July, 2008
Inspection News and Views from the American Society of Home Inspectors

Commercial Inspection Tips: Open- and Closed-Flame Heating Systems


Expand your business with commercial building information from Carson Dunlop Weldon

Welcome to Commercial Corner, an ASHI Reporter feature written by a leading commercial inspection training firm, Carson Dunlop Weldon & Associates. Each month, we feature a new article that provides useful technical and business information on commercial building inspections. Our goal is to stimulate your interest in diversifying into the field of commercial building inspections as a way to expand and grow your business.

When describing the heating systems in a commercial building, there are a number of variables to consider, including:

  • The type of fuel used.
  • The medium used for heating (air, water, steam).
  • The burner configuration — open- vs. closed-flame.
  • The presence of a heat exchanger — direct- vs. indirect-fired.
  • The type of heat transfer — convection vs. radiant.

This article will discuss the difference between open-flame and closed-flame heating systems and when one would expect to find each type.


These types of heating systems are typically natural gas, propane or oil-fired. With an open-flame system, you can see the flame. The air inside the building can come in contact with the flame. It has to as the building provides the combustion air.

A closed flame system, on the other hand, does not allow the air inside the building to interact with the flame. Closed flame heating systems typically use outside air for combustion.


Photo: Closed-flame unit heater

Open-flame systems are by far the most common. These would include most types of interior furnaces, boilers or unit heaters. Closed-flame systems would include certain types of high-efficiency furnaces, boilers, unit heaters, radiant heaters and all rooftop units.

Open-flame systems are typically used, as they are less expensive. The circumstances where closed-flamed systems would be required involve buildings that contain explosive dust or vapors. For example, some types of woodworking or wood manufacturing plants would require closed-flame systems because of the amount of sawdust generated inside. Another example would be a system near spray-paint booths.


Photo: Open-flame unit heater.

Another way to look at it is that closed-flame heating systems are more energy-efficient, as they use outdoor air for combustion. While this is true, it is typically not the reason for this type of equipment in the building.

This article and accompanying diagrams have been taken from the yet-to-be-released, updated Commercial Building Inspection textbook written by Carson Dunlop Weldon & Associates Ltd.

Visit www.CDWengineering.com.