Generally, it is understood that direct fired makeup air units (DFMAH) must be interlocked with an exhaust fan. But, ask around and you’ll get varying responses on the amount of exhaust required relative to the makeup-air supply. Recently, I went back to CSA Standard B149.1, the Gas Code, to reaffirm these requirements for a kitchen environment. The code is really quite clear, so, not sure where the variance in the industry has come from?
There are two situations that are outlined in the Gas Code for applying DFMAH in kitchens. The criteria for exhaust interlock is determined based off how the the kitchen is interconnected to the dining area. Where there is a normally closed door, or an opening not exceeding 16-ft2 (1.5-m2), the exhaust capacity must be minimum 90% of the make-up air supply. Otherwise, the exhaust system shall be 100 to 105% of the makeup-air supply. I’m sure this is going to catch a number of engineers off guard! I’ll summarize using an example.
| Space Interconnection Criteria |
MUA (cfm) |
EF (cfm) |
| Closed door, transfer <16-ft2 |
1,000 |
900+ |
| Kitchen “open” to dining area |
1,000 |
1,000-1,050 |
Applying a DFMAH in a kitchen is very attractive as it is an economical an efficient option for the owner. By ensuring we understand the code and apply it properly, we can also maintain a safe and healthy environment.
Oh yeah! Cardinal rule – don’t apply a DFMAH to anywhere there are sleeping quarters. I’ve seen this mistake too many times.
