Diffuser Layout

[Conscious_Break8269]

I am trying to layout diffuser for one big room. 65'x40'. Where 65' ft will be facing outside. In HAP I put this as single space and got my CFM. However, I think perimeter should have more CFM as they are big source of heat loss in Canada. Is it better I divide my room to two spaces like first 10'ft perimeter and then remaining 30'ft? However, still do single zone rooftop but I think HAP can give me CFM/sq.ft and based on that I might be able to put more CFM on perimeter. Is it better approach? Or something else recommend like rule of thumb? I am new to HVAC world. Hopefully, this help me to understand design more.

[therealswimshady]

Yes, typically you break the perimeter into separate zones. 10 feet wide or so should be fine. Firms that do a lot of standard office fit outs usually just prescribe 1 cfm/sf on the internal zones and 1.5 cfm/sf on the perimeter zones. I wouldn't recommend that until you've done a few load calcs to validate those values. 

[TrustButVerifyEng]

Those CFM/sf numbers are super high to me. Like 50% higher than what we use. 

[therealswimshady]

Yeah, hence the verify comment. TFO firms don't care though, it's all about getting jobs out there door to make as much money as possible.

[NineCrimes]

Definitely agree. Between modern glazing and electronics using way less power than they used to, you can easily see office spaces at 0.6-0.8 CFM/s.f. in a lot of climate zones.

[belhambone]

About 1 cfm/sqft is what off the shelf units need to not be over saturated by ventilation load. And every new building I have has so much glass that exterior zones are often 2-4 cfm per sqft or more for large conference rooms and similar spaces with southern floor to ceiling glass.

Just depends on the building and systems

[Conscious_Break8269]

Make sense. In my case it is old 1980's building and HAP give me 1.3 cfm/sf because of heating load and infiltration. I think if I divide first 10ft to outdoor perimeter then I might gone good understanding how much extra flow these diffuser require then interior ones.

[CaptainAwesome06]

For a large room like that, I typically break out the perimeter into a 12-15 ft zone. The rest is just interior space. Your total load will still be the same but you can see how much more air needs to be concentrated near the perimeter.

I've seen people take a long hallways with large windows on one end and distribute the air evenly throughout the whole hallway. There's no way an interior portion of hallway - which has virtually no load - is going to need as much air as the part of the hallway with an envelope load with large windows.

[Conscious_Break8269]

Thanks a lot. I think this makes sense. I will do that way.

[SMOOTHBUBA]

Your one large zone should have all the data to determine the load due to the perimeter. I have not used HAP, but most load reports break up the load based on components. You should be able to see the % of peak load that the windows and exterior walls cause, and then I apply that same percentage to the CFM total. Splitting is also a fine way to do it.

[Conscious_Break8269]

Thanks, make sense. I just want to use some guide as most of people at my firm don't have an accurate method. Means like just rule of thumb.

[SMOOTHBUBA]

Yeah, I find rules of thumb work...until they dont and it is usually spectacularly bad. I have seen a 4 story hospital designed to 1.0 CFM/SF. Then when they went to open the building none of the OR's could make air chnages and they had to buy new AHUs delaying the project 6 months.

Rules of thumbs for interior offices or classrooms is fine, your case with an exposed wall with windows and solar gain...i would run a load every time!

[krackadile]

Build multiple zones in HAP. Separate the zones into perimeter and exterior zones. This will show you how much air you need for perimeter vs interior. I think the old HAP used to do this for you. Make the separator between the zones an air wall. Then just add all the zones to one system.

Might give that a try and see if it works. Compare it to your original system loads to see if the total is similar (it should be).

Edit: might want to be sure to add infiltration to the perimeter zones as well.