Air distribution in high skylights

[Vicsidhu]

I have a 54”x30” skylight @ 17’ above the floor of a single storey building (Doctor’s office). Suspended ceiling is @ 9’6” above finished floor. The skylight is above a 4’ wide corridor. The light enters the corridor through a cone drywalled from the suspended ceiling to the underside of the skylight in the roof. The wide of this slant narrows from 4’ at the bottom to 30” on the top. If you do the mathematics – I have a huge ceiling space of 7’ to play with.

My dilemma is – the skylights are notorious for condensation and non-circulation of the air. I have never done a skylight ventilation before. So here I am – seeking help of the HVAC veterans.

 

[fredb]

Hello,

My suggestion would be to install a free hanging rigid decorative panel to split the volume vertically-lengthwise leaving a space between the top vertical edge and the skylight surface. Mount several architectural "can" style fans on the wall opposite one side of the central divider and taking air from below discharge it up to the skylight and down the other side of the divider. To size fans treat each half of the divided space as a large rectangular duct, and select an appropriate airflow. In this way you will be continually taking room air from below and conditioning the entire skylight space. A somewhat complex solution, but depends how bad problem is.

 

[Vicsidhu]

Thanks Fredb,

Architect is done with his design and would not change anything as this is a low low low budget job. So my options are none other than to play with my air.

What about linear diffusers along the slant (drywall) which connect the skylights to the corridor walls and throwing part conditioned air upwards? Throw being enough to go from one linear to another where it is returned to the plenum (ceiling space acts as a return air plenum). This way I will have some circulation in problem area and can retain the original design.

Any comments?

 

[ChasBean1]

Hi Vicsidhu. You'll only get interior condensation during outdoor cold weather and indoor humid conditions. Is this a humidified space? If you duct a constant (small, e.g., 20-50 cfm) volume of cool, primary, non-humidified supply air in the vertical drywall portion leading to the skylight, I believe this would be helpful in that it lowers the delta-T across the glass and displaces humid air generated by the people load within the space. The same design will have an adverse condensation effect during summer - but condensation formed will be on the exterior. So what? The thing's gotta keep out rain anyway, right?

I can't say that this is what I do because I'm not a design engineer. Design engineers, however, sometimes have the liberty of thinking outside the box, and I like your thoughts on this. Go for it. -CB

 

[235zzzo]

Hello Vicsdhu

If i understood, first, let me ask:- will (?)the architect allow, (as your budget!) to install a double skylight, with two or three sheets of glass (with a very low conductance coefficient) or just to add one more single, I mean the same model of the skylight, from outside, just to form an isolated small airbox in-between, with a very small and hidden ventilator and a very small hidden electrical resistance, as well, (both placed in a local outside technical box) for taking care of the air, of that small in-between space, controlled by thermostatic/enthalpic device related to the wet-temperature.

In a second concern, is it (?) possible to improve the passive isolation perfomance of that space towards the skylight, to turn down all the eventual thermal bridges and all the cold points or surfaces. These second measures are not really expensive to implement.

Finally, i agree with Chasbean the outside condensation in the summer time does not matter at all!

Kind regards
zzzo

 

[SAK9]

Sidhu,

I have done an atrium space about six storeys high with a glass roof on top.I located the AHUs on the roof.Ducted the supply air all the way down to 1st storey and distributed it from side registers along the periphery of the atrium.Collected return air just below the skylight at sixth storey again through side registers.I accounted for about 70 % percent of skylight gain in the return air and the balance 30% in room sensible gain.This design has worked for me.

 

[Vicsidhu]

SAK and Chasebean - I agree with your solution and explanation. I was thinking of going a bit higher with side linears. By deflection of 45 degree I would throw the air to the skylight which will be returned through the other diffuser directly opposite to the supply. But if your system works for a 6 storey building. I am sure it will work for mine. BUT - what are the outdoor conditions in your case? I am doing this building in Canada - extreme summers and winters.

ZZZ - Thanks for the feedback - but my architect won't do that. I think I will just go with the solution aboce.