The Development Of Modern Primary Controls

664849-1Advances in electronics technology are making an exciting impact in our oilheat industry. These developments have enabled control manufacturers to put high performance features into residential primary controls. Residential controls are now being made available to you that have features that were previously found only with the expensive primary control units for commercial or industrial applications. Features like valve-on delay (pre-purge), burner motor-off delay (post-purge), and interrupted duty ignition are becoming universal. Preignition, limited reset and recycle, and alarms contacts are also helping to make today’s controls advanced and powerful tools.

This technical bulletin will discuss some of these important improvements and present a list of descriptive terms to help you become familiar with this new generation of controls. We will include a discussion of the Honeywell R7184 series control, the most recent entry into our marketplace. A handy R7184 quick reference guide for technicians that is included on page 4 will help you become knowledgeable with its operation and features.


As the electronics industry constantly develops, many of its products and techniques have been applied to burner controls. Some of the developments that are key to us are the following:

  • Relays have become smaller and more reliable. It is now common to see several relays inside each control, so that the motor, ignitor, and valve may all be controlled by separate relays. Solid state relays are increasingly being used instead of the traditional electromechanical relays.
  • Microcontrollers (small computer chips designed specifically to control other electronics) have increased the ability to control the burner’s components separately and with more intelligence. A microcontroller’s software code can easily add control functions and make complex decisions, eliminating the need for large decision-making circuits. Circuits are now needed mostly to provide processor and 24 Volt power, convert sensor signals, and drive relays.
  • Timers have developed from being controlled by bimetal switches to being controlled by semiconductors. Now, many different timers can all be running at the same time. Timings are less affected by temperature, vibration, light, line voltage, etc. They can be accurate to within fractions of a second, instead of varying by 10% to over 50% for some traditional controls.
  • Flame sensing has progressed from stack mounted bimetal switches to cadmium sulfide sensors (cad cells) and ultraviolet sensors. These sensors can more quickly and accurately sense the flame, and they also have the ability to sense the varying brightness of a flame. Software programming can monitor the rise and fall of a cad cell resistance to better interpret its signal. For instance, the cad cell signal can be smoothed out to help prevent nuisance lockouts. The Honeywell R7184 can display the cad cell resistance (see the Quick Reference Guide on page 4), so you do not need to use an Ohmmeter.
    NOTE: For proper operation, it is important that the cad cell resistance is below 1600 Ohms.
  • LED indicators offer you an increasing amount of diagnostic information such as recycling, flame status, or different lockout modes. Indicators like these are making it easier than ever to know what was happening before you arrived at the installation.
  • Self checking and system checking to ensure a safe, proper starting and operation of the burner are now standard procedures in microcontroller-based controls.
  • Advanced features, both new and borrowed from more expensive controls, are being added to residential controls. In the near future, look for increased use of the reset button, special pump priming procedures, redundant safety features, brownout protection, advanced recycle methods, and other features that will make controls better and your job easier.


You are probably familiar with the definition of most of the control-related terms below. However, you may not be aware of their benefits or importance to service technicians. As you read on, remember that many of these features were not readily available to us until recently.

1 BURNER MOTOR-OFF DELAY (POST-PURGE) is the time period after the fuel valve closes during which the burner motor continues to run. Don’t forget: post-purge controls typically need one wire to limit (commonly red) and one wire to constant L1 power (commonly black)!
2 DATA COMMUNICATION PORT is a small electrical port connected to the primary control’s computer chip. Comm. ports will enable the control to communicate with flame quality and other service instruments, home security systems, or data acquisition equipment, as they become available.
3 DIP SWITCHES (Dual Inline Package) are sets of switches that can be manually set to an open or closed position. DIP switches provide the serviceman with an easy way to select different control options (for instance, burner motor-off delay times).
4 DRY ALARM CONTACTS are isolated (non-powered) relay contacts that are closed when the burner is in safety lockout. Alarm or home security systems can use the alarm contacts to alert the homeowner or service company that the burner has locked out.
5 FLAME FAILURE RESPONSE TIME (FFRT) is the time it takes for a control to sense that the flame has been lost. The FFRT is one of the key safety timings of a control. If it is too short, you could have nuisance lockouts. If it is too long, the chamber could become flooded with fuel if the ignition fails.
6 IGNITION CARRYOVER is the period after a flame has first been sensed that the ignition source is left on (only applicable to controls that have interrupted duty ignition). Ignition carryover helps to ensure that the flame is strongly established before the ignition source is turned off.
7 INTERMITTENT DUTY IGNITION is the ignition method in which the ignition source is energized the entire time the burner is firing. In older, non-flame retention burners, intermittent ignition is usually required to sustain the stability of the flame.
8 INTERRUPTED DUTY IGNITION is the ignition method where the ignition source comes on only to light the flame. After the flame is established, the ignition is turned off and the flame keeps burning. Interrupted duty ignition saves on electrical energy and significantly increases the lifetime of the electrodes and the ignition source.
9 LED INDICATOR (Light Emitting Diode) is a small low power light that can be easily turned on and off by the primary control. The LED is a service tool that can be used to indicate that the burner sees light, is in a recycle or lockout mode, etc.
10 LIMITED RECYCLE means that the control will only recycle and try to restart the burner a limited number of times if the flame is repeatedly established and then lost. Limited recycle keeps the appliance from being excessively sooted due to repeated failing combustion.
11 LIMITED RESET (or RESTRICTED LOCKOUT) means that a simple press will only work a certain number of times to reset the control from lockout. After that, a special service reset procedure is required (for instance, holding the button longer). Limited reset prohibits those who are unqualified from repeatedly resetting the control and increasing burner system problems.
12 PREIGNITION is the term for energizing the ignition source before the motor / pump, to give the ignition source time to establish its arc. Pre-ignition decreases difficult ignitions that are due to delayed arc establishment.
13 RECYCLE is the waiting period after a burner has lost its flame before the control can try to restart the burner. The recycle (delay) period helps to ensure that the burner will safely try to restart the burner.
14 SAFETY LOCKOUT is the action a primary control takes to shut down the burner if no flame is detected within its safety lockout time (TFI). Safety lockout prohibits the burner from continuing to spray unburned fuel into the appliance.
15 SAFE START CHECK is an electronic procedure in which the control makes sure it is safe to start the burner when a call for heat is initiated. Safe start checks enable the control to ensure that the line voltage is sufficient, the cad cell is sensing darkness, and internal software is error free.
16 STANDBY is the period during which the burner is off, while the primary control waits for a call for heat. The R7184’s LED will turn on even in Standby if the cad cell senses light – helpful if you have stray light in the burner.
17 TRIAL FOR IGNITION (TFI) is the period during which the burner is attempting to ignite the fuel it is delivering to the nozzle. A shorter trial for ignition time (safety lockout time) keeps the appliance from being flooded with fuel if it does not light.
18 VALVE-ON DELAY (PREPURGE) is the time period during which the burner motor is running before the fuel valve is opened. Valve-on delay allows the burner to establish air flow and to bring the pump to full speed, helping to keep ignitions smooth and clean.



As electronics and control technology continue to develop, our industry will continue to reap the benefits in the form of new controls and features. Controls will have more high performance features integrated into one package. They will be able to better diagnose the burner and communicate with service personnel.