Question on Makeup Air for hazardous Exhaust System

IanVG · Oct 1, 2023

Hey everyone,

I've got a question about a piece of code from the 2018 International Mechanical Code (IMC). Section 510.6.5 states for makeup air for hazardous exhaust systems that:

Makeup air shall be provided at a rate approximately equal to the rate that air is exhausted by the hazardous exhaust system. Makeup air intakes shall be located in accordance with Section 401.4.

In addition section 501.4 on pressure equalization states that:

Mechanical exhaust systems shall be sized to remove the quantity of air required by this chapter to be exhausted. The system shall operate when air is required to be exhausted. Where mechanical exhaust is required in a room or space in other than occupancies in Group R-3 and dwelling units in Group R-2, such space shall be maintained with a neutral or negative pressure. If a greater quantity of air is supplied by a mechanical ventilating supply system than is removed by a mechanical exhaust for a room, adequate means shall be provided for the natural or mechanical exhaust of the excess air supplied. If only a mechanical exhaust system is installed for a room or if a greater quantity of air is removed by a mechanical exhaust system than is supplied by a mechanical ventilating supply system for a room, adequate makeup air shall be provided to satisfy the deficiency.

I thought that good practice concerning exhaust systems is to maintain relative negative pressure relative to spaces where the relevant air-borne hazards are lesser. When I read that makeup air shall be provided at an approximately equal rate that air is exhausted, I am not sure how it is possible to keep a space both relatively negative and where makeup air equals exhaust air. The only other way I see this making sense, is if the makeup air enters the space through an indirect system, e.g. makeup air infiltration through transfer ductwork, or through/under doors. With that interpretation you could practically rewrite section 510.6.5 to simply that "Think about where and how your air is entering your space." Any thoughts?

Snickster · Oct 1, 2023

The mechanically exhausted air must be greater than the mechanically supplied air in order for a negative pressure to be developed in a space. However the total air entering the room must be equal to the exhaust air as if not eventually there will be no air left in the space or at some point the exhaust air will stop flowing at some negative space pressure. The other air comes from flow through the building construction - cracks in doors, windows, walls. This air flow from the outside across the building construction produces the pressure drop that causes the internal negative design pressure.

A calculation is performed per ASHRAE methods that has equations that predict the air flow across each path of the building envelope to determine the total flow at given internal space design pressure. The mechanical supply flow plus this infiltration flow though building construction will equal the exhaust air flow at the design space internal pressure. This is because at the design internal pressure you calculate the infiltration flow through the byuulding envelope, subtract this from the exhaust flow, then set you mechanical suppied air flowrate to this value.

Attached is a sample calculation from a project I worked on for negative pressurization of a space.

Note that to have a space positively pressurized you would do the opposite - set the mechanical exhaust flow lower than the mechanical supply flow with the difference flowing through the building to the outside at the desired space positive pressure (using the same pressure drop calcs from ASHRAE).

Snickster · Oct 1, 2023

In regard to the Code statement shown in your post I would interpret that when they refer to mechanical ventilation or exhaust it means by a fan/blower. And that the required Code exhaust rate must be provided mechanically by blower/fan and not some of it partially by such as natural exhaust ventilation. So your exhaust fan has to be sized for the entire exhuast rate required by the Code. When they refer to makeup air it can mean either mechanical fans/blowers or flow through cracks in building envelope or even louvers installed in the walls. So as far as supply air is concerned it does not matter if it is all mechanical or all natural through cracks or louvers, or a combination of both. In the end total flow of air in has to equal total flow of air out. Usually spaces with hazardous materials are kept at a negative pressure relative to other spaces. I believe maybe they left it as negative or neutral here in the Code so as to not to conflict with that part of the same Code that specifies it somewhere else. Also note that a room can be maintained with a neutral pressure and still contain the hazardous gases if the surrounding spaces have positive pressures so this is maybe why they include neutral as a possibility.

ChasBean1 · Oct 6, 2023

Snickster articulated well on this - but the whole meaning of these 184 words of mechanical code is "Make sure the contaminated room is not positively pressurized to abutting spaces by appropriately balancing supply and exhaust airflows." I prefer my 19 words, but things that are under-articulated in code lingo I guess tend to be misinterpreted.

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Question on Makeup Air for hazardous Exhaust System

IanVG

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