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Limit Switches for Boilers 101

switches for boilers a temperature sensor honeywell i4006 honeywell l6006c honeywell l91b temperature limit switches gas pressure limit switches

This blog explains how Limit switches are critical safety devices on commercial boilers and burners. Their job is simple: if something moves outside a safe operating range, they shut the burner down to prevent damage or unsafe conditions.

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Limit Switches Questions?

There are two main types

Temperature limit switches

A temperature limit switch shuts the burner off if the water or steam gets too hot.

Gas pressure limit switches

Protect the burner if gas pressure is too high or too low.



Temperature Limit Switches (High Limits)

How They Work?

If the boiler is supposed to run at 180°F and something goes wrong (bad control, stuck valve, failed pump), the high limit might shut the burner off at 200°F to prevent overheating. 
Inside the control is:
  • A temperature sensor (bulb or probe)
  • An adjustable setpoint
  • Electrical contacts
When temperature rises:
  • The sensor expands or electronically detects temperature
  • At the set point, the contacts open
  • Opening the contacts shuts off the burner circuit

Manual reset is common for safety high limits so someone investigates the cause.

Most are:
  • Manual reset (must be physically reset after tripping)
  • Some are auto reset

Common Temp Limit switch models

Important: Make sure the probe and mounting style match the application 
Honeywell L4006A
  • Immersion high limit aquastat
  • Common on hot water boilers
  • Adjustable dial
Honeywell L6006C
  • Mounts on pipe
  • Often used as secondary high limit
Honeywell L91B

  • Manual reset safety limit
  • Common in commercial systems

Gas Pressure Limit Switches

There are usually two:  Low Gas Pressure Switch &  High Gas Pressure Switch. Think of gas pressure like water pressure in a hose.
What They Do
  • Pressure is too low > flame becomes unstable or goes out
  • Pressure is too high > unsafe combustion or overfiring

So these switches monitor gas pressure and shut the burner down if it's outside the safe range.

How They Work

These are wired into the burner safety circuit.

  • Inside the switch is: A diaphragm, adjustable spring, electrical contacts
  • When gas pressure: Pushes against the diaphragm, If pressure crosses the setpoint the contacts open and burner shuts off.

Gas Pressure Switch Models  ( Common )

Important: verify the range and switching action for the application
Honeywell
  • Model C6097A
  • High or low gas pressure
  • Very common on commercial burners
  • Adjustable 

Kromschroder (very common on larger industrial boilers/burners)
  • Kromschroder DG Series 
  • DG 10, DG 50, DG 150 etc. 
  • Available for high and low pressure 
  • Widely used on industrial boilers 

Dungs
  • Dungs GW Series
  • GW 3A5, GW 10A5, etc.
  • Very common in packaged burner gas trains
  • Adjustable setpoints

Where These are installed?

The Pressure limit switches will be installed in the boiler’s “gas train” while the temp limit switches and controllers will be installed on the boiler vessel.

The Big Takeaway

  • Temperature limit   “Too hot? Shut it down.”
  • Gas pressure limit   “Gas pressure wrong? Shut it down.”
Limits are: Safety devices - Wired in series with burner controls - Required by code - Critical for safe operation

If one trips: The burner stops, Controls may lock out, and you must find the cause before resetting.

Limit Switches Questions?

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