Hello. What influence if any may have opening (a) window(s) on a well balanced air supply/exhaust system provided pressure, temperature, and humidity are the same inside and outside the building ?  

It will disrupt the air balance. It is easier for air to come through a window than an exhaust grille. If the system is already well balanced, why would you open a window?

When there is no pressure difference between the room and the outside, why do you think the air will go through the window ?

If the room is being exhausted, it is easier for makeup air to come into the room by an open window than by a comparatively small grille.

The rooms with well balanced ventilation, I believe, have both, some air incoming, and, the same amount of air going to the outside. It seems to me now, and you hint at it, the direction of the air "disrupting" the balance will depend on what is closer to the window : the exhaust or the supply.

Yes. Do a calculation on static pressure drop for an open window vs. a return or supply grille for the same CFM. That will tell the tale.

Typical practice is to have slight postive balance against infiltration. If that is the case, then conditioned air is blown out the building which would otherwise be returned. The difference would then be made up by either the heating coil or cooling coil. If the AHU is in economizer mode, it wouldn't make much energy difference. Otherwise, it would lead to a higher utilities bill. As to why someone would open the window on a well balanced system, the Darwin Award is still given out each year.

I never heard about that "award" till now. Have just googled it ... disgusting undertaking.

[b]Dear. When comfort conditions inside and outside the building is same. Taking the due advantage of outside weather, mechanical AC system should be stopped. So why worry about pressure effect when you are enjoying the same comfort conditions from natural wind pressure?.

H-VAC

An AHU should operate independent of the weather outside, I do not understand you.

I would guess a reference to economizer, since interior zones aren't going to get much use out of natural draft. Unnatural drafts usually call for more ventilation.

depending on the volume of air that is being supplied and exhausted, you would remain equal relative pressure. That is if in fact, as you state, the press inside and outside are equal. The supply fan must be 100% outside air type and the exhaust fan is equal to the supply fan, as you elude to in your question. If air comes in from outside, then the exhaust fan would need to be on the increase. If air was going out the window, then make up air would need to be on the increase. Since you say the inside and outside conditions are the same, equal pressure would remain. Should outside conditions remain the same except the wind begins to blow, then you can expect a change in volume of exhaust air to compensate for the added, although minimal, pressure increase and air volume from outside. What is it that you are attempting to prove. Sounds like a human resources argument from another employee.

Thanks for your answers to my (old) student qwestions !