Fresh air supply for building 1

[mechanicaldup]

Regulations normaly state a certain rate (say liter/s per occupant) for a certain type of building.

Is there a certain min. number of air changes per hour (ACH) one should also adhere to?

 

[tombmech]

No. Exchange rates were dropped from most standards for commercial ventilation. Since volumes may vary widely, it was a rather imprecise and wasteful way to specify occupant ventilation. Changes were incorporated to specify ventilation rates per occupant, so that the building envelope could not influence the rates.

Even so, the use of AQ or CO2 sensors is becoming more prevalent. Such sensors, when incorporated in a comparative control logic with outdoor air, can establish minimum ventilation flow rates automatically. They also provide an additional protection from one of the byproducts of modern society: the outside air may be of less quality than the indoor air.

If your application is industrial or laboratory/health, then air change rates are still applicable. You are trying to dilute contaminants - regardless of the number of occupants.

 

[willard3]

ASHRAE 62-1 has a ventilation guide for people-occupied buildings.

There are many nbuilding types that would not require fresh air.

 

[KiwiMace]

Some codes specify in L/m2 in air-conditioned spaces, which is occupant independant, but does allow for variations in ceiling height.

Usually Air Changes are specified in non-conditioned spaces.

I've also seen minimum air change requirements for purging duty (2ach) in a/c spaces but these are for removing VOC's and are not required to run all the time at this flowrate. This is good design practice, as I've seen a fortune 500 company have to temporarily house 800 people for two months after nasty voc levels kept them out of the new building after the old lease ran out. heads will roll....

 

[willard3]

There are requirements for fresh air in building codes.

Fresh air increases the size and cost of heating and cooling equipmment but it is the only way to have a comfortable person-occupied building.

Nothing nastier smelling than a building that's not well ventilated.

 

[imok2]

If you can keep the CO2 level below 1000 ppm in all areas of a building you will moe then likely have adaquite amount of ventilation

 

[willard3]

I was involved in making a system with CO2 sensors work and, while the concept is very cool, the equipment, sensors and logic to control it are poor and frequently badly misapplied.

Simple ventilation systems do a good job and everybody, including maintenance personnel, understands how they work. Simple is almost always better.

 

[tombmech]

There is hardly anything simpler - two sensors, compare the two, and vary OA accordingly. Most systems with Outside Air economizers already include all the equipment necessary to make it work. Only additional controls logic and the sensors themselves are all that's required. There is often a problem in execution, though, and that depends on the controls vendor and their experience. The writing's on the wall, though - it is fast on its way to becoming a standard.

Having said that, I personally would not choose to use a CO2 sensor. Rather, a "general" Air Quality sensor is better. Typically, they read CO2 and a whole host of other contaminants. Strictly speaking, CO2 level is the definition of ventilation effectiveness, but specific CO2 sensors tend to be expensive and finicky.

As with anything "new" technique (10+ years old, at least), there can be unanticipated pitfalls. I have seen systems trip the safeties on equipment because the OA dampers shut on the passing of a truck. That's exactly what you want from an occupant point-of-view, but not by Maintenance.

The controls vendor needs to anticipate such consequences, and ensure that necessary deadbands or delays are incorporated - something often left out without even involving CO2 ventilation control.

 

[mechanicaldup]

Thks Gents
CinciMace what do you mean with VOC's?

 

[tombmech]

Volatile Organic Compounds ... solvents in the paints and glues of building materials that outgas for months or years after a building is built. As in acetone, toluene, formaldehyde, etc., etc. Recent initiatives have started specifying measured concentrations of VOC's (among other things) before acceptance of a newly built building.