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What is the Difference Between Blow Through and Draw Through?


This blog was guest written by Chris Leach, Applications Engineer.

There are two main non-recirculating direct gas-fired heater designs for commercial and industrial applications and those are blow-through and draw-through. The main difference in these two systems is the location of the blower relative to the burner. While a simple difference, blow-through units provide multiple benefits for large industrial space heating driven applications.

Blow-through, as defined by the DOE (Department of Energy) as HTHV (High Temperature Heating & Ventilation) and employed in our S-Series heaters, places the blower before the burner and benefits from a higher LAT (Leaving Air Temperature) and temperature rise (∆T). HTHV units are not limited by the fan components in the hot airstream and can safely discharge at the maximum allowable limit for CSA certified direct fired applications. Draw-through units place the blower after the burner and have a lower LAT and ∆T due to limitations of the fan components in the hot air stream.

Each of these systems has a specific purpose for large industrial spaces. For heating driven applications, maximizing the BTU to CFM ratio maximizes efficiency while satisfying space temperature. Cambridge S-Series HTHV units are best suited for heating applications because they provide the maximum amount of BTUs per CFM. For airflow (make-up air) driven applications, consistent, tempered CFM is required to satisfy process exhaust or other consistent airflow losses. Cambridge M-Series draw-through units do just this - providing the proper balance of tempered CFM to the space. 

When looking at heat driven applications, higher temperature rise and discharge temperature means less airflow required to meet a space’s heat load. Less airflow equals reduced unit size, smaller motor, and lower electrical operating costs. The specific heat capacity equation reveals how much less CFM is required to heat a theoretical building by an HTHV unit:

HTotal = 60 × Cp × ρ × CFM × ∆T 


HTotal = Total heat transferred (BTU / hr)

60 = Time conversion factor from minutes to hours

Cp = Specific heat of air handled by the blower

ρ = Density of air handled by the blower

CFM = Volumetric flow rate of the blower∆T = Discharge Temp - Outside Temp (°F) (temperature rise) 

This equation solves for the total heating capacity for a specified application. HTHV has a maximum temperature rise of 160°F while draw through only has around a 140°F. The densities between these systems are also different because HTHV unit blowers (in the cold air stream) process more dense air than draw-through unit blowers. For a hypothetical building requiring 5,000 MBH at 0°F outside air:

HTotal Blow Through = 60 × 0.24 × 0.0862 × 25,176 × (160 - 0) = 5,000 MBH

HTotal Draw Through = 60 × 0.24 × 0.0662 × 37,465 × (140 - 0) = 5,000 MBH

When appropriately sized to meet the heat load of this building, a draw-through unit requires an additional 49% more CFM to meet the heat load than a blow-through unit. More CFM equals more unit horsepower (HP), therefore a draw-through unit requires more HP to heat the same building. This is simply wasted energy and when it comes to heating large industrial spaces efficiently.

Another benefit of HTHV technology is the non-ducted, high velocity discharge. Cambridge S units discharge air at a 45° angle around 1500-2000 FPM and move a large volume of fresh, warm air throughout the building, eliminating higher ceiling temperatures and uncomfortable drafts. Utilizing this hot air at the ceiling allows HTHV to benefit from free energy that would otherwise stratify at the ceiling. Other technologies do not mix the air as effectively, further reducing their overall efficiency.

The combination of these factors is what makes HTHV the most energy efficient way to heat large industrial spaces. Cambridge helps leaders in manufacturing and warehousing create healthy working environments for their people – contact a rep to learn more.