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Millis, MA, USA (800) 553-3554

Open Flame vs. Concealed Flame Direct Fired Heaters

Written by Babfar on . Posted in News

When choosing heating systems for a construction site or any other commercial project, there are several variables to prioritize. One such variable is the heating configuration. The three most common options are direct fired heaters with an open flame, direct fired heaters with a concealed flame, and indirect fired heaters. Understanding how each work and the differences between these options is key to selecting the ideal temporary heating rental for your next project.

What is a Direct Fired Heater?

A direct fired heater uses natural gas or propane and typically features an open flame design. Fuel is fed directly into the burner, where it mixes with air forced through the burner baffle to generate heat.

While open flame heaters are the most common, a direct fired heater can also feature a closed or concealed flame. Each option has pros and cons, as discussed later in this article.

What is an Open Flame Heater?

An open flame heater is a direct fired heating system in which all intake air is forced through the burner for heating purposes. That means air must pass through the combustion chamber. Due to their straightforward heating process, open flame heaters typically feature a simpler construction. An open flame heater often resembles a cylinder turned on its side with a flame inside and a fan pushing the heat out.


  • Energy-efficient, since all combusted fuel is captured as heat
  • Seamless installation process
  • Requires minimal maintenance
  • Price-friendly
  • Produces high heat


  • Not powerful enough for duct so need to be placed inside the enclosure.
  • The open flame design makes this type of heater a potential fire hazard.
  • Flames often extend beyond the burn chamber.
  • They require proper ventilation to prevent carbon monoxide buildup as well as trap harmful particulates.
  • Combustion process and lack of ventilation builds up moisture inside the enclosure and walls.
  • Inside placement creates negative pressure pulling cold air through exterior walls.

What is a Concealed Flame Heater?

At its core, a concealed flame heater is like an open flame heater enclosed in a large, durable shell. Combustion air is drawn from outside and forced through a burner fixed on the exterior end of the heater. Hot air is then pulled into the heater, where it mixes with air from the burn chamber and is discharged through a spiral duct.


  • Since air is passed directly through the burner for combustion, concealed flame heaters are also energy efficient because all the fuel is converted into heat.
  • Concealed flame heaters promote heating equipment safety, since the flame is enclosed.
  • Because they use exterior air for combustion and blow it into buildings under high pressure, concealed gas heater equipment distributes heat efficiently and keeps airborne pathogens at bay.
  • Outside placement and pressurization allows for the heater to heat and ventilate the enclosure.


  • More working parts make installation, uninstallation, and maintenance of concealed flame heaters cumbersome.

Differences Between Open Flame and Concealed Flame Direct Fired Heaters

Common differences between open and concealed flame gas heater equipment lie in the following areas:

Construction and Maintenance

Due to their straightforward heating process, open flame construction equipment rentals feature simpler construction. The main parts usually include a heat exchanger, burner, exhaust, and openings for combustion air. An open flame heater’s fewer working parts mean easier maintenance.

Concealed industrial air movers are more intricate and often feature:

  • Heater enclosure
  • Burner on the end of the heater where air combustion happens
  • Firetube
  • Blower
  • Ductwork

Installation and Pressurization

Because of their open flame design, open flame heaters aren’t compatible with ductwork. That means they are often installed inside buildings and require proper ventilation, because they tend to create negative pressurization and trap potentially harmful particulates in the atmosphere.

Concealed flame heaters, on the other hand, feature powerful blowers and can be used with ductwork. As a result, they are typically installed outside and connected to commercial buildings via ridged spiral ducts. Unlike their open flame variants, concealed flame temporary heaters create positive pressurization, meaning that heat is distributed efficiently and airborne particulates are kept at bay.

BABFAR Has All Your Heating Needs

Whether interested in a direct fired heater with an open flame or a concealed flame design, BABFAR can help. Our selection of high-quality direct fired temporary heaters for MA construction sites will keep you and your team safe and comfortable all through your project.

Contact the heating equipment experts to request a quote or schedule an appointment today.


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