Natural Ventilation Heat Load

[Mech]

2006 / 2009 IBC / IMC

I got into a discussion today with a co-worker about sizing an HVAC system when utilizing natural ventilation.

I'm curious, when figuring the size of a heating or cooling system, how many people include outside air (***uming natural ventilation) as a heat loss or gain and how many people ignore it?

If you do include it, how do you figure the amount of air? The same amount required by mechanical ventilation?

If you do not want to answer for legal reasons, I'll understand. But can you tell me if 'another person in your office' regards the includes the air or not? LOL!

Thanks in advance.

 

[Paul Sweet]

You always have to include outside air, whether it is brought in mechanically or through cracks & other gaps. Older HVAC text books had ways to calculate infiltration based on length of cracks around windows & doors, if they had storms or weatherstripping, etc. I was usually lazy and just used 1 - 2 air changes per hour in older looser buildings and 1/2 air change per hour in newer tighter buildings.

 

[Dr. J]

By natural ventilation, I ***ume you mean operable windows.

The code requirement is to calculate the heat loss of an open window on the coldest day of the year. This is due to IMC 401.3 which requires ventilation to be provided (not just available) whenever the space is occupied, IMC 402.2 which requires an opening of 4% of the floor area, and IBC 1204 which requires the space to be 68 deg at the design outside air temp. This will be huge, and unless you have some sort of magic that intercepts that cold breeze before it can cool the air in the immediate vicinity of the window you still won't meet IBC 1204.

There are plenty of AHJs out there that just ignore these requirements, so the answer to your first question is "not many" (other than covering infiltration).

To answer your second set of questions, there is no way to limit the amount of air coming in an opening that is 4% of the ***ociated floor area (a 2'x2' hole in the wall of a 10'x10' room) to just the cfm/person of Table 403, so that would be inappropriate.

 

[klarenbeek]

Mech,

check out the thread on natural ventilation about 6 items down the list. There is some controversy over whether the windows need to be open or just operable. If, as Dr. J says, the windows have to be figured as open, then of course that would have to be included in the heat loss. I'm sure Dr. J knows my interpretation of natural ventilation differs from his, though. Natural ventilation is to be provided through operable window--windows that can be opened. The code does not state that the windows have to be opened during occupied hours, only operable.

If it is your interpretation that the windows need to be open during occupied hours, then that needs to be figured in the heat loss. If it is your interpretation that they only need to be operable, not open, fully open windows would not have to be figured in the heat loss.

Dr. J,

As an AHJ, I DO NOT ignore the natural ventilation requirement. My interpretation may be different than yours, but that does NOT mean I ignore it.

 

[Mech]

For this project, I'm actually using overhead garage doors and personnel doors as my only means of natural ventilation. No screens on any of them.

klarenbeek: As an AHJ, would you allow doors, without screens, as the only means of natural ventilation?

 

[Builder Bob]

That depends would be the best answer..... Not enough info to really give a good answer.... Such as type of occupancy, occupant capacity, etc. However, IMHO, if you install a mechanical means of conditioning air, the fresh air for the unit would be based upon zero natural ventilation being provided. Reasoning, painting of windows, winter - nobody wants an open window in winter, etc..... The natural ventilation is an option that you can use with heating.... but in the sunny south, windows are closed to keep the heat out, and thus, fresh air is extremely important ---- can you say legionnaire’s disease?

 

[klarenbeek]

Mech,

I don' see section 401.5 as applying to doors and windows, but openings designed specifically for HVAC. As long as the doors and windows meet IBC requirements, I require nothing else. It sounds like the building you're dealilng with is a warehouse or shop of some kind? We allow overhead doors to be used for natural ventilation regularly here. Keep in mind, these occupancies are not typically cooled around here. The sheet metal shop where I used to work would get a little stick in July and August, but it didn't pay to cool the building for 2-3 months of the year. The overhead doors would be open fom late march or april through october. A sunny 50 degree day in march would be a nice day, so the doors would be open. Also keep in mind that if windows are used for ventilation in an office, I never see operable windows without screens. After the pheasant, mosquitos and gnats are our other state birds

 

[Mech]

klarenbeek,

You are correct, we have a warehouse / shop that we are designing - no cooling. The office portion will have windows with screens; this area gets heating AND cooling. I'm not sure if this will be natural or mechanical ventilation though.

I intentionally kept things vague since the code does not address natural ventilation based on specific occupancies.

Thanks for the replies.