Natural ventilation question....... 1

[agilityrider]

So, I am just starting out in the HVAC industry, and had a question regarding the effectiveness of natural ventilation. We do some schools and residential buildings which have natural ventilation through operable windows and doors. I was wondering though, what if the occupants leave the windows closed for long periods of time? I rarely open the windows in my own home. I guess air can enter the building through small cracks and leaks through older windows, but what about new construction which will have sealed and weatherproof windows? With out leaving them open how will they get the necessary outside air?

 

[GMcD]

Natural ventilation "sucks". And by that I mean that the only way to get fresh air into the building is to have the inside of the building under a negative air pressure relative to the outdoors to suck the outdoor into the building. This may be fine in spring and fall and very mild climates, but in colder and hotter/humid climate zones you give the building envelope people the willies. Go to

http://www.building

science.com and study up on some basic building science, there are some good ventilation articles there.

You are absolutely on the right track - the current "to Code" building envelopes are being built to a very tight, higher standard than the old leaky envelopes, so a dedicated source of outdoor air IS required to make sure that you can satisfy the ASHRAE 62.1 (or REHVA if you are in Europe) requirements - how can you prove that the natural ventilation is delivering the required minimum fresh air to all spaces? Computational Fluid Dynamics software (CFD) modeling - very expensive and cannot account for all the "what-if" situations of wind directions, wind speeds, and how the air will move around inside the building. And don't forget that the outdoor air that is coming in through the windows is un-filtered, raw outdoor air that may have pollen, dust and whatever else is out there. In some urban environments, the recirculated air inside the building is cleaner than the outdoor air around the building.

Natural ventilation is a "nice to have" thing, but for a non-residential building, you MUST have a powered means of getting fresh air delivered into all the spaces, even with a natural ventilation approach. Here in my area, the local City Code is now requiring HRV/ERV units in ALL residential dwellings, including high-rise MURBs that don't have air conditioning and central DOAS systems.

 

[EnergyProfessional]

Check your code if natural ventilation is allowed at all and under what conditions. typically it requires a % of the opening area, and occupant access to open the windows. code review may stop you right there.

Insulate tight, ventilate right..... with mech. ventialtion you can control ventilation, filter, and condition the incoming air (and recover energy from exhaust). air jsut being sucked through some cracks (along with al the mold and other pollutants in the cracks) isn't real ventilation.

In a school I would be concerned since the student's well-being depends on if the teacher feels like opening the window or not. In cold/humid climate the occupant has to chose between ventilation and comfort....

 

[ChasBean1]

If the window open area exceeds 4% of the floor area, you can meet most natural ventilation guides.

Don’t “over-code” this issue. The ventilation regs are guidelines, and if you don’t have a mechanical system the operable window area will suffice. If you are in a classroom in Paterson NJ that can barely afford two maintenance mechanics for 42 school buildings, and Billy farts, you can open the window and not have a $500,000 HVAC system to take care of it.

 

[EnergyProfessional]

I would strongly recommend meeting code.. the code is not just a guideline, it is the law. Talk to JHA about options in your area.
IAq is too often neglected... but it isn't just about the farts, it is about CO2 levels, and contamninants etc. Class rooms are densely populated and you can watcht he CO2 level rising.

I realize nowadays they use iPads in school to not even attempt to use the brain, but they still need some oxygen for vital functions.

 

[willard3]

Codes are minimum and, as I tell my clients, if all I had to do was meet the code, I would not be meeting my professional obligations.

It is possible to meet ventilation requirements without mechanical ventilation and Engineers did it for years, it just takes lots and lots of big ducts.

 

[317069]

ChasBean1: you are 100000000000000% right.

 

[ChasBean1]

317069, gimme a star GoshDarnit! haha

 

[irfansiddiqui]

check the local codes, Now a days sealed pack windows/door are designed in a building which must have mechanical ventilation system or simply HVAC with in them. a sealed pack building with out HVAC system always flops