Here’s your Friendly Friday Fail, wonderful restaurant with an amazing menu.  However as always we are on duty so lets start from the top!

While we were waiting to be seated I noticed the grease filter missing on the very right side corner of the hood.  I also noticed every time someone entered the restaurant there was a cold draft from outside.

Conclusion:  the make-up air unit is not operating correctly.  The reason why is the kitchen exhaust fan connected to the hood is exhausting easily up to 3,000 – 5,000 cubic feet per minute.  If this air is not replaced with a make-up air from outside you will have cold drafts and poor kitchen exhaust.

To put it into context take a standard condo down town at 600 square feet by 9 foot tall ceiling.  Total volume of the condo is thus 600 x 9 = 5,400 cubic feet.

Lets pick the smaller of the kitchen exhaust fans at 3000 cubic feet per minute.  That is every 60 seconds we are removing 3,000 cubic feet of air.  So if we installed this fan in your condo, closed all the windows and doors and turn on the fan we would create a vacuum in 5,400/3,000 = 1:48 minutes!

Now the reason this doesn’t happen is you have leakages of air into the building that provide ‘relief’ air.  Further as the negative pressure builds up into the space the performance of the kitchen exhaust fan drops down exponentially… which brings me to my observation of the grease filter missing:

The cook thinks the kitchen exhaust fan is not working hard enough. So he removes the filter to allow more exhaust through.  However the problem is the ‘condo’ situation I described above: no make-up air.  We are trying to exhaust a space with no make-up air thus the cold drafts when the door opens!

Second issue here is the hood to the far right is to shallow. The Rational Oven ( doesn’t fit under the outer front face of the hood.  Every time the door is opened steam and heat will escape and pour out into the space not being captured by the hood.

As per ASHRAE std. 154 Table 3 Class II hoods are to maintain 12” over hang in front of any cooking appliances.  Ok ok so you say well James this is Class I cooking so it doesn’t apply right?  Wrong as per section 4.2.3 Class I hoods are to be installed with same overhangs as Class II.

Third and Fourth issue is the proximity of the kitchen exhaust fan to an operable window and to wooden power poll. As per NFPA-96 operable windows and air intakes need to be over 32’-6” away if directly above.  The reason is the same as described above: in the event of a fire, smoke and flames will be exhausted out.  In this case it appears to be less than 10’-0” away and if open during a fire would be very dangerous.  Further the proximity of the kitchen exhaust fans to the wooden power poll.  In the event of a fire the kitchen exhaust fan turns on to full speed to remove the smoke and flames.  In this case if there is a significant amount of flames that continues for a long period of time this power poll could catch fire.  To add fuel to the fire (pun intended) what if the power poll is supplying electricity to the restaurant.  What if it caught fire and burns down?  There will be no power to the kitchen exhaust fan and thus smoke and flames would build up within the space and building. 


7.8.3 Wall Terminations. Wall terminations shall be arranged with or provided with the following properties:

(1) The termination shall be through a noncombustible wall with a minimum of 3 m (10 ft) of clearance from the outlet to adjacent buildings, property lines, grade level, combustible construction, electrical equipment or lines, and with the closest point of any air intake or operable door or window at or below the plane of the exhaust termination.

(2) The closest point of any air intake or operable door or window above the plane of the exhaust termination shall be a minimum of 3 m (10 ft) in distance, plus 76 mm (3 in.) for each 1 degree from horizontal, the angle of degree being measured from the center of the exhaust termination to the center of the air intake or operable door or window, as indicated in Figure 7.8.3.

5. Lastly the kitchen exhaust fans were installed in the incorrect orientation as the grease drains are on the sides.  They need to be facing downward with the flow of gravity and a grease cup installed.

NFPA-96: Upblast fans shall have a drain directed to a readily accessible and visible grease receptacle not to exceed 3.8 L (1 gal).

James Hicks P.Eng

Evolve Mechanical Solutions


#Restaurant #HVAC #Fail