/differential cycle will continue until the thermostat setting is satisfied and the low limit setting is reached. For a longer stoking period, the differential setting can be increased, but the circulator will still stop when the boiler water temperature drops to 150 degrees.
**Note** It is understood that the above description of the controlling aquastat is for fully ideal conditions, i.e. a perfectly sized boiler and radiation for the house, with an unchanging outside climate and perfectly adjusted coal feed and air settings.
If the boiler water temperature would reach the high limit setting of 200 before the thermostat setting is satisfied, the stoker will shut down on safety. It will not start up again until the circulator returns enough cool water to lower the boiler water temperature to 190 degrees. This situation can be caused by overfiring or impeded heat extraction of the hot water. Examples: Curtains hanging over the top of the baseboard; a rug too high under the baseboard; radiator covers; not enough radiation.
Using the R845A relay in conjunction with the triple aquastat for multiple circulators on a boiler with domestic coil:
Use the R845A relay with multiple circulators on a boiler with a domestic coil. On a call for heat, the R845A will work in conjunction with the triple aquastat and turn off the circulator on low boiler water temperature, thus allowing hot water for domestic water use and preventing the circulation of cold water through the heating loop. This is preferable to the RA889A/RA89A which will turn on the circulator regardless of boiler water temperature on a call for heat.
Because it is wired to the ZC and ZR terminals of the triple aquastat, the R845A relay, upon a call for heat at the thermostat, will start both the circulator and the burner. When the boiler’s triple aquastat senses low boiler water temperature, but the thermostat is still calling for heat, the R845A relay will allow the burner to run and stop the circulator so the burner can keep the boiler water temperature high. whereas the RA889A/RA89A will not shut down the circulator, causing low temperature water in the domestic water coil.
The stoker timer:
When used with the triple aquastat, will send power to the ZR terminal of the aquastat, which will cause the stoker to run for a pre-determined amount of time, set by the timer, usually 2 ½ minutes per half hour. The timer will cause the stoker to run regardless of the temperature of the boiler water. If the aquastat high limit is reached at any time during the 2 1/2 minutes of runtime, the stoker will shut down on high limit. This is one reason for keeping a 40-50 degree spread between the low and high limit settings. If the high limit is set too low, there will be a problem with the fire going out.