RTU roof curb too low! Possible solutions besides curb extension?

[RandomUserName]

I have some rooftop units already set in place on a building. I specified a roof curb thinking we were going to have a sloped deck and standard roof insulation. Ends up the architect specified a flat deck and the sloping for storm water to be made with insulation so we have 8-10" of insulation on the roof. Now my units are way to close to the roof and I am concerned about sucking up snow in winter months.

We could do roof curb extensions of course, but that would require a crane to lift each unit, the extension itself, plumbing time to extend natural gas, and most expensive electrical wanting to run completely new power up to each unit. Roughly $10k-12k.

My question is as such: has anyone ever removed the factory hood (for rain protection) and had a sheet metal contractor fabricate a snorkel of sorts to extend our outdoor air intake a little bit higher up?

Just looking for ideas.

 

[EnergyProfessional]

WE had a regualr RTU MAU isntalled at normal height and it sucked in snow all the time. No surprise with the fpm.

We eneded up isntalling a huge "snow cather"... basically a longer duct as large as the MAU (so much more crossection thant he air intake) to slow down the snow and to fall down and drain out. Worked good lst weinter.

You don;t need to do soemthign that extreme, but can design some sheetmetal structure... no problem. IF the unit sucks in air at full speed in witer, the ehigth above snow line isn't so critical, it is how many fpm becasue whne it snows it will draw the air in like a vaccum.

Reduction of velocity is the key.

 

[RandomUserName]

A quick Google research leads me to this study (

http://www.inive.org/members_area/medias/pdf/Inive/clima2007/A10/A10K1733.pdf)

that says to limit face velocity to 3.23 ft/s. My largest unit is pulling in 590 CFM OA. I am having the sheet metal contractor field verify bolt hole locations for existing outside air hood but we can assume the approximate dimensions are 18" x 40" or 5 sq. ft. That gives me a face velocity of just under 2 ft/s.

I feel terrible, this is my first big mistake as an engineer 2 years out of undergrad.

Glad to hear that something can be fabbed up to connect on the OA side.

 

[EnergyProfessional]

Most designs don't take snow into account... don't beat yourself up to bad and learn from this.
On another project (not designed by me!) we had to locate an MAU damper inside the conditioned space because it would ice up. Most likely from a combination indoor air creeping up and condensing (the original damper was on intake side of RTU). what I did install class 1A low leakage damper inside to a) prevent air creeping up, and b) prevent it from freezing.

In my opinion no equipment should be outside. All air should be taken in from very large louvers (they have good rain-tight ones) and all equipment be inside. It isn't only the damper freezing, what about mechanics that have to fix a broken RTU in a blizzard?

 

[Drazen]

why are you sure that higher curb is definite solution? have you accessed predictable snow height compared to curb level?

it could be possible to install defrosting heaters on nearby surface, or to make fresh air ducting up to roof edge, if fan arrangement allows that.

 

[EnergyProfessional]

The code often gives a minimum snow height for your area. imho this is not conservative enough and only protects from ground (roof) snow, not from snow flakes in the air getting drawn in.

the heater won't help much as it melt the snow and then the water will be drawn in. the only thing that helps is a very large intake to reduce velocity.

 

[ChasBean1]

RandomUserName (great name BTW), you didn't specify what your intake velocity is and what "way to close to the roof" is, nor did you say how many RTUs. Holy unknowns! If you are talking about 5 or more units the 10-12k sounds like very short money.

 

[RandomUserName]

ChasBean1,

Stated in second post: facevelocity is under 2 ft/s.

Height from bottom of OA intake to roof is approx. 7-10 inches , varies with unit location.

5 RTUs in total.

 

[ChasBean1]

Sorry, missed the velocity. You sure the units don't economize and max is only 590 cfm? 2 fps is pretty low and you might just cross your fingers, wait and see. Otherwise I think maybe (like you said) a sheet metal fab with with an up-turn and goose neck down might be the best option.