Low limit aquastat not calling for heat
I recently purchased a building with a 250.000 btu Slat Fin Galaxy cast iron gas fired boiler installed in 1994. during demolition of the building the low level aquastat was accidentally removed (it was located outside the mechanical room). When we went to fire up the boiler this winter it was apparent something was missing and a new Resideo model L4008A was installed along with a new temp/pressure gauge. Because the previous immersion probe was seized in the well tube and we didnt have a new one on hand, we installed the immersion probe to the side of the hot water supply line and wrapped with insulating tape. I have the high limit aquastat set to 180 and the low level set to 160 with a 20 degree differential. The boiler has never reached 180 and the low level aquastat never seems to call for heat when the water temp dips down, sometimes nesr 120 degrees.
THE BOILER ONLY FIRES WHEN THE THERMOSTAT CALLS FOR HEAT, NEVER WHEN LOW LEVEL TEMP DIPS BELOW DESIRED TEMP.
Im hoping the issue is becuase i have an immersion style aquastat probe not properly installed inside the well. however i still feel like the low level shouuld be calling for heat, even if its not accurate.
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The reverse setting on a dual aquastat is used to maintain temperature if the boiler has a coil inside to supply domestic hot water. The low side also shuts off the heat circulator from robbing the supply of hot water ..
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there are 2 aquatats installed. one high limit and one low limit.
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do you have a picture of how the aquastat itself is wired? If it’s wired to the bottom & top terminals then it’s shutting off at 160 and isn’t wired as a low limit, it’s wired as a high limit
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What is the B aquastat wired to ? What happens if you jump out the B ?
The B makes on temperature rise . It is not a low limit …
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Im not following you, Im also not a mechanical guy so you have to speak in dummy terms… what do you mean by B? Are you referring to the external mount resideo?
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This photo is a picture of a Reverse Aquastat also known as a B aquastat. It appears to be new by the date code in the red box H2542. That indicates the 42nd week of 2025. So sometime in October that control was manufactured.
On the other side of the metal chassis (indicated by red arrows) is printed a model number that might be L4006BXXXX (with four numbers after it.). Is there a way to post those numbers so we can look up and know what that control is capable of
From that photo I can tell you that it is connected by wires to a 24v. control circuit, but I can not tell you what it is connected to. That same metal chassis can be used for other Aquastat controls with very long probes that can measure the temperature from outside the building or from a room that is several feet away or from a pipe that is somewhere behind that wall the control is attached to. The probe might look like this:
By the dial setting and the wire terminal designation I can tell you that the control pictured will turn on something when the temperature probe is at or above 160° and that the control will turn off that device when the temperature is at or below 155° if the control has a fixed differential of 5°. If you are expecting that control to turn on a boiler when the boilere temperature is below 160°, You selected the incorrect control.
Here is the terminal designation of the L4006 and L6006 controls. W at the top R in the middle and B at the bottom.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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ok the "B" aquastat (pictured) is mounted outside the mechanical room with the temp probe passing through the wall and connected to the hot water supply pipe.
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OK that control os a operation control that will power on a control if the temperature at the probe is lower than the set point of the dial. That means that you should be able to use that to turn on a burner the burner should stop when the temperature exceeds the 160° set point and then the burner should start once the temperature drops to 155° if the differentia is set to 5°. That control has an adjustable differential from 5° to 30°
To determine what is going wrong, can you show me where the other end of the red and white wire on that control are connected to?
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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if you scroll up you can see the photo i posted where the stat wires are labeled. "stat" being the thermostat and "Aqual L" being the low limit. that photo shows the high limit aquastat mounted behind the access panel on the boiler.
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What does it say on the inside of the cover? There's only 2 terminals, so its SPST. If it says "R-B opens on temperature rise" then you've got another High (A) Limit. You need a B Limit. An L6008B might work. You could also get an L6006C strap on if its feasible.
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This is your wiring diagram to the best I can see from the pictures:
There is no control that will operate a circulator. So I guess that is constant operation. The operator or low limit, the high limit, all the other safety switches, and the thermostat need to be closed (turned on) for the vent damper to operate. Any one of them opening will keep the vent damper closed. Once the vent damper is open and the end switch inside the vent damper closes to indicate the damper is open, then the gas valve will get power to operate the burner.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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