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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7542
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Teledyne Laars boiler model number PW0400CN12KBASN,

This answer was rated:

I have a Teledyne Laars boiler model number PW0400CN12KBASN, a
two stage unit. The gas valve is a Robertson Grayson 7000DER2HC, it
has a Honeywell S8600F electronic control and a Honeywell two stage
temperature controller L6008G1009 (Laars part number RE0093200).

The problem is the boiler fires O.K. and the flames come up, but after
operating a little while the temperature increases the the point where
the high limit cuts the unit off. Then after it cools down the unit will
work again and go through the same cycle. If I turn the temp controller
down during this time the boiler will NOT cut off (even whenever it is turned all the way down to the stop).

I was thinking of replacing the temp controller and the immersion well
(I figured I would not be able to get the old bulb out).
Welcome to Just Answer!

The temperature controller is most likely bad... to test that remove one of the two wires attached to it while the boiler is firing...if the burner goes off... the controller is bad. Thats a test intended for experienced technicians. Be careful not to let the wire touch anything when you lift it off of the terminal during the test.

If you are not experienced, let me know I will show you another way to do it..its a bit more time consuming and complex.

You should have no problem getting its sensor bulb removed from the well...look for a clip at the top of the well you have to remove .. the bulb should slide right out..

I will hold this question open for follow up with no time limit as long as you rate my responses *positively.


Phil and other HVAC Specialists are ready to help you
Customer: replied 3 years ago.

Thanks for responding.
The controller has two "R" terminals with nothing connected.

The "B" terminal has a wire on each terminal. I believe this makes up the

24 volt circuit to start and fire the boiler. The "W" terminal has a wire

on each terminal - I believe one is stage one and the other is stage two.

I did lift what I believed was stage one wire and the boiler turned off.

Please explain why this would indicate changing the temp controller?

Also whenever I had stage two going, I turned the controller all the way down and it would NOT TURN off.

These parts cost approx. $277.00, I would like to get this right. Is there any other part that would cause this problem?

I pulled the brown wire off the gas valve and the unit shut down - which is what it is supposed to do.

What is the other way to verify this is the part that needs replacing?

Hello again! Thanks for the excellent rating.

The control operates by breaking the circuit the two wires that go to the switch ... verify the switch by looking at the wiring diagram on the temperature control.... by removing one of the wires you are simulating the switch opening. So we know that it is the control failing to open the switch when it should.

You should try that a dozen times or so... it takes a while to format ones thinking patterns in these regards.... then it becomes obvious.


.if it is failing to break the circuit that energizes the gas valve when the temperature of the boiler water is above the thermostats set point.... then either the contacts are fused closed...or the sensing element is not sensing the water temperature.

If the sensing element is 3/8 or 1/2" diameter copper bulb, it is full of refrigerant. When the refrigerant warms up it puts pressure on a diaphragm inside the controller and that is mechanically linked to the switch that the wires are connected do.

If the sensing element is electronic it is called a 'thermistor'.. and is 1/4" in diameter, chrome plated as a rule and has an insulated set of wires going to it.... *not a thin copper tube.

Let me know which you have, . We can go from there.


Customer: replied 3 years ago.

I have the 3/8" diameter copper bulb. According to the schematic one "W" terminal on the controller goes to the first stage and the other

"W" terminal goes to the second stage. It looks as though the two "R"

terminals make and break the 24V control voltage.


Please tell me if you think there is any other part in this system that could give this symptom before I purchase the controller, etc. tomorrow. This would be an expensive mistake and very disappointing.


The only other components to choose from, in my opinion, would be the

gas valve and maybe the electronic controller. It seems as though the

temp controller is the component that actually removes the 24V to turn

the boiler off ---- IS THIS CORRECT?


You are correct... the chances that we are mistaken here are zero. Be very sure you wire the new one exactly as this one is.... make a large, overly clear and unmistakable diagram of the wiring... a photo would be good too.

Note: the 'terminals' such as R do not make or break... it is the switch connected between R and W that make and break... you can test that by killing power to the boiler, removing the wire to R and to W then operating the switch by adjusting the dial to its extremes... if the switch does not open and close, the controller is bad.

either its contacts are burnt closed or more likely the sensing element has lost its charge. Stay in touch as needed, I will hold the question open.

Customer: replied 3 years ago.


Thanks for your help. I will be installing the temp controller tomorrow,

so I may have a couple more questions.




Good plan. Follow up as needed.

Hello again,

When you put the new sensing bulb into the thermo-well use some heat conductive paste made for the purpose, they will carry it at the place you buy the temperature control...its made of grease and aluminum powder. about 95% aluminum... lacking that the control will tend to lag quite a bit.

Customer: replied 3 years ago.

Hi Phil,

Thanks for the follow-up suggestion about using the heat conductive

paste. I figured I needed this whenever I removed the old one. I was

hoping there would be enough residue from the old bulb! I did not

realize I needed this.


I installed the temp controller on Monday and everything has been

fine. I will keep you posted.


Thanks for being so conscientious - your help is appreciated.



Hello again... thanks for letting me know it all worked out well on your end... By working with us you saved yourself $300 to $500 in shop service charges, or a lot more if they had made a mistake in diagnosis.

and you learned something... that actually creates more dendron arm connections in the brain and makes you exponentially smarter over time.