Fresh Air into a classroom

[marc5180]

I was performing maintenance on a A/C unit in a classroom today and as i was above the ceiling looking at the pump i noticed a fresh air ductwork which was open ended oposite the indoor 4 way blow cassette.
My question is, i thought that there should be fresh air pumped directly into the classroom or through the air con unit but this was doing neither. The fresh air was vented above the ceiling. I've argued with my boss that it should go into the room but he says it shouldn't?

[Pooh]

Marc
the usual is to duct into the cassette if there is a spigot available if not the fresh air should go into the room. Is there a fan in the fresh air duct? If the fresh air goes in above the ceiling tiles how is it supposed to get into the room or is there vents?

Ian

[superswill]

I was performing maintenance on a A/C unit in a classroom today and as i was above the ceiling looking at the pump i noticed a fresh air ductwork which was open ended oposite the indoor 4 way blow cassette.
My question is, i thought that there should be fresh air pumped directly into the classroom or through the air con unit but this was doing neither. The fresh air was vented above the ceiling. I've argued with my boss that it should go into the room but he says it shouldn't?

was the freash air being dumped into the void or the 4 way?

[marc5180]

Marc
the usual is to duct into the cassette if there is a spigot available if not the fresh air should go into the room. Is there a fan in the fresh air duct? If the fresh air goes in above the ceiling tiles how is it supposed to get into the room or is there vents?

Ian

From what i can see the ductwork runs along the main corridor then branches off into each of the classrooms. It is about 3ft away from the spigot entrance of the 4 way cassette and it does not go onto the cassette it is just open ended above the ceiling.
There are 2 grills inthe room, the 600x600 ceiling tile ones but they are not connected to anything. Just seems strange to me to have fresh air above ceiling not in the room.

[marc5180]

was the freash air being dumped into the void or the 4 way?
It was being dumped into the void above the ceiling, my boss says this is normal, i don't agree.

[Pooh]

Marc
is it flexible ducting? and is there an actual open spigot on the cassette? if the answer to both questions is yes you will probably find the flexible ducting has shrunk and pulled off the spigot, I have known this before especially if there are tie wraps round the end of the flexi instead of a proper clip.

Ian

[Brian_UK]

Marc, you will probably find that there used to be ventilation ductwork installed before the AC was even considered.

When the AC was put in someone in great authority said that the ventilation wasn't required anymore so just leave it blowing above the ceiling. The theory being that it will find it's own way through the grilles.

[marc5180]

No it isn't flexible ductwork, its solid and no there isn't an open ended spigot on the cassette. Sorry if it didn't make sense. I meant the solid ductwork is about 3 foot away from where the spigot would go on the side of the cassette (if there was a spigot) which there isnt:confused:

[chillin out]

Your boss is right, the duct work is creating a plenum above the false ceiling....
http://searchdatacenter.techtarget.com/sDefinition/0,,sid80_gci213716,00.html

I used to be quite common but don't really see it now. A lot of supermarkets will draw air from below the cases, heat it up and then blow it into the plenum above.

Chillin:):)

[superswill]

the other thing to look out for is at what temp is the supply air,if you refix the duct (if indeed thats where its from) eg summers day with an outside temp of 23c going over the coil in cooling mode will lead to loss of duty

ok i type too slow and missed the last few post!

[marc5180]

Your boss is right
Great, why can i not be right just once:(

[Pooh]

Marc
I am still with you, I cannot see why you would want to use a dusty ceiling void above a suspende ceiling as a plenum when the facility is there to do the job properly for a few quid more and be able to balance the system too.

Ian

[Brian_UK]

Marc
I am still with you, I cannot see why you would want to use a dusty ceiling void above a suspende ceiling as a plenum when the facility is there to do the job properly for a few quid more and be able to balance the system too.

Ian
There's your answer everytime. ;)

[TRASH101]

It was being dumped into the void above the ceiling, my boss says this is normal, i don't agree


Its all about the return/extract air path or whether they have natural ventilation i.e. windows that open or trickle vents.

It would probably do no harm to connect onto the unit but you must consider how it might affect the whole vent system.

Stick with your boss on this one as re balancing a system for the sake of a connection to an a/c unit is difficult one to justify, especially when you don't know if there is a problem in the first instance.

[MikeA]

It would probably do no harm to connect onto the unit but you must consider how it might affect the whole vent system.

This is correct. Introducing fresh air in any form expecially to a classroom is a good idea even with opening windows. Through the unit is the best form of creating an effective mix with the return air and there is usually a knock out panel on the side of the cassette to make the connection.

This type of fresh air installation however, is normally associated with an in-ceiling ducted system where the ceiling void is used as the return air plenum and the conditioned air is supplied to the room thorugh flexible ducts and ceiling grilles for distribution. This only works as long as the system is on the lower floors of the building where the external fabric ie roof, is not contributing to the return air heat load.

The boss is correct if there is no extra money to be made (eg Don't fix what ain't broke) but introducing the fresh air either into the unit or into the room is the best solution. Have completed some installations in Camden and Harlow to the same effect and the students stay awake now much to their combined disappointment.:D

Cheers Mike

[2007eng]

I think this fresh air should has its way through in the cassette unit(mixed with the return air) or discharged to the room as a supply diffusers.For all cases the system should has a return fans, and grilles for air balance, But the existing situation is not accepted.

[R1976]

Hi,

It seems strange to use the ceiling void as a pleneum for supply ventilation.

It is normal for extract ventilation. Supply air is pulled under doors or through trickle vents.

It may be that it is supposed to be extract but someone fitted the fan the wrong way round. I have seen this before.:p

Just my thoughts.

[Abby Normal]

that would be a disastrous set up in a humid climate, it would create a fungal jungle in the ceiling space

[Argus]

.


It’s been an interesting technical discussion so far, mainly on the inadequacies of this installation and the inability of small split systems to do something for which, quite frankly, they were never intended – that is ventilation by proxy through a plenum.

There will probably be 30 odd pairs of lungs constantly delivering CO2 into an enclosed space in this classroom.
It strikes me that one crucial thing that been overlooked is the effects of minimal fresh air make-up and constantly re-circulated air containing high CO2 levels in the classroom.

Research has indicated a direct correlation between static CO2 levels in the air in schools and learning ability.

In Scandinavia, I believe, legislation exists to introduce a minimum replenishment of air in this type of application to dispel the build up of stale respiration and they have noted an improvement in classroom performance.



.

[Greengrocer]

Be carefull using Cassettes for delivering Fresh Air. They can only take approx 10% of the Cassettes total air volume. Any more and they get noisy. Also if the Cassette fan is off and the F/A is still being delivered dust from the Cassette filter can be blown into the room.
My guess is that the F/Air volume for a classroom (8-10l/s/person) would be way too high for a single Cassette to handle. Connect the F/A to the 600 x 600 grilles.
What about extract air? Is there another stub duct above the ceiling. If so the F/A will never get past the ceiling into the room as currently set up.

[Andy AC]

As superswill mentioned earlier, the temperature of the fresh air is important if going through the cassette.
Got called to a gym a few years back that was like an iceberg in the summer because the incoming fresh air was so warm it wouldn't allow the cassettes stop on thermostat, so they just kept going and going. In the winter, the fresh air was so cold that it stopped the cassettes from doing any cooling whatsoever, which resulted in 50+ hot sweaty bodies having to go outside to cool down.

Stick it in it's own grille with a damper on it and it should be ok.

Andy

[Abby Normal]

.


It’s been an interesting technical discussion so far, mainly on the inadequacies of this installation and the inability of small split systems to do something for which, quite frankly, they were never intended – that is ventilation by proxy through a plenum.

There will probably be 30 odd pairs of lungs constantly delivering CO2 into an enclosed space in this classroom.
It strikes me that one crucial thing that been overlooked is the effects of minimal fresh air make-up and constantly re-circulated air containing high CO2 levels in the classroom.

Research has indicated a direct correlation between static CO2 levels in the air in schools and learning ability.

In Scandinavia, I believe, legislation exists to introduce a minimum replenishment of air in this type of application to dispel the build up of stale respiration and they have noted an improvement in classroom performance.



.

I am really wondering though, is it the fact that the CO2 level is high, as in the CO2 is bad for you, or is it simply an indicator of not enough fresh air.

[Thermatech]

For UK engineers
This weeks H&V News did a 39 page colour supliment title HVAC for schools.
Some very interesting best practice facts & figures for ventilation in school applications.

[yinmorrison]

Marc,

I have watched this thread and if you really want to get the answer, what you should do is check if there is a positive airflow through the ceiling grilles and if there is, measure what it is and if it meets the requirements of the occupancy levels in the room then your Boss is correct ( approx 10l/s/person ) This is the best way to introduce fresh air to the room via the system that is installed rather than connect directly to the cassette as long as it is controlled via a heater battery for winter operation.If you connect directly to cassette it causes other problems as the manufacturers do not lend enough detail to the control of the way the air enters the cassette as discussed earlier by Andy AC

[Argus]

I am really wondering though, is it the fact that the CO2 level is high, as in the CO2 is bad for you, or is it simply an indicator of not enough fresh air.

Both.

The thrust of the studies in Scandinavia was that higher than normal CO2 levels due to poor ventilation in classrooms impairs rates of learning in children.
It follows that the same is true for adults.

It is customary to situate 30 - 40 kids in a class in a much greater density level than would be tolerated in a commercial environment so, using accepted adult rates of ventilation as a guide that were developed for a commercial environment may not be the answer.

.

[Abby Normal]

what ventilation rate were they recommending in Sweden?

They were also probably having excessive humidity in the classrooms in the winter there also causing condensation on windows

[Sridhar1312]

It should go through the cassette so that it is treated. Leaving just above may create sweating of the places above ceiling

[HVAC_GAY]

fresh air intake shall connect to return air duct & via this to suction side of blower fan.But In your case may be the return air duct is sucking air from above ceiling or above false ceiling & fresh air duct is terminated upto above false ceiling only so that the return air causing the fresh air to enter by forced darft force that causing due to retun air going out of calssroom hence the same time fresh air drawing inside.

[Brian_UK]

It should go through the cassette so that it is treated. Leaving just above may create sweating of the places above ceilingWe don't have that sort of problem here in the UK due to our lower ambient conditions, otherwise a valid point.

[S_Line]

A good thread this one.

Ive seen this method of supply air often, its a cheaper way and a bit of a naughty way. The School should think themselves lucking getting fresh air. We have installed into classrooms, with no fresh air input.

I think they rely on trickle vents in the windows. Its a disgrace.

Things will get shaken up as soon as these Compulsory Tests are carried out on all public buildings and schools.
To monitor the level of efficiency.

[kyaw htoo]

I was performing maintenance on a A/C unit in a classroom today and as i was above the ceiling looking at the pump i noticed a fresh air ductwork which was open ended oposite the indoor 4 way blow cassette.
My question is, i thought that there should be fresh air pumped directly into the classroom or through the air con unit but this was doing neither. The fresh air was vented above the ceiling. I've argued with my boss that it should go into the room but he says it shouldn't?
I think your boss is right.There's no way to supply the fresh air directly into the room and how did you know that it was a fresh air duct?