What is the Difference Between Blow-Through and Draw-Through Heaters?
When you are designing or upgrading heat for a large industrial or commercial building, the type of direct gas fired heater you choose has a big impact on comfort and operating costs. Two of the most common non-recirculating designs are blow-through and draw-through units. At first glance, the difference can seem minor, the blower simply sits on one side of the burner or the other. That simple change affects temperature rise, airflow, horsepower, and energy use across the entire system.
In this helpful guide, the industrial heating and cooling experts at Cambridge Air Solutions will break down how each heating system works, the best applications for each, and how Cambridge’s own
S-Series and
M-Series are the perfect solution for a wide variety of heating needs.
What is Blow-Through Heating?
In a blow-through HTHV system:
- The blower pulls in cold, dense outdoor air.
- Air passes through the burner after it leaves the blower.
- The unit can safely reach the maximum allowable leaving air temperature for CSA-certified direct-fired equipment because the fan components are not exposed to the hottest part of the airstream.
- As a result, blow-through units deliver a higher leaving air temperature (LAT) and a higher temperature rise (ΔT).
Because the air entering the blower is colder and denser, the blower moves a mass of air that carries more heat for a given volumetric flow rate.
What is Draw-Through Heating?
In a draw-through system:
- Air first passes through the burner.
- The blower sits in the hot airstream and pulls the heated air through.
- To protect the fan components, the leaving air temperature and temperature rise are lower than in a blow-through design.
These units are designed to deliver consistent, tempered airflow for processes that need reliable
make-up air rather than maximum BTUs per CFM.
Which Heater Design Works Best?
Blow-through and draw-through systems are both direct gas-fired, non-recirculating heaters, but they are optimized for very different jobs inside the building. The easiest way to separate them is to decide whther your project is primarily heating-driven or airflow-driven.
Heating Driven Building Applications
Heating-driven applications are projects where the number one goal is to raise and maintain the temperature of the occupied space. The system is sized and selected around BTUs and comfort first, and air changes or make-up air are secondary.
Common examples include:
- Large warehouses and distribution centers that must keep employees comfortable in Winter
- Manufacturing facilities with high ceilings and open floor space
- Maintenance shops and service bays
- Logistics and cross-dock facilities with frequent door openings
In these spaces:
- The building often has high stratification, with hot air collecting at the ceiling and cooling air at the floor.
- The priority is to deliver as much usable heat as possible per CFM, so you get more heating effect from a smaller volume of air.
- Reducing fan horsepower and electrical operating costs is very important because the equipment will run many hours per year.
Because blow-through HTHV units can operate at a a higher temperature rise and process denser inlet air, they deliver more BTUs for each cubic foot of air they move. Cambridge S-Series HTHV blow-through heaters are designed for the heating-driven projects. They:
- Provide a higher leaving air temperature and temperature rise
- Reduce the total CFM required to meet the heating load
- Use high velocity discharge and a carefully designed pattern to push warm air down into the occupied zone and mix stratification heat
For a facility manager, that translates into fewer units, smaller fans, lower electrical usage, and more even comfort throughout the building.
Airflow-Driven Building Applications
Airflow-driven applications are projects where the system must move a specific volume of air to support a process, code requirement, or air quality target, and the heating function is important but secondary. The system is sized and selected around CFM and ventilation first, and BTUs follow.
Common examples include:
- Facilities with significant process exhaust, such as welding operations, metal fabrication, or industrial processes that remove large volumes of air
- Paint booths and finishing areas that require controlled, consistent airflow patterns
- Industrial kitchens and food production spaces with high exhaust rates
- Building where codes or standards require a certain number of fresh air changes per hour
In these spaces:
- The system must reliably replace exhaust air to maintain building pressure and protect indoor air quality.
- A consistent supply airflow rate and discharge temperature matter more than achieving the absolute maximum temperature rise.
- Process stability, fume control, and occupant health are key outcomes.
Cambridge M-Series draw-through make-up air units are built for airflow-driven applications. The M-Series:
- Deliver a controlled CFM of one hundred percent outdoor air into the space
- Provide a tempered discharge temperature that supports comfort without overheating the process area
- Help maintain neutral or slightly positive building pressure relative to the outdoors
For a facility that depends on exhaust for safety or product quality, an M-Series draw-through unit ensures that every cubic foot of exhausted air is replaced with tempered, clean outdoor air on a predictable basis.
How to Calculate Building Heating Capacity?
In order to help choose between a draw-through or blow-through heating system, it's important to first know your building's heating capacity. To calculate the heating capacity of a given building, use: HTotal =60×Cp ×ρ×CFM×ΔT
Where:
- HTotal = total heating transferred (BTU per hour)
- 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
- ΔT = discharge temperature minus outside temperature (temperature rise in °F)
For a given heating requirement (HTotal), you can meet the load with different combinations of CFM, air density, and temperature rise.
HTHV has a maximum temperature rise of 160°F, while draw-through only has around 140°F temperature rise. The densities between these systems are alos different becuase HTHV unit blowers (in the cold air stream) process more dense air than draw-through unit blowers.
A Practical Example: Heating Capacity in Action
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 when it comes to heating large industrial spaces efficiently.
Another benefit of HTHV technology is the non-ducted, high velocity discharge. Cambridge S-Series discharge air at a 45°F angle around 1500-2000 FPM and move a large volume of fresh, warm air throughout the building, eliminating higher ceiling temperatures and uncomfortable drafts.
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 (ΔT). 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 ΔT 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 × ? × CFM × ?T
where
HTotal = Total heat transferred (BTU / hr)
60 = Time conversion factor from minutes to hours
Cp = Specific heat of air handled by the blower
? = 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.