As safety monitoring is the primary function of a burner management system, there are many ways that a burner trip can occur. While this is the desired response to an unsafe condition, if a burner management system with a series interlock circuit is being used, it can often be difficult to troubleshoot why the lockout is occurring. This can lead to frustration and loss of heat or the boiler plant. Luckily there are better ways of monitoring interlock circuits by using parallel annunciation of lockouts.

Troubleshooting a series limit circuit

To understand the advantage of a parallel annunciated circuit, we must first look at how series limits circuits work and how to troubleshoot them. As the name suggests, in series limits circuits all safety interlocks are wired in a single series with power being source on one end and senses on the other end by the burner management controller. If any single interlock opens, the burner management will see the loss of voltage and will trip the burner. The problem here is that the controller only knows that there was no voltage at the end of the circuit, and it has no way of knowing where the voltage was lost and which interlock opened.

The way to troubleshoot this is to use a voltmeter and find which interlock broke the circuit. If you’re lucky there will be terminals in the main control panel where each limit switch is landed. If you’re unlucky, you have to check for voltage at each individual switch on the boiler. Another issue is that in the process of shutting down the burner, a switch may open that was not the one that originally tripped the boiler. For example, the burner may have tripped on low gas pressure, but once the burner was shut down the minimum combustion air flow switch opened because air flow had stopped. Additionally, by shutting down the burner, the low gas pressure condition may go away, so in this case it would look like the gas pressure was fine and the combustion air flow switch is what caused the trip. You’re no closer to solving the problem, the boiler is still unreliable, and the customer is becoming impatient.

Parallel interlocks and annunciators to the rescue!

The solution to this is to install an interlock annunciator or a burner controller that includes parallel interlock wiring. A first-out annunciator is wired in such a way that the output of each interlock switch is sensed by the annunciator. Because of this, when an interlock switch opens, the annunciator can sense which switch or switches opened, and which opened first. This switch is then displayed for the user who now knows exactly what the problem is. A burner control system that utilizes parallel interlock circuits can detect which switch opened by default and does not need an external annunciator.

Summary and Preferred products

As we have seen, interlock annunciation and parallel interlock circuits are vital for burner troubleshooting. Preferred’s QA5004-85 and QA5004-FF are designed to work seamlessly with the M-85 flame safeguard and the FlexFit boiler control system. Additionally, the Burnermate Universal burner control system utilizes parallel interlock circuits with built-in annunciation.