Retrotec Multi-Fan Blower Door - Manual (Page 5)

Page 5 of 37
©Retrotec Inc. 2015
1.2 Calculate number of fans needed based on leakage
requirement
Simply knowing the volume of the building is not enough information to estimate how many fans are
needed, because for the same volume, an extremely leaky enclosure will require more fans to pressurize
the enclosure than a very tight enclosure.
To more accurately estimate how many fans you need, you can perform a calculation based on
enclosure area. This calculation can depend on the allowable leakage specified in the standard you are
trying to comply with (e.g., ATTMA TSL1, ASTM E779 – 10, CGSB, USACE Protocol) or another leakage
requirement that you are specifically trying to achieve (e.g., Air leakage, Permeability, Air Change rate).
For typical leakage requirements around the world, refer to Table 4 in the Appendix (page 35).
To estimate how many fans you need, here’s an example of the calculation to perform:
Let’s assume you are testing a 70,000 sq ft enclosure area and the Standard you are trying to comply
with requires that you can only allow a maximum permeability of 0.25 CFM/sq ft @ 75 Pa (compliance
requirement for USACE).
𝐸𝑛𝑐𝑙𝑜𝑠𝑢𝑟𝑒 𝐴𝑟𝑒𝑎 = 70,000 𝑠𝑞 𝑓𝑡
𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡 = 0.25
𝐶𝐹𝑀
𝑠𝑞 𝑓𝑡
Multiplying this permeability requirement by the enclosure area, you can estimate that you would need
a total flow rate of 17,500 CFM to reach 75 Pa.
𝐹𝑙𝑜𝑤 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 𝑃𝑒𝑟𝑚𝑒𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑚𝑒𝑛𝑡 𝑥 𝑒𝑛𝑐𝑙𝑜𝑠𝑢𝑟𝑒 𝑎𝑟𝑒𝑎
𝐹𝑙𝑜𝑤 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 0.25
𝐶𝐹𝑀
𝑠𝑞 𝑓𝑡
𝑥 70,000𝑠𝑞 𝑓𝑡
𝐹𝑙𝑜𝑤 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 = 17,500 𝐶𝐹𝑀
If you are using Retrotec’s high output fans, they generate a maximum flow rate of approximately 7,000
CFM at 75 Pa (see Figure 1 for maximum flows of Retrotec fans), so you can divide the Flow required by
the Flow capacity of the fan at 75 Pa to determine how many fans you need.
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑎𝑛𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 (𝑎𝑡 75 𝑃𝑎) =
𝐹𝑙𝑜𝑤 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
𝐹𝑙𝑜𝑤 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝑓𝑎𝑛 (𝑎𝑡 75 𝑃𝑎)
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑎𝑛𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 (𝑎𝑡 75 𝑃𝑎) =
17,500
7,000
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑎𝑛𝑠 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 (𝑎𝑡 75 𝑃𝑎) = 2.5 = 3 𝑓𝑎𝑛𝑠
In this example, the quotient is 2.5, meaning you would need at least 3 fans. (If the quotient is not a
whole number, you would round it up to a whole number). In the event that you run a test using 3 fans
and find that you cannot achieve a pressure of 75 Pa, you know that there is too much leakage in the
building to comply with the permeability requirement for the standard (0.25 CFM/sq ft @ 75 Pa); hence,
seal up some leakages and try the test again.
This example makes the assumption that the permeability requirement of the building is 0.25 CFM/sq ft
to comply with USACE. If you are testing to a different standard, you are to use the requirement from
the correct standard.
Keep in mind that the size of the building cannot tell you in advance what the permeability will be. If
you need to determine what the permeability is, and are not doing sealing to make the enclosure