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Portable Air Conditioners • Industrial Cooling Systems • Dehumidifiers • Heaters • Generators

Heaters 101: Make-up Air vs. Recirculating Heating

25 January 2021       Bookmark and Share


In our last installment, we went over how some of our most popular types of heaters work, and how their characteristics make them appropriate for one application or another. These distinctions are important for understanding the value of heaters and the role they play in numerous applications, but there is another important concept that’s worth learning about heat solutions. Today, we’ll go over the difference between recirculation and make-up air heating, and why make-up air is usually a superior type of temporary heating solution.


To understand make-up air, let’s first consider how air moves around and through a building. If you picture any building, such as a store or an office, the air inside the space is separated from the outside by the building’s envelope. This envelope consists of all the materials that create this separation, such as exterior walls, windows, and doors. However, envelopes usually aren’t airtight, meaning there are cracks, crevices, and openings where outside air can enter the building.


This infiltration has its advantages and disadvantages, but it’s important to note that the greatest part of a typical building’s heating load goes towards heating air that has infiltrated the inside space. And when it comes to temporary heating solutions, natural infiltration of air also creates problems in the distribution of heat throughout a space. To prevent any number of issues that are essentially caused by openings in a building envelope and other unfavorable conditions, make-up air systems are used to better distribute heat and to “make up” for any air that’s leaving a building. These systems come in many different configurations, but today we’ll focus on their use in temporary heating solutions.


In a typical recirculating heat solution, a direct-fired heater is staged inside a building, where it recirculates heated air around the enclosed space. However, since there isn’t any air being mechanically ventilated inside, outside air naturally infiltrates the building. In environments where cold air infiltrates the building envelope, this cold air acts like a barrier around the inside perimeter of the space, confining the heated air to a “bubble” that centers around the heater. This is problematic because the cold air barrier hinders the even distribution of heat throughout the space. Additionally, the heater creates moisture as a combustion byproduct, which causes undesirable condensation on the envelope and wherever the warm, humid air meets the cold air. If the conditions caused by recirculating heat solutions must be avoided for a given application, Mobile Air & Power Rentals offers a valuable alternative: make-up air solutions.


In a make-up air solution, heaters are installed so that they draw 100% outside air across their burners. Most importantly, the solution is also designed with enough inward airflow to create positive pressure inside the building, meaning that the pressure inside is higher than the pressure outside. Therefore, most air going through the cracks and crevices of a building envelope is leaving the building, not entering it. The high airflow and positive air pressure created by this type of solution ensures that heat is evenly distributed throughout the space, and that any excess moisture is being effectively ventilated out of the building. Ultimately, make-up air is the best way to utilize the heat provided by equipment and avoid condensation.


Now you know how heaters work, as well as the advantages offered by make-up air solutions. In general, buildings that have tight envelopes benefit the most from make-up air solutions. With this knowledge, it’s time to take a specific look into the rental side of heating solutions, starting with why you would rent a heater from a solutions provider such as Mobile Air & Power Rentals.

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