# Gravity makeup air



## Rick18071 (Jul 17, 2010)

Someone wants to put in "passive makeup air" for a small 4' commercial kitchen type 1 hood by installing a vent (no fan)on the wall behind with lovers that open when the exhaust fan is turned on. The code (2009 IMC 508.1) allows makeup air to be provided by gravity means. I never saw a gravity system. I told him that he must prove that the makeup air does not exceed the capacity of the HVAC system (508.1.1). Can't find anything in IECC about it.

Does anyone know if this is allowed?


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## klarenbeek (Jul 19, 2010)

It is allowed.  I would also require a balance report proving that enough air is coming in to replace the air exhausted by the hood.  The problem with sizing the HVAC system is that, while there is a temperature that must be maintained for heating (68), in the code there is no temperature that must be maintained for cooling.  The contractor could say he designed the system to maintain 95 degrees.  There is nothing in the code to say he can't do that.


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## Builder Bob (Jul 19, 2010)

This would have to be shown on a balanced HVAC system report....Also what deoes the installer's guide for the Hood System state? BTW what is the exhaust rate for the hood, makeup air cannot exceed 300 cfm using gravity dampers/ 2006 IECC

503.2.4.4 Shutoff damper controls. Both outdoor air

supply and exhaust ducts shall be equipped with motorized

dampers that will automatically shut when the systems

or spaces served are not in use.

Exceptions:

1. Gravity dampers shall be permitted in buildings

less than three stories in height.

2. Gravity dampers shall be permitted for buildings

of any height located in climate zones 1, 2,

and 3.

3. Gravity dampers shall be permitted for outside

air intake or exhaust airflows of 300 cfm (.14

m3/s) or less.


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## Paul Sweet (Jul 19, 2010)

I hope this section of the IECC wasn't meant to apply to kitchen hood exhaust systems.  A motorized damper in the exhaust would make a wonderful grease trap.

A 4 ft. hood is only going to require a few hundred CFM.  If there's a dining area, it's possible that the required fresh air for the dining space would be adequate.

If the vent is near the hood the unconditioned air will be pulled right out, so it shouldn't have much effect on the comfort conditions of the occupied space.


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## TimNY (Jul 19, 2010)

My concern would be 508.1.1 [2003 IMC], how do you keep the temperature differential less than 10 degrees with gravity ventilation.  Exception 2 leaves a bit of a chance to make it work.  Depends on your location and how the kitchen is used.


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## peach (Jul 24, 2010)

I don't know that you're going to find in the IMC a requirement that the make up air has anything to do with comfort level.


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## TimNY (Jul 25, 2010)

peach said:
			
		

> I don't know that you're going to find in the IMC a requirement that the make up air has anything to do with comfort level.


508.1.1 ex 2 "Makeup air that does not decrease the comfort conditions of the occupied space." [2003 IMC]

Comfort is subjective term, which is why they placed in it there I suppose.  Give the AHJ some discretion imho.


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