Low limit aquastat not calling for heat

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 ..

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

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 …

You may be in over your head. Be careful. Mad Dog

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.

@ericthor said: "ok I checked the temp on my hot water supply pipe and it does match the brand new thermometer."

This statement is ambiguous and needs clarification.

What pipe and what thermometer? How was it "checked". Describe the procedure and what "Known Good" device was the test instrument?

What were the test results? What temperature was the pipe and what temperature did the new thermometer say the pipe was? The numbers please.

In the original post @ericthor said: "THE BOILER ONLY FIRES WHEN THE THERMOSTAT CALLS FOR HEAT, NEVER WHEN LOW LEVEL TEMP DIPS BELOW DESIRED TEMP."

After all the pictures and wires are analyzed this statement is TRUE.   Why would you need anything more?  Are the rooms too cold?  Then the thermostat is set too low.  Turn up the thermostat.

There is a concern with steel and cast iron boilers that operate at low temperatures of something call Flue Gas Condensation.   That condition exists when the flue gas temperature is below the due point of the flue gasses.  A large contributor to that problem is cold water (lower than 140°) in the return pipe from the radiators.   That can cause premature failure of the metal resulting in a leaking boiler.   I am wondering how old your boiler is?   If it is over 20 years and has been operating this way the entire time, then I don't believe that you have to worry about premature failure.  You are already past that.   How long will your boiler last from now?  Only time will tell.  

If you are interested in getting the water temperature in the boiler to be somewhat higher, then you need to do a little repiping and rewiring.    

1. You don't want the water temperature in the radiators to change because that is keeping the rooms comfortable
2. You want the boiler temperature to get above 140° quickly
3. You don't want the circulator to operate constantly especially if the boiler water is going to be over 140° because of reason #1
4. You don't want the off cycle 140+ water to stay in the boiler after the call for heat is satisfied because that is wasting energy.
5.  You don't want the post-purge heated water to cause the rooms to overheat.  

So the control design you want is a little more complicated than selecting an aquastat to keep the circulator off.  It will involve having a bypass pipe from the boiler supply to the boiler return to facilitate quick warm up of the boiler water, then you want something to blend cooler return water with the water that is going to the radiators in order to keep them from overheating the rooms.   This might involve a second circulator and a combination aquastat relay with post purge capabilities.  The Hydrolevel 3200 series of controls come to mind

Once the call for heat is satisfied, then flue gas condensation is no longer a problem. So you can allow the boiler to go cold with the post purge operation.  That would involve shutting off the boiler circulator and operating the system circulator thru a mixing system that delivers lower than 150° water to the radiators and continuing that flow until the boiler temperature is depleted to a lower temperature of say 110°

Are you interested in such a system design?  If so, then you should also consider outdoor reset to keep the room temperature even more even than you currently have with a thermostat only system.   We are talking thousands of dollars for such a design.  I’m not sure that is in the budget for most building owners, until you need to replace the boiler for some other reason.  That is when you consider taking the lowest bid which usually ends up with the highest operating cost or getting a sophisticated control and piping system to lower the cost of operation which will save you in the long run. 

Low limit and the Thermostat connected in parallel.

I'm glad it works as you want.

I think there must be more to the overall story. I would think with either call Thermostat or Low limit the circulator would run. Since it does not, which is probably good in this case, I suspect there may be additional control for the circulator between the actual thermostat and the thermostat wires pictured above.