Low limit aquastat not calling for heat

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.

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?

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.

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.

I believe that is proper usage of that L4008A to be an operator. So you have an operator and a high limit. That is no problem. (Yes @HVACNUT he has 2 high limits).

There is no better Aquastat control for operating the circulator. If you don't want to have your circulator operate all the time, then you need a circulator relay. Are there any other controls that you have not posted a picture?

There are relays that the thermostat can operate to turn on the circulator and the burner. That way the circulator only operates when there is a call for heat from the thermostat. Do you want me to send you the info for the control you need, and the wiring diagram that will work for you?

@ericthor said: "I can feel the current low level aquastat click when i move the dial and it hits what it thinks the boiler temp is at. it also clicks accordingly to how i set the variation dial. currently the low level was set to 160 and the temp gauge was showing 120. I increased the low level up to 200 and the flame kicks on heating the boiler temp to 145-150 then shuts off."

You need to be careful with reading the temperatures with the way you have the system wired up. The thermometer that tells you the boiler temperature is 145° is 50° different from the temperature setting at the control. That tells me three things.

• The water temperature at the thermometer and the water temperature at the pipe where the operating control is different.
• The control is not accurately reading the correct temperature
• The boiler thermometer is not accurately reading the correct temperature.

Or a combination of all three. So BE CAREFUL with your adjustments. If you are setting the control on the pipe to 200° and that is the most accurate control because it is the newest part on the system, and the boiler temperature is actually over 200° but the thermometer is not correct, you may end up using more fuel than you need to… or …over heating the boiler and the relief valve discharges… or …worse.

When professionals do service calls and this happens, we have a "Known Good" accurate thermometer to check the temperatures at the different boiler locations to see which device is accurate and which device is out of calibration.  That way we don't make any dangerous mistakes.

This is the diagram that will get you operating the circulator with the thermostat.

This is the same diagram as before with a General Purpose relay added. WR 90-130 Fan Center is a little less expensive than the Honeywell in the diagram and the wire colors may be different, but the diagram is identical.

First: Turn off power

There are 2 sets of relay contacts including a transformer on the same electric box plate, so you can remove the existing transformer and replace it with the new combination transformer/relay. The C terminal and the R terminal will stay the same. Just take all the wires from the C and put them back on the C and take all the wires off of the R and place them back on the R. Then connect the Black and White 120 VAC wires in the same way, and you should have exactly the same set up. Test that this works for you as it did with the old transformer and verify that the system operates the same as it did before.

Now turn off the power again and remove the thermostat from the system and just put the wires from the wires from the Low limit and the High limit that are left behind and connect them together.

Connect the R wire from the thermostat to the R on the transformer

Connect the W from the thermostat to the G on the transformer. Now when the thermostat calls for heat the relay will pull in and close the relay switch contacts closed (turned on). See how that completes a circuit from R on the transformer to R on the thermostat to W on the thermostat the G on the transformer to the Relay coil on the transformer to C on the transformer to make a complete circuit?

Remove the R wire from the transformer that goes to the spill switch should be connected to the violet wire on the relay and the red/yel wire on the relay should be connected to R on the transformer.

Once you have all those wires connected you can take the 120 v. Black wire from the circulator motor and connect it to the Black wire on the relay. Take the Red wire on the relay and connect it to the place you removed the black wire of the circulator from.

That should complete the rewiring.

Power up and test. Make sure all the limits stop the burner when there is a call for heat. You don't want to have the flame continue to burn if there is a problem with flue gas spilling, or the water getting too hot.

If this sounds too involved, then get a professional to do this for you.