# Natural Ventilation



## duckbill (Jun 10, 2010)

IMC 2009, Section 401.3.

Many DPs take advantage of the minimum openable area to the outdoors at 4% of the floor area being ventilated for the outdoor air requirements.  IMC 2009, Section 401.3 requires the ventilation to be provided during the periods the room or space is occupied.  in reality, no one is going to open the windows and doors when the temps are 20 or 90 degress.   How do others deal with this scenario?


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## klarenbeek (Jun 10, 2010)

There was a previous thread on this topic that was, well, interesting.  Code states natural ventilation can be provided through operable windows and doors.  It does not say the windows must be open during occupied hours, only operable.  The occupants must have the ability to open them at their own discretion.  At least that's my interpretation.


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## Dr. J (Jun 10, 2010)

IMC states _"401.3 When required.  Ventilation shall be provided during the periods that the room or space is occupied."_  IBC states _"1204.1 Equipment and systems.  Interior spaces intended for human occupancy shall be provided with active or passive space-heating systems capable of maintaining a minimum indoor temperature of 68°F (20°C) at a point 3 feet (914 mm) above the floor on the design heating day."_

The code requires the windows to be open whenever the space is occupied, and at the same time, for the inside temperture to be 68 deg on a design heating day.  Ask the DP how that is being accomplished.

According to the other "interesting" thread, the way some AHJs handle this is to ignore IMC 401.3, or to justify somehow that IMC 402.1 ("_The operating mechanism for such openings shall be provided with ready access so that the openings are readily controllable by the building occupants."_) supercedes IMC 401.  Sorry, but 401.3 clearly states when ventilation is required, (whenever occupied).  402.1 just states the requirement for controlability.  This is no different than the controllability requirement for mechanical ventilation in IMC 405.1.


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## mtlogcabin (Jun 10, 2010)

> The occupants must have the ability to open them at their own discretion.


Agree and I have never lived in a house with air conditioning (spent 42 years in south Fl) so the windows where always open when the temps got above 70's.


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## klarenbeek (Jun 11, 2010)

Once again I'm going to respectfully disagree with you, Dr. J.  401.3 only states ventilation is to be provided when the space it occupied, it does not say how it is to be provided.  For HOW it is provided, you go to either section 402 for natural ventilation or 403 for mechanical.  Section 403 gives specific CFM requirements for ventilation, so the system has to be operating during occupied hours to deliver required CFM.  If natural ventilation is chosen, window and door openings must be openable and readily controllable by the occupants.  Nowhere in section 402 does it say the windows must be open during occupied hours, only operable.

There are days here in SD where we could probably get the cfm required for mechanical ventilation through a window opened only a couple inches.


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## Dr. J (Jun 11, 2010)

> 401.3 only states ventilation is to be provided when the space it occupied


  No argument there.





> Nowhere in section 402 does it say the windows must be open during occupied hours, only operable.


 No argment there either.  Like you said, it's 401.3 that requires ventilation to be provided (not just the means, but the actual ventilation) when occupied.  403 requires a switch, 402 requires a handle, 401 requires the switch to be on or the handle to be open when occupied.  





> There are days here in SD where we could probably get the cfm required for mechanical ventilation through a window opened only a couple inches.


  The DP needs to calculate the heat loss of a window open to at least 4% of the floor space being ventilated.  Each 100 sf needs a 4 sf hole in the wall.If a DP can show that a space is not occupied when it is cold, there is no obligation to prove that 68 can be met with the window open.  Perhaps an outdoor pool cabanna.  However, for most spaces, it is probably going to be occupied on a design heating day.  So a means of keeping the space at 68 with the window open is required.

I view this as a code writing fail.  There should be nothing wrong with ventilating some occupancies with open windows in some climates, but there should have been some limitations.  One of my favorite bars has one entire wall as overhead doors.  They are open to the patio in good weather.  The extent of the opening is big enough cover the 4% for the entire interior seating area.  Should they be allowed to have absolutly no outside air brought into the HVAC system?


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## vegas paul (Jun 11, 2010)

Suppose all the ventilation is mechanically provided, and maintaining the required temperature.  What happens when the thermostat reaches the programmed temp and the unit stops?  Are you in non-compliance?  There MUST be a difference in ventilation provided vs. ventilation in operation.  After all, the code requires a minimum artificial lighting value, but doesn't require the lights to be on!


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## Dr. J (Jun 11, 2010)

> What happens when the thermostat reaches the programmed temp and the unit stops? Are you in non-compliance?


 Yep, not in compliance._405.1 General._

_Mechanical ventilation systems shall be provided with manual or automatic controls that will operate such systems whenever the spaces are occupied. Air-conditioning systems that supply required ventilation air shall be provided with controls designed to automatically maintain the required outdoor air supply rate during occupancy._



> After all, the code requires a minimum artificial lighting value, but doesn't require the lights to be on!


True.  If IBC 1205 had said "Artificial lighting shall be provided whenever the space is occupied", then it would be the same as IMC 401.3, and lights would have to be on all the time unless turned off by an occupancy sensor.  But it doesn't, so IBC 1205 and IMC 401.3 are not equivalent.


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## klarenbeek (Jun 14, 2010)

So a 10x10 shop or warehouse office would require either mechanical ventilation or the windows to be open all day long no matter the temperature?  Sounds a little extreme to me.  As an AHJ, I have to apply the same rules to that office as to a large office building. I'm not the window police. I can't force building occupants to open their windows all day long, every day, for the life of the building.  Once again, "operating mechanisms for such openings shall be provided with ready access so that the openings are readily controllable by the building occupants." (2009IMC Sec. 402.1) The occupants provide the ventilation themselves.  If you would size the heating system as if the windows were open all the time even on the coldest days, the system would be so grossly oversized it wouldn't work properly when the windows are closed (remember the occupants are to have control of the windows).  Not only would it have to be able to keep the air 68 degrees, it would also have to be able to melt the 2' of snow that blows in throught the open window.  For cooling it would have to be so large NO humidity would get removed.

I agree with Dr. J that if mechanical ventilation is used, the system is supposed to be operating during occupancy and needs to be sized accordingly.  I make sure what the damper minimum opening is set at and the fan hase the ability to run continuous, but I don't take the thermostat apart to see if the fan switch is jumpered.  The service tech or building owner would simply remove the jumper (or change the control computer settings) before I made it to the next job.  At least it would take a little more effort than that to reset minimum damper positions. I tend not to try to enforce the unenforceable.


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