Burner Flame Sensors and Safeguard Controls

Boiler flame sensors and safeguards are tools to help ensure your burner is working properly. They keep you safe from start up to shut down by confirming your burner system is burning the fuel it is requesting. This prevents the system from pumping in fuel when a flame has gone away and prevents a potentially explosive situation. Before we jump into the different types of sensors, let’s look at the sequence of operations of safeguard controls.

Sequence Of Operation

All flame safeguard controls have a sequence of operations which are mandated by insurance providers. This sequence requires flame safeguards to send information to the mod motor to begin operation. The order in which these sequences of operation occur can vary based on the size and horsepower of the equipment. Typical sequence is as follows:

  1. Call For Heat
  2. Pre-Purge Or Start Blower
  3. Modulate Combustion Air Damper From Low To High and High To Low
  4. Low Fire Start Switch Mode
  5. Start Ignition And Pilot
  6. Prove Pilot (Dictated by insurance but is usually around 10 seconds.)
  7. Ignition Drops Out
  8. Main Fuel Valve Energized
  9. Main Flame Proven
  10. Release to Modulate
  11. Demand or Call For Heat Satisfied
  12. Post Purge (Usually 60 seconds)

Flame scanner diagram showing a UV or infrared scanner Flame Sensing

Now that we understand how a typical safeguard control operates, let’s take a look at the flame sensing portion of the control process. There are 3 types of flame sensing controls:

  1. Rectification (Flame Rod, Photocell)
  2. Infrared
  3. Ultraviolet

Flame Rectification (Flame Rods and Photocells)

Flame rectification is when a flame can act as an electrical rectifier. We commonly see the principle of flame rectification in both flame rods and photocells.

Flame RodsDiagram of how a flame rod works

Flame rods sit directly in the flame. The flame will carry a small current between the flame rod and the grounding plate letting the system know that it has a flame. This works on the principal that a flame will actually conduct a small amount of electricity. When the sensor detects the electrical signal, the system adds fuel.

If the flame flickers away or is diverted away from the sensor, the flame guard will show a pilot failure. This will then cause the system to cycle off.

Note: Flame flickering is often the result of too much air or gas velocity in the system.

Close up of a photocell burner sensor in black and white


A photocell is another flame sensor that uses a rectification amplifier. Oil burning systems commonly use this type of sensor and it’s also commonly seen in ovens and furnaces as well. Photocells look for visible light. When the sensor picks up the light, electrons are emitted and tell the flame safeguard that a flame is present.


This form of scanner picks up the invisible infrared rays created by the flame. It picks up these invisible wavelengths using a pyroelectric sensor which detects electromagnetic radiation.

Ultra Violet scanner on a Johnson BoilerCareful aiming of the infrared scanner, directed at the pilot and main flame, is imperative. This is because hot refractory in a boiler or oven can reflect these infrared rays as well. If improperly aimed this can lead to the flame safeguard falsely indicating a flame is present. This type of sensor works well even in smokey environments.

Technician can test to see if an infrared scanner is working by removing the scanner remove the scanner and running a flashlight back and forth over the sensor. The system should cycle on and off with each pass as it detects the infrared waves coming from the flashlight.


Ultra Violet Flame sensor in the boxThis is the most common scanner used today and is great for clean burning fuels like natural gas and propane. UV scanners see the ultraviolet radiation that the flame emits. This can be detected on the inner core or first 1/3rd of the flame. This sensor will not work if it is aimed at the outer edges of the flame.

It is imperative that UV scanners are not able to see the igniter arc. This is because the igniter arch is rich in UV and can lead to a false reading. One good way to eliminate this problem is to use an interruptible igniter. Interruptible igniters shut off once a pilot is established. This eliminates the potential of a false positive indicating a strong pilot when instead it is picking up the arc. An intermittent ignitor on the other hand is when the igniter stays on as long as the pilot system stays on. This can often be the entire time the burner is on.

When you have a burner which cycles on and off as it satisfies the connected load, a UV scanner is a good choice. However, if your system simply modulates from low fire to high fire and back to low, a dynamic self-checking scanner should be used. Self-checking scanners have a shutter that closes every 6 seconds to ensure the system is sensing the UV light correctly.

Burner Flame Sensors and Safeguard Help

As always, we recommend a good PM program to test flame sensors and safeguards regularly.

Burner on the fritz? We can help. Reach out to the pro’s at Rasmussen Mechanical Services, our team is here to help! Receive a free estimate, call us at 1-800-237-3141 or chat with an agent.

Check out a recent burner retrofit we completed on this Hurst Boiler! New Autoflame controls on a Limpsfield burner.

A burner retrofit with autoflame controls on a limpsfield burner