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Direct vs indirect-fired heaters: Advantages and disadvantages

8 min read
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In this article, we will discuss the differences between direct-fired and indirect-fired heaters, and the pros and cons of each technology, to help you determine which system is ideal for your needs.

Whether you are a factory, a hospital or a construction firm, keeping employees warm is crucial to their happiness, health and productivity.

While enhancing worker safety is a heater’s primary job, these solutions can also be used to achieve optimal conditions for products in storage, dry out wet concrete and prevent equipment from malfunctioning in cold conditions, among other industrial and commercial applications.

However, delivering a consistent stream of heat is sometimes easier said than done. Depending on the size of your workspace and the location of your site, choosing a heater small enough to move, powerful enough to warm workers and versatile enough to operate without power may seem too big an ask.

Both direct and indirect-fired heaters can fulfil this responsibility. But, choosing the best type of heater for your situation depends on a variety of factors. Here we will explore the pros and cons of each unit type to make your decision that much simpler.

What is a direct-fired heater?

Direct-fired heaters work by using fuel, such as gas or oil, to heat the air. To do this, an open flame inside the unit warms incoming air before it is exhausted, delivering a vast amount of heat output quickly.

How direct fired heaters work - infographic by Master Climate Solutions

Advantages of direct-fired heating

One of the standout advantages of a direct-fired heater is that, because the air exhausted makes direct contact with the open flame, the vast majority of its fuel is converted into valuable heat energy. This makes these units an almost 100% efficient option.

Beyond that, contractors often choose direct heaters because:

  • The cost of purchase is low, due to the relatively simple construction of these units and low number of moving parts

  • This technology can deliver large amounts of warm air while still remaining a portable heating system

  • They are easy to service, often presenting owners with small maintenance costs due to their straightforward design

  • Their high efficiency means running costs are minimal, especially when compared to electric heaters

Limitations of direct-fired heating

While direct contact with an open flame ensures that fuel is used efficiently, it also means that byproducts of combustion, such as CO2 and some humidity, will be present in the heat expelled. For this reason, direct-fired heaters should ideally be used in areas with good ventilation, such as semi-open spaces or in large buildings with a proper ventilation system in place.

These units can be used in enclosed spaces, but only if there are no people or animals present, as the exhaust could put their health at risk. In addition, numerous materials may also absorb the smells or moisture exhausted by direct heaters, so this must also be considered before using these units.

Furthermore, due to the presence of a direct flame, it is not recommended to use these heaters in areas containing flammable or combustible materials.

What is an indirect-fired heater?

While direct heaters allow for direct contact between flame and air, in an indirect heater the flame is enclosed within a sealed combustion chamber. The heat inside this chamber is transferred to the air passing through via a heat exchanger, guaranteeing that the flame never touches or reacts with the exhaust air.

Infographic showing how indirect-fired heaters work by heating and cleaning cold air

Advantages of indirect-fired heaters

Although pollutants are still generated with these systems, they are separated and expelled via the chimney, in turn delivering hot, clean air. This makes indirect fired heaters perfectly suited to warming enclosed spaces with people, animals and objects, with no additional ventilation required.

However, it is important to remember that the heater itself must be installed in a place where there will be enough fresh air supply for the combustion process.

Beyond that, indirect-fired systems are the solution of choice for many commercial and industrial applications because:

  • They can be safely used in a wide variety of scenarios, both indoors and outdoors

  • Outdoor operation is associated with a decrease of heating efficiency, but much of the energy can be recovered by recirculating clean air

  • Through the use of tubes, the warm air generated by these units can be distributed to multiple spaces at once

  • The hot air produced is free from noxious fumes and potent smells

Disadvantages of indirect-fired heaters

Although the prospect of clean and fresh hot air is the main reason why many opt to utilise these temporary heaters, the efficiency of indirect solutions is not as high as direct alternatives, as the products of combustion are separated from the heated air stream.

Additionally, due to the more complex construction, the cost of ownership and maintenance is comparably higher. However, when it comes to the health and wellbeing of anyone in these spaces, this is an investment worth making.

Comparing direct and indirect-fired heaters

Although direct and indirect-fired heaters are similar in many ways, there are numerous differences between each type of unit.

Infographic showing the differences between indirect-fired heaters and direct-fired heaters

These key differences mean the use cases for each type of warm air heating system are separate.

Due to the pollutant-free warm air, an indirect-fired heater would be ideally suited for use in a storage facility full of products that are sensitive to low temperatures and foreign odours. A direct heater, on the other hand, due to its high energy efficiency and low running costs, would be perfect for protection of bottled waters from frost damage.

Which type of heater is right for my circumstances?

To further demonstrate the separate uses for both technologies, and help give you the information you need to make the right choice for your situation, consider the following examples.

Industrial halls

To keep teams and equipment at a comfortable and productive temperature in a large factory, a powerful direct-fired solution, such as a Master B 360, may be suitable. This is because large interiors often have a sufficient air exchange rate that will naturally regulate air quality, especially if windows and doors are left open.

This is relatively easy when the heater is meant to protect properties stored in a warehouse.

However, you should always put safety first and be careful to compromise when human health is at stake.

Temporary accommodation

Ensuring refugees in tents and other types of temporary accommodation are kept safe and warm, smaller indirect-fired heaters, like the BV 77, are optimal. This technology provides sufficient heating energy without exposing occupants to the risk of overheating.

Construction sites

Construction projects, on the other hand, go through multiple stages, often requiring both indirect and direct solutions throughout the project.

For operation in enclosed spaces without dedicated ventilation, you will want to use a powerful indirect-fired heater capable of warming large spaces, such as a BV 290. To keep workers warm outdoors and in shell buildings, direct-fired heaters, like the B 150, offer an ideal balance between efficiency and capability.

Find your ideal heater at Master

Factors to consider when buying direct or indirect fired heaters - Image and infographic

Choosing the right heater for your industrial or commercial application is essential. Not only can it help keep workers safe, happy and productive, but it can also be used to ensure equipment remains operational and the drying process on-site is expedited, among other uses.

To help you make such an important decision, you should consider the following factors when picking the right direct or indirect-fired heater:

Master is a well-renowned name in the world of portable climate control solutions. In the 60 years since we were established, we have developed a range of robust direct and indirect-fired heaters that put a real focus on reliability, sustainability, efficiency and usability.

B 150

One such unit is the B 150, a medium direct-fired heater capable of producing up to 44kW of heat energy, ideal for warehouses and factories.

Built to withstand harsh conditions, such as construction sites, this unit features an optional thermostat to maintain optimal temperatures as well as a wheeled trolley for easy transport.


The AIR-BUS 471 is a powerful indirect-fired heater that can produce up to 136kW of heat energy. This unit can be equipped with one, two or four ports for ducting, allowing multiple in-construction rooms or tents to be heated at once.

Due to this heater’s large size, it comes fitted with crane hooks and wheels for added mobility. Beyond the robust long-lasting components, the AIR-BUS can operate smoothly in cold environments, thanks to a dedicated ‘cold start’ button.

Product comparison of a Master Direct Fired Heater Vs a Master Indirect Fired Heater

Speak to the experts today

When it comes to finding the right direct and indirect-fired heating, there is no ‘one size fits all’ solution. The needs of your space will ultimately dictate which system is best for you. To help guide you through this process, the experts at Master can help.

Learn more about our portable heating solutions, and discover which is best suited to you by getting in touch today or filling out the contact form below.

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