Capturing Cooking Effluent with the Exhaust Hood

Q: I’m having issues with my exhaust hood not catching all the discharge from my cooking equipment in the restaurant I just took over from a failed operator. Any ideas?

Exhaust Hood End Panels

Illustration of partial (top drawing) and full (bottom drawing) side panels. Used with permission. From Design Guide 1: Improving Commercial Kitchen Ventilation System Performance, Selecting & Sizing Exhaust Hoods

A: Issues involving exhaust hoods can be complicated and expensive. Start with the easy things first:

Check the fan to be sure it is operating. If the fan has a belt, also make sure the belt is in place.

Make sure the filters are clean and in place.

Ensure you have the proper overhang on the hood beyond the cooking equipment. The current code states that the hood must overhang the cooking surface a minimum of 6 inches.

Make sure there are no cross drafts from fans used to cool the staff or from diffusers in the ceiling for heating/air conditioning supplies. Cross drafts from diffusers, even those located many feet away from the hood, can have adverse effects on capture.

Experiment with side (or end) panels. Old cardboard boxes make great temporary trial capture improvers. Start by placing a sheet of cardboard at each end of the hood forming a partial end panel on the hood. Blocking off just 25-50% of the ends of the hood can make dramatic changes in capture along the front of the hood. If the temporary fix works, have the ends fabricated out of stainless steel and install them in place of the cardboard.

If all this fails consider hiring an expert to evaluate the situation. Hood design has advanced very rapidly recently and the new designs save energy and work better. Pay back can be more rapid than with other energy saving improvements so it is work the investment to hire a knowledgeable professional and budget for the best system for your circumstances.



Frost Prevention in a Walk-In Freezer

Q: I have frost forming in my walk-in freezer. Can you tell me how to stop the frost buildup?

A: Below are the most common reasons for frost formation in the walk-in freezer and how to stop it in each situation.

  1. Propping open the door: Frost accumulates when warm air is allowed to enter the walk-in. Make sure the employees are not propping the door open for extended periods of time while loading or unloading the freezer.
  2. Seam leak: If your walk-in is a new installation and the frost is forming along seam lines, you may have an air leak at the seam. This leak can be repaired by sealing the seam with caulk from the outside only (the warm side). Do not caulk on the inside or you will make the problem worse as the moisture will accumulate in the seam and freeze and the expanding ice will further separate the panels.
  3. Leaky door or wiper gasket: If the frost is around the door you likely have a leaking door gasket or wiper gasket on the bottom of the door. Go inside the freezer, have someone turn the lights off, wait a minute or two for your eyes to adjust and look around the door for light entering from the kitchen. If you see light you have a leak at the gaskets that will need to be repaired.
  4. Pressure relief port issue: Another leak at the door can be caused by an iced up pressure relief port. Walk-in freezers are generally equipped with a small electrically-heated vent to allow the warm air that enters the box to shrink as it cools without creating a low pressure that will have to equalize. If you notice the door is difficult to open immediately after being closed you may have a failed pressure port or none at all. Find the port, which is usually a louvered vent located near the door and visible on both the inside and outside of the box, and see if it is blocked with ice. If it is, clear it and check the small heater inside to see if it’s working. If the heater doesn’t seem to be working, or if you do not have a pressure relief port, call your refrigeration technician for repairs.
  5. Fan delay relay failure: If the frost is on the ceiling, particularly near the unit cooler (coil), you may have a failed fan delay relay. The fan delay relay functions to delay the coil fans from restarting after a defrost cycle until the coil refreezes. If there is no delay the water on the fins of the coil will evaporate and turn to frost on the ceiling of the walk-in freezer. You’ll need a refrigeration tech to confirm the problem and replace the fan delay relay.

Good luck!


Welcome to Doc Kitchens

DocKitchensImageWelcome to Doc Kitchens, where all your commercial and institutional kitchen design questions will be answered. We have a list of blog topics that we’ll begin addressing soon. These entries will answer some of the questions that we get repeatedly from end-users and architects, such as why we use 10″ wide solid tray slides.

If you have a specific design question you’d like us to answer, feel free to let us know, either by posting your question in a comment here or emailing it to