December 2007
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Innovations in Comfort, Efficiency, and Safety Solutions.

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Wiring For Combustion Air

Prove the presence of the combustion air source before allowing the boiler plant to operate

Steven R. Calabrese

Steven R. Calabrese
Automated Logic Chicago

Read Steve's previous AutomatedBuildings columns:
November - Terms & Terminologies – Avoiding Confusion 
October - Electric Duct Heaters – Application & Control 
September - Pressure Transmitters – Selection & Placement
August - Rooftop Unit Economizer – Operation & Control 
July - RTU Operation Via Conventional & Digital Controls
June - Interlocking of AHU Safety Devices

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The requirement of combustion air for indoor gas-fired heating appliances such as boilers and furnaces is one that is code-mandated, but more importantly, one that is mandated by the operational nature of the gas-fired equipment. For combustion to take place, there must be three components present. The first two, fuel and ignition, are provided via the equipment. The third, which is oxygen, must be introduced into the space housing the equipment at a rate dictated by the size and capacity of the gas-fired equipment. An insufficient amount of combustion air will result in reduced combustion, in which the flame is actually choked down due to the lack of oxygen. The resulting situation is undesirable, for two reasons: reduced heating capacity, and perhaps more importantly, compromised operational safety!

To meet the requirements for combustion air in a typical boiler room, a hole (or two) will be cut through the outside wall of the room, and terminated with a louver and normally closed motorized damper. Operation is preferably such that the damper is closed when there is no equipment being fired, commanded open when there is a call for boiler operation, and proven open before actually allowing boiler operation.

Another means of handling combustion air requirements, one that’s normally reserved for the larger scale boiler plants, is to have a “forced air” ventilation system. This is typically in the form of a 100 percent outside air unit (make up air unit). Operation of the unit, again preferably, is such that the unit is off when no equipment is being fired, commanded to run upon a call for boiler operation, and proven to be running before combustion is allowed to take place.

With either method of providing combustion air, we see that the “preference” is to confirm combustion air before allowing boiler operation. With the simple combustion air damper, this is done via an end switch on the damper actuator, set to close when the damper has stroked substantially open. With the forced air system, this will typically be done by confirming airflow, via an air proving switch, or by confirming fan operation, via a current sensing switch.

When a mechanical system, be it hot water or steam, is served by a single boiler, interlocking the boiler and combustion air is a relatively straightforward task. The trick is to intercept the boiler’s operating control, and use it as a means to energize the combustion air damper (or the make up air unit). Wait a minute, we’re not done yet. We then need to get the “means of proof” into the boiler’s limit circuit, after the operating control. Now, when the operating control calls for boiler operation, the combustion air system is placed into operation, but the boiler doesn’t fire right away, because its limit circuit is not quite made. Once the operation of the combustion air system is confirmed, the proving switch makes, and the boiler’s limit circuit is completed. And away we go!

If, for whatever reason, the means of providing combustion air fails, the boiler is prohibited from operating. This is normally considered “the lesser of two evils”. In other words, a better scenario than allowing the boiler to fire without sufficient combustion air. At least it will prompt a maintenance inspection or a service call, and the source of the failure will be discovered and rectified.

When a mechanical system is served by more than one boiler, and there is only a single source of combustion air common to the boilers, interlocking boiler and combustion air is not so straightforward. To demonstrate the relative complexity of this scenario, consider a two-boiler system, with each boiler operating via its own operating control. The preference, remember, is to prove combustion air before allowing combustion to take place.

Reliable Controls First of all, we need to determine whether either boiler is calling for operation. Therefore, we need to monitor the status of each boiler’s operating control. This can be done with a relay at each boiler. The coil of the relay is energized when its respective boiler is calling for operation. The normally open contacts of each relay can be wired in parallel, and the combustion air source can be called for by this parallel connection of contacts. In other words, if either contact closes, the combustion air system will be placed into operation. Now, when combustion air is proven, the “proof” must be in the form of not one but two switches, with each switch wired into each boiler’s limit circuit. Recapping, on a call for boiler operation by either boiler, a field mounted relay local to the calling boiler is energized. The contacts close, thus pressing the combustion air system into operation. Once combustion air is proven, two switches or contacts are made, one in each boiler’s limit circuit. At this point, both boilers are enabled for operation. The boiler whose operating control is calling proceeds through its ignition process, and fires.

So you can see how you need to perform some logical gymnastics with relays and switches, in order to prove combustion air before allowing boiler operation, when two (or more) boilers serve the same system and are not centrally controlled. What is the scenario when they are controlled by a central boiler sequencer (i.e., digital controller)? The logic would become much easier, if combustion air control could be incorporated into the sequencer. Upon a drop in temperature (or pressure) below setpoint, the sequencer will request the operation of the lead boiler. But the sequencer can also send out a request for combustion air (via a binary output), and wait for confirmation (via an input), before allowing lead boiler operation.

Though on paper this looks to be an attractive and relatively straightforward approach to combustion air control, in practice it leaves a bit to be desired, and (in the opinion of this writer) shouldn’t be considered as an option. The concern is what takes place if and when the sequencer fails or is taken out of service for some reason. Be that the case, the boilers will need to temporarily be controlled by their own operating controls, at least until the sequencer is pressed back into operation. What about combustion air during this period? If the sequencer has command over the operation and confirmation of the combustion air system, then what happens when the sequencer is taken out of service? At the very least, combustion air won’t be initiated automatically. It can be implemented manually, by a maintenance attendant throwing a switch to get the damper open or to get the air handler running. However, the maintenance personnel must be made aware of this manual procedure, in order for the boilers to get the required combustion air. In the case of the simple damper, this raises concerns about the damper being open when neither boiler is firing, and cold outside air migrating into the boiler room for no reason.

Combustion air is a matter of operational safety, and the implementation of it should be integrated into the boilers’ control systems, and not solely by any higher tier system that can be taken out of service and yet still allow the boilers to operate. Furthermore, confirmation of combustion air should be wired into the boilers’ limit circuits, and not back to a sequencer whose role in the operation of the boilers is subject to manual intervention.

Combustion air, whether provided simply by a motorized damper or by a forced air fan system, is a requirement in virtually all gas-fired heating plant applications. The interlock between heating plant and combustion air system is an important one, and one that must be addressed on a job-to-job basis. While it’s a simpler task to fire the boiler and simultaneously send out a request for combustion air, most designers and engineers prefer that proof of combustion air be received before allowing heating plant operation.

Tip of the Month: When controlling a single combustion air damper that serves two (or more) boilers, fabricate a “relay bank” panel using a 12” x 12” enclosure and three 120-volt relays. Install the panel between the boilers or in an otherwise suitable location. Install conduit between the panel and the boilers’ burner controls compartments, and from the panel to the C.A.D. actuator. Label the relays inside the panel as B-1 CALL FOR C.A., B-2 CALL FOR C.A., AND C.A.D. OPEN. Wire the first two relays such that they energize upon a call for boiler operation. Wire the normally open contacts of these two relays in parallel, and deliver 120 volts through the parallel circuit and out to the 120-volt damper actuator. Wire the damper actuator end switch back into the panel, such that when the switch closes, the third relay energizes. Wire each normally open contact of this DPDT relay back to either boiler, into each boiler’s respective limit circuit. Keep everything line voltage (120 volts), so as to eliminate any need for control transformers. Most mechanical rooms require most everything to be in conduit anyway, so you don’t gain any advantage by running low voltage wiring.






 The relay panel serves as a central “junction point” for the interlock of the combustion air system to the boilers. Provide relays with built-in LEDs (indicator lights), and you can know at a glance if either boiler is calling for operation, and if the combustion air damper is open or closed. A little more field work perhaps than the alternative method of installing the relays within the burner controls compartments, but from a maintenance and operation standpoint, you can’t beat it!

About the Author

Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS).  You can visit his website at

In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.


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