Air volume

[ajk1]

There are 7 units along the exterior wall of a room that is roughly about 50 feet x 25 feet. The room is at grade level on the east side of the building. This used to be 3 separate rooms, and has 4 thermostats controlling the 7 units which are fed from an underfloor (slab-on-grade) "dual duct" system (constructed in the 1965, but new controls put in the central mechanical room about 3 years ago), and controlled from an algorithm on a centrally located computer in the mechanical room.

Generally it operates fine, when the room has no more than about say 50 people in it in the morning and again in the evening. But on one day a year in late September when there are about 120 people in the room all day, it reportedly grossly overheats. There is reportedly (by a mechanical engineer) to be adequate cooling capacity of the equipment.

I am told that the starting point is to verify that the volume of air coming from each unit is about right, and that the temperature of the air from each unit is the same. I have so far measured the the temperature of air from 3 of the 7 units and it is 20.9 C, 19.6 C and 20.2 C so I expect that is as good as one can expect. I am going to measure the temperature at the remaining 4 units shortly.

Question:

Is there any simple way for a layman like me to measure the volume of air coming from each unit?

 

[Occupant]

How is the warm air dissipated?

 

[ajk1]

Return air grilles in top of wall opposite from wall with air supply units.

 

[TheTick]

Nope. Hire a balancer.

Honesty may be the best policy, but insanity is a better defense.

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[ajk1]

ok

 

[racookpe1978]

Sounds like his problem is rare (late September), but predictable.

Cheaper to add a single "add-on" room type AC unit on the wall,or just to change the September meeting to another location?

Are there ANY winter heating/cooling problems, or just this Sept cooling problem?

 

[ajk1]

There are no winter heating/cooling problems.

 

[TheTick]

Long answer: A balancer will have all the right equipment AND now how to use it. An experienced balancer may also spot problems and be able to offer simple solutions.

p.s. I once worked as an apprentice balancer.

 

[dbill74]

If you want to verify the amount of air coming from a unit, you could get yourself a vane anemometer, but they can be expensive for a one-time use situation.

20°C (68°F) is actually high for air from an HVAC unit. I would expect the air temperature to be 12.8-15.6°C (55-60°F). This is an indicator that either the units are not working as intended or you have a controls issue. When/how did you make these temperature measurements?

Other questions to ask regarding the one day a year there are ~120 people in the space:
Are the occupants doing anything different than normal?
Is this day occurring on a weekend? (Is the building controls in 'occupied' mode when these people are in the space.)
Do you know how much outside air the
HVAC units are bringing in? Is it adequate?
Did your mechanical engineer consider that one this one day you have more than double the normal occupancy?

 

[chicopee]

With 120 people in the room, water vapor from respiration could be a problem and increase the relative humidity. You can estimate the sensible and latent heat loads from these people. Each person would generate about 100 watts of sensible heat load and an amount of moisture of 2 liters per hour.With that information you can estimate the condition within the room.

 

[ajk1]

I am surprised that you would expect the air coming from the units should be down at 12.8 to 15.6 C. That sounds awfully cold to me. I can see that this is way over my head as I am a structural engineer. I will leave it to the house committee to deal with it. Thanks for all your comments. Much appreciated.