Last Updated on January 14, 2026
Keeping workers safe and warm is a top priority for business owners and contractors during chilly days and nights. Cold weather can negatively impact worker productivity and damage sensitive materials. A simple solution is to rent a temporary heater. Direct fired heaters and indirect fired heaters are two of the most common worksite heaters used across industries, from construction to manufacturing.
But how do you know which is right for your project? In this article, you’ll learn the difference between direct fired heaters and indirect fired heaters.
Why You Need the Proper Worksite Heater
Temporary heaters are a necessity in cold climates. Employers are required to protect workers from cold stress hazards, according to OSHA. Installing temporary heating can be a game changer in preventing the effects of cold stress on workers and freezing conditions on sensitive materials, like paint and concrete.
However, not every heater is right for every site. Workers can be seriously injured from carbon monoxide poisoning. This usually occurs while using fuel-burning equipment (such as the wrong heater) in buildings or semi-enclosed spaces without adequate ventilation.
To combat the cold and protect workers from cold stress and carbon monoxide poisoning, you need to follow all safety standards for temporary heaters. You also need to understand the pros and cons of two commonly used mobile heaters: direct fired and indirect fired heaters.
What Are Indirect and Direct Fired Heaters?
Both direct fired heaters and indirect fired heaters warm the air by burning fuel, like diesel, propane or natural gas. The main differences are how cold air moves through the heater and what happens with the exhaust.
- Direct Fired Heaters: Direct fired heaters, also known as direct gas fired heaters, are similar to gas grills, where the flame comes into direct contact with the air. Cold air is drawn into the heater and across the flame, which warms the air as it leaves the heater.
- Indirect Fired Heaters: Indirect fired heaters, sometimes called “torpedo heaters,” operate like a mobile furnace and are more complex. Powered by oil, diesel, propane, natural gas or dual fuel, these heaters — which are usually set up outdoors — draw fresh, outside air through intake vents. The air then passes over the burn chamber where it’s heated and forced by blowers into a building through ducting. The heater’s unwanted fumes are expelled through a “chimney.”
Pros and Cons of Direct Fired Heaters vs. Indirect Fired Heaters
Most business owners and contractors will use one of two types of heaters – direct or indirect fired heaters. While both types of heaters do the same thing ― heat the air and keep work stations comfortable ― each have their own benefits and work better in different situations.
Pros of Direct Fired Heaters
- High Portability: Direct fired heaters feature a smaller footprint than indirect fired heaters and are easy to move, set up and adjust. This makes them ideal for teams that move from site to site.
- Heat Large, Open Spaces Quickly: These heaters are designed to heat large, open spaces varying in size from 2,875 square feet up to 19,000 square feet. This makes them great for outdoor construction projects, agriculture and outdoor events.
- Efficient Heating: Direct fired heaters operate much more efficiently than indirect fired heaters with nearly 100% of the fuel being used to convert heat.This efficiency can help lower fuel costs.
- Multiple Fuel Options and Wide BTU Range: Depending on the model, you can find a direct fired heater that uses natural gas, kerosene or propane to generate a wide range of heat. Small units may put out 115,000 BTU or less. Industrial models can have an output of more than 1.5 million BTU.
Cons of Direct Fired Heaters
- Ventilation and Air Quality Concerns: The open flame and blower design of direct fired heaters mean they can’t be used with ductwork. The warm air and exhaust are one in the same. To avoid carbon monoxide poisoning, these heaters must be set up and used in open air locations or buildings with large, well-ventilated open spaces. For example, outdoor construction sites, outdoor sports arenas or car dealership service bays fit the bill.
- Can Create Moist Air: Water and moisture are byproducts of direct fired heating. This means that the warm air they produce can have more moisture than the warm air from an indirect fired heater. This can be an important consideration if you have products or materials that require dry air.
Pros of Indirect Fired Heaters
- Creates Clean, Dry Air: The heating process of an indirect fired heater greatly reduces safety concerns related to carbon monoxide poisoning. They maintain optimal temperatures without adding moisture or exposing the heated air to combustion gases or other contaminants.
- Can Be Used Indoors and with Sensitive Materials: Indirect fired heaters are often used for warming offices, manufacturing facilities and other facilities because no combustion byproducts are introduced into the airstream. The clean, dry air produced by an indirect fired heater is great for processing, drying or maintaining sensitive materials, such as food products or concrete that is curing.
- Ductwork and Recirculating Air: Because the heated air never comes into direct contact with the flame, air can be recirculated. You can also use ductwork to make sure all parts of your facility stay warm.
- Multiple Fuel Options and Wide BTU Range: Indirect fired heaters can run on diesel, propane, natural gas or oil. Some units can accommodate both propane or gas. They also generate a wide range of heat with some models putting out 2 million BTU.
Cons of Indirect Fired Heaters
- Less Efficient Heating: Because the air doesn’t touch the flame, indirect fired heaters are less efficient than direct fired models. You can expect an estimated loss of fuel efficiency when compared to direct fired heaters.
- Larger Footprint That Requires Indoor and Outdoor Space: Indirect fired heaters are often larger and less portable than their direct fired counterparts. They must be positioned outdoors to exhaust unwanted fumes through the chimney. Then you use ducting attachments to push the warm air indoors.
- Can Be More Expensive: Since indirect fired heaters are more complicated machines than direct fired heaters, they are often more expensive to rent or buy. A reliable equipment rental provider can help mitigate this by helping you find the ideal model and rental terms for your project.
How to Choose Between Types of Heaters
There are a lot of advantages to using either a direct fired or indirect fired heater, and it can be a challenge to figure out which one is best for your project. Here are the top three ways to help you decide.
1. Safety Always Comes First: Analyze Ventilation Needs
Small, enclosed spaces without means to expel potentially deadly fumes are no place for a direct fired heater. In fact, some sites may prohibit them.
Indirect fired heaters are designed to keep carbon monoxide, carbon dioxide and nitrogen dioxide outside a space. They do so by redirecting toxic fumes through a chimney or stack. That makes them safe to use indoors and in tight spaces, as long as you have an exhaust outlet.
2. Consider How the Site Is Used
If your employees are working in a large or well-ventilated space, the simplicity and cost-savings in terms of fuel efficiency from direct fired heaters will be substantial.
However, excess humidity coming from direct fired heaters can create drying and curing problems for sensitive materials. In extreme cases, it could lead to mold development. In these cases, indirect heaters may be a better choice. They may also help speed up some construction tasks like the curing of concrete.
3. Consult an Equipment Expert
Unsure which model is best for your site or project? Enlist a reliable equipment rental provider. A trusted provider can help you maximize safety and efficiency with the right technology, support services and rental terms.
- Still can’t decide? Learn more by reading How to Find the Right Temporary Heater Rental to Keep Your Worksite Warm.
Stay Warm This Winter with Worksite Heaters from Herc Rentals
Safe, comfortable working conditions are paramount no matter what time of year. But the winter months create a different set of challenges for business owners, contractors and workers in the field. When temperatures drop, you can find high quality direct fired heaters and indirect fired heaters at Herc Rentals.
Explore Herc Rentals’ selection of climate control solutions, or call 1-800-432-6215 to speak with an expert today.
