F/A supply systems for buildings

[mechanicaldup]

what would say, when to supply a central fresh air system to a building and when to rely on the infiltration through windows etc. what is your opinion?

what is your opinion of using the ceiling void as a fresh air plenum box (blowing the air in to the ceiling void from where it distribute to each room via ceiling grilles on top of each room)?

 

[Engineer6512]

Infiltration through windows etc. will actually usually "overventilate" a space. The problem is that it is not controlled... some areas will receive an excess and some will not receive enough ventilation.

Using ceiling spaces is an option, but again you are losing a lot of the control that you would have when compared with running duct to each space. There is no easy way to control the exact amount of air going to each room. A more common use of the ceiling space is as a return plenum.

Steamguy2

http://www.SteamPlantEngineering.com

- Discussion & resources for professionals in the Power Generation Industry

 

[garnerengr]

One consideration is the fact that infiltration does nothing to dehumidify the air before it enters the space. This isn't much of a concern in drier climes, but in more humid environments it is of major concern. Positive pressurization of the building through a pre-conditioned central supply system can help in alleviating infiltration caused problems, but in any exterior wind condition over about 5 MPH, the positive aspects of this effect are easily overcome by the velocity pressure of the prevailing wind.

Another thing to take into account is that most building codes (for commercial construction) refer to and require compliance with ASHRAE 62 provisions, which, while offering some calculation alternatives (determining pre-charging and post-charging of the space to achieve adequate air quality on average during occupied periods) to the prescriptive ventilation rates contained therein, are in fact, quite hard to quantify and utilize in most cases, leaving the prescriptive methods to prevail. The quantities of ventilation required to satisfy these prescriptive rates are, by specific requirement of ASHRAE 62, to be supplied continuously during occupied periods. Once again, if some means of positive dehumidification/pre-treatment isn't employed, this can become problematic, at best. Such a condition is of particular problem in DX systems with only simple thermostatic control systems.

Just my 2 cents worth.

 

[AEBuck]

mechanicaldup ... I've got to meet you someday. Talk about thinking out of the box! Limiting to one opinion for each question.

Residential (single or multi) would be one of the very few upon which one should rely upon natural ventilation, but I would not generally endorse that either. I have a current client who's (grammar?)low-income tenants refuse to open their windows in order to reduce their heating costs, and then wonder why their units smell, why the walls are green and why the brick and/or block exterior walls are spalding due to freeze/thaw cycles within the wall. And, just how much air do you think a 1/50th HP Nutone bathroom fan will suck through a closed window, and then discharge through a 15-story riser? Some tenants even hang wet laundry in their units as well, again to save the quarters. I just love some of these projects.

When you mentioned an outside air plenum above the entire ceiling, the first thing I thought of was an array of buckets on the floor to capture black water from frozen and ruptured sprinkler pipes.

Please find it in your heart to forgive me for picking on you. If it's any consolation, I weighed a few loads of my own laundry, dry and wet, and then clocked natural drying time, to calculate the dehumidication load and resulting fresh air requirement. I'm pondering upon a submission to ASHRAE, but they'd just ask for money to study the issue and publish another chapter full of charts and tables.

 

[mechanicaldup]

AEBuck ,

What exactly is your concern?

 

[AEBuck]

After participating in this forum for a few months, I have the following concerns:

I have noticed more than a few posts by authors who lack common sense, and/or the ability or desire to take 60 seconds and reflect upon what they are asking before doing so.

I have noticed posts by authors who appear to be ill-equipped to even begin the design for which they are posing questions. I applaud those who have attempted an initial investigation on their own, and then come prepared with intelligient questions and sufficient background to comprehend responses. I'll help them well into the night. You might have noticed that I do not wish to assist those who lack initiative.

I am even more concerned about an apparent lack of supervision of those described above, and wonder of the whereabouts of senior engineers in their firms. Most of my clients deserve and receive much better service than I perceive to be offered by some participants in our forum.

I suppose my previous response was just too subtle.

 

[lilliput1]

Consider a 100% OA unit with exhaust air heat recover energy wheel, heating coil and cooling coil and duct the preconditioned OA to the return air intake of the room AC unit. See SEMCO or Greenheck.

 

[cme]

or AAON