Humidity -- There is No Escape

This post discusses the importance of controlling humidity in a kitchen ventilation system.

Humidity – There Is No Escape

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The building code specifies that make-up air (Mua) must be returned to the room within 10 degrees of the ambient room temperature; UNLESS, the ventilation system (HVAC) is designed with enough capacity to handle the additional heating and cooling loads added by the MUA.

The problem with this reasoning is that the only way the HVAC system can modify the MUA is for it to pass across the coil of the unit. This means it needs to be taken out of the room through the return grille. The return grille is typically located as far away from the hood as possible, so it does not compete with the hood’s capture. So the MUA has to mix with the conditioned air by design, meaning the kitchen will never be at the desired temperature.

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The other issue is humidity. Humidity is a vapor pressure. The best way to describe this is if you let one drop of blue ink fall into a bucket of water, in no time all the water is blue. There is nothing that can be done to stop it. The ink spreads through the water until all the water is the same shade. Similarly, humidity seeks to find equilibrium in the kitchen. And hot air holds more moisture than cold air. So if the MUA is coming in at 91 degrees and 60% relative humidity (RH) from outside and the kitchen HVAC is set at 75 degrees, the kitchen will now be at 99% RH. A sweat box with condensation dripping off the diffusers, damaging the ceiling, and causing mold.

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The solution is to temper the MUA to remove as much of the heat and humidity as possible to limit the impact on the kitchen environment. Kitchens are notoriously hot due to all the people and equipment, so air coming in at 55 degrees feels like air conditioning to the staff. Don’t overheat the MUA when you return it.