Stoker boiler operating and temperature controls

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.

stoker-man wrote:Stoker boiler operating and temperature controls

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.

Stoker man, There a problem with this statement if where talking about Honeywell Triples like a L8124 or a L7124, when there is a call for heat weather it be a thermo on the TT terms or a 845 relay connected to ZC/ZR the boiler will run till it hits high limit and then shut down being these have a 10 diff they will start the boiler again at 170F if there is a call for heat. Once there is no call for heat the water will be maintained by the low limit setting till the next call for heat.

Even with the differential setting at the maximum of 25, the stoker will shut down at 15 degrees above the low limit setting. Allowing a 30 degree rise in boiler water temperature from the hot coals in the pot after the stoker shuts down and using the low limit setting in the original posting, you get the following numbers: A low limit setting of 160, plus 15 degrees because of the differential setting of 25, and a 30 degree increase in boiler temperature from the hot coal in the pot, ending up with a boiler temperature of 205 resting. This may be what you are seeing, but the aquastat is not designed to run the stoker until the high limit is reached.

Several reasons are given in the original posting as to why a stoker boiler may be reaching the high limit too quickly.

My boiler has a Honeywell L8124A and according to the manual that is the factory installed aquastat. On a call for heat the stoker motor is energized regardless of where the low limit is set. I verified this with the Honeywell schematic; As long as the temperature of the boiler water isn't above the high limit the stoker will run when the thermostat calls for heat. To avoid lack of heat the feed has to be set to match the BTU requirement of the building it is heating based on standard heat loss calculations. Lets say that the boiler is set at 4 teeth to provide ~90k BTU's, which would heat an average house on a 0 degree day. On a 30 degree day on a call for heat the high limit will stop the stoker from overheating the boiler, if the boiler feed rate was set as to not hit high limit on a 30 degree day it would run constantly on a 0 degree day and the house would be cold. So basically whatever the high limit is set to during the winter is what the boiler water temperature is going to be on a warmer than average day, on a colder than average day the high limit will not be reached as the boiler would be operating at it's design setting. During the summer the high limit also shouldn't be reached as the boiler will run off the low limit and the differential to maintain the boiler temp. Keep in mind that the coal will continue to add heat to the boiler, this is why setting the high limit at 170 will result in a boiler running at 180-190. How much radiation is in the house, the temperature of the return water, and the size of the pump among other things will determine how hot the boiler should be. The hotter the boiler water the more standby losses and the more coal you use. Stoker-Man's description is of a Honeywell L8124M which only runs the circulator on a call for heat. This seems to be a better setup but the 8124M isn't readily available as far as I can tell.

e.alleg wrote:My boiler has a Honeywell L8124A and according to the manual that is the factory installed aquastat. On a call for heat the stoker motor is energized regardless of where the low limit is set. I verified this with the Honeywell schematic; As long as the temperature of the boiler water isn't above the high limit the stoker will run when the thermostat calls for heat. To avoid lack of heat the feed has to be set to match the BTU requirement of the building it is heating based on standard heat loss calculations. Lets say that the boiler is set at 4 teeth to provide ~90k BTU's, which would heat an average house on a 0 degree day. On a 30 degree day on a call for heat the high limit will stop the stoker from overheating the boiler, if the boiler feed rate was set as to not hit high limit on a 30 degree day it would run constantly on a 0 degree day and the house would be cold. So basically whatever the high limit is set to during the winter is what the boiler water temperature is going to be on a warmer than average day, on a colder than average day the high limit will not be reached as the boiler would be operating at it's design setting. During the summer the high limit also shouldn't be reached as the boiler will run off the low limit and the differential to maintain the boiler temp. Keep in mind that the coal will continue to add heat to the boiler, this is why setting the high limit at 170 will result in a boiler running at 180-190. How much radiation is in the house, the temperature of the return water, and the size of the pump among other things will determine how hot the boiler should be. The hotter the boiler water the more standby losses and the more coal you use. Stoker-Man's description is of a Honeywell L8124M which only runs the circulator on a call for heat. This seems to be a better setup but the 8124M isn't readily available as far as I can tell.

hi guys mw setup is alittle differentI am running the 8124A, Honeywell timer and a TaccoSR506EXP zone control...
The way it is setup (wether it is right or wrong) when the zone calls for heat it turns on the circulator until satisfied, the aquastat low is set for 155, diferential 15, and the blower and feed starts at about 146 and kicks of at about 165, we are getting domestic hot water and have cast iron radiators, the zones are all setback programable thermostats which makes for comfortable house... the boiler is only fireing when it senses a loss of heat from water temp. I am presentl running 4 teeth and about 4.5 on the blower air. When all the thermos call for heat at the same time in the moorning it will dropboiler to about 135 but it recoveres and maintains 165 all day temps have been in the 20sto 0 past week burning 80 to 100lbs coal day.

ted wrote:When all the thermos call for heat at the same time in the moorning it will dropboiler to about 135 but it recoveres and maintains 165 all day temps have been in the 20sto 0 past week burning 80 to 100lbs coal day.

Why don't you stagger the zone start up times some? ... let one zone get warmed up, before the next calls for heat.

stoker-man wrote:Even with the differential setting at the maximum of 25, the stoker will shut down at 15 degrees above the low limit setting. Allowing a 30 degree rise in boiler water temperature from the hot coals in the pot after the stoker shuts down and using the low limit setting in the original posting, you get the following numbers: A low limit setting of 160, plus 15 degrees because of the differential setting of 25, and a 30 degree increase in boiler temperature from the hot coal in the pot, ending up with a boiler temperature of 205 resting. This may be what you are seeing, but the aquastat is not designed to run the stoker until the high limit is reached.

Several reasons are given in the original posting as to why a stoker boiler may be reaching the high limit too quickly.

Stoker-man, As Ed stated you are correct with a 8124m which I totaly forgot about since I haven't seen one in a long time but that is also why I posted I didn't want someone like in Ed's case to raise the High limit to much and create a unsafe situation.

ted wrote:

e.alleg wrote:My boiler has a Honeywell L8124A and according to the manual that is the factory installed aquastat. On a call for heat the stoker motor is energized regardless of where the low limit is set. I verified this with the Honeywell schematic; As long as the temperature of the boiler water isn't above the high limit the stoker will run when the thermostat calls for heat. To avoid lack of heat the feed has to be set to match the BTU requirement of the building it is heating based on standard heat loss calculations. Lets say that the boiler is set at 4 teeth to provide ~90k BTU's, which would heat an average house on a 0 degree day. On a 30 degree day on a call for heat the high limit will stop the stoker from overheating the boiler, if the boiler feed rate was set as to not hit high limit on a 30 degree day it would run constantly on a 0 degree day and the house would be cold. So basically whatever the high limit is set to during the winter is what the boiler water temperature is going to be on a warmer than average day, on a colder than average day the high limit will not be reached as the boiler would be operating at it's design setting. During the summer the high limit also shouldn't be reached as the boiler will run off the low limit and the differential to maintain the boiler temp. Keep in mind that the coal will continue to add heat to the boiler, this is why setting the high limit at 170 will result in a boiler running at 180-190. How much radiation is in the house, the temperature of the return water, and the size of the pump among other things will determine how hot the boiler should be. The hotter the boiler water the more standby losses and the more coal you use. Stoker-Man's description is of a Honeywell L8124M which only runs the circulator on a call for heat. This seems to be a better setup but the 8124M isn't readily available as far as I can tell.

hi guys mw setup is alittle differentI am running the 8124A, Honeywell timer and a TaccoSR506EXP zone control...
The way it is setup (wether it is right or wrong) when the zone calls for heat it turns on the circulator until satisfied, the aquastat low is set for 155, diferential 15, and the blower and feed starts at about 146 and kicks of at about 165, we are getting domestic hot water and have cast iron radiators, the zones are all setback programable thermostats which makes for comfortable house... the boiler is only fireing when it senses a loss of heat from water temp. I am presentl running 4 teeth and about 4.5 on the blower air. When all the thermos call for heat at the same time in the moorning it will dropboiler to about 135 but it recoveres and maintains 165 all day temps have been in the 20sto 0 past week burning 80 to 100lbs coal day.

I'm not familier with the Taco zone control but if it has boiler out connection you would want to run those to the TT terms on the 8124A that way it would fire the boiler when there is a call for heat run up to the high limit so if your low is 155 you would at least put the high limit 20F higher so there is no diff over lap which would be 175F. This is of course your choice. I wanted to add that the way you are wired now your 8124A pretty much works like a 8124M with the low limit controling the boiler temp and the high limit shutting down the boiler if it gets to hot. If your not wire to ZR or ZC on the 8124A your circulators are going to run even if the boiler drops below low limit this would make domestic water get cold if your using a coil inside the boiler.

Dallas wrote:

ted wrote:When all the thermos call for heat at the same time in the moorning it will dropboiler to about 135 but it recoveres and maintains 165 all day temps have been in the 20sto 0 past week burning 80 to 100lbs coal day.

Why don't you stagger the zone start up times some? ... let one zone get warmed up, before the next calls for heat.

Tom as I was typeing that comment that thought was going through my head....
what would be really nice if I could set up a program on my computer that I could access all of the thermostats and adjust temps rather than at each thermostat...Just a dream and am sure that there is technology to do that..Right now my daughter is home from college so all the setbacks during the day have to be overridden in order for hheat in house... I still think that it is easyer than hand feeding the fire last year
thanks for your input
Ted

My bro-in-law does exactly that. He can monitor the OR temps and conditions in the Philadelphia hospital. If there is the slightest variation in room temp, he knows about it via computer and can make adjustments via computer.

this is not as pretty as u2345s but it is my second attempt at boiler instalation last one 18 yrs ago

I decided to test my oil boiler and compare the results to the Honeywell specs for the L8124.

I calibrated the aquastat and made the low and high limit setpoints at the same time. The test began with the boiler at 110 degrees resting, the house was at 72 degrees and the outside temperature was 38. The boiler is about 90K BTUH net. I turned up all three thermostats (3 floors) to 80 degree and the burner started.

With the low limit set at 120, the burner heated the boiler water to 120 on the gauge and then shut off. The water temperature then drifted up to 132. Then I disconnected the circulator and set the high limit at 150. The burner took the water up to 150 and shut down on high limit. I hooked up the circulator again and it ran until the boiler temperature went down 10 degrees, the burner started up again and ran up to the high limit and shut down. This high limit, then minus 10, then high limit cycle continued the entire time there was a call for heat. Over three cycles, beginning at the high limit temperature of 150, it took 11 minutes of circulation to all three zones (2200 sq ft) to reduce the boiler water temperature by 10 degrees to 140. It took the burner 6 minutes to raise the water temperature back up to the high limit of 150, where it again took another 11 minutes to reduce the water by 10 degrees. I'm adding that half of the return water is bypassed back into the supply line to moderate the water to the baseboard.

Conclusion: The aquastat worked exactly as it is designed to work. It ran up to the high limit and stayed in that zone because the boiler is able to create heat at twice the rate that I can dissipate it. Apparently, the boiler is oversized for the amount of radiation in my house. As long as I can maintain a stack temperature of at least 350 degrees, I can reduce the firing rate of the boiler.

If this were a coal stoker, I would have the ability to reduce the air and feed rate.

Figure 22 on page 13 describes exactly how the switches perform and make on temperature rise and fall.

http://www.exqheat.com/images/aquastatL8124_honeyw.pdf

Apparently, the boiler is oversized for the amount of radiation in my house. As long as I can maintain a stack temperature of at least 350 degrees, I can reduce the firing rate of the boiler.

On a 38 degree day you are exactly correct. When it is 38 degrees outside your house only needs about 30k btu's to heat it. Once the temp drops down to zero and the wind is howling you'll be using 90k btu's as the heat is pulled from the water much faster.