Question for ductwork experts.

[chillrdude]

for starters I am no duct work expert, I have never really had to learn a whole lot about it, I can read a ductulator but have never had to size duct runs. That being said does the duct in these pics look OK to you, looks kinda cheesy to me. They ran 27" x 26" vertically about 20 feet then kicked it down to 12" x 27" thru the wall then back up to 26x27 horizontally about 50 feet. The unit is an 15 ton Voyager, this is a buddy of mines building and he has 4 more systems all about the same, any thoughts?

http://i93.photobucket.com/albums/l53/slick813/IMG_4966s.jpg

http://i93.photobucket.com/albums/l53/slick813/IMG_4967s.jpg

[amickracing]

EEps! Even with out measuring it, it'd say that system can't possibly be runnign anywhere near full performace.

[jason_c]

im no expert onducts by any means, but necking the duct down will increase velosity onlly to slow it don on the other side. anyway you look at it, efficiency is greatly deminished

[du mech eng]

thats one hell of a bottleneck. not ideal but i'm guessing that unit could handle the static as long as the layout inside isnt as creative. is there some structural issue preventing them from running it full size?

[ralphtheplumber]

Yep, it looks like sheet. Probably performs the same way.

I understand that sometimes you have to squeeze things down to fit, but couldn't they have used a change elbow with vanes or something?

[chillrdude]

Nothing structural in the way just did a half assed job, it is one big clusterf*#k, here is the supply trunk running through a drop ceiling with common return, the duct rains all over the place.

http://i93.photobucket.com/albums/l53/slick813/P7290001.jpg

[master mechanic]

Buy a magnehelic gauge or digital manometer and learn how radical fittings cause extreme preasure drops.You can not put 10 lbs of potatoes in a 5 lb sack.

[jayhawker]

Turning vanes will help reduce the effective lenght and pressure drop of the fittings at the RTU and the turn into the building, without two turning vanes that fitting is equal to 60 feet feet of duct with it is equal to 30 feet. As stated before checking the pressure drop across the fitting at the top will tell you how severe the problem is.

[core]

Maybe they just needed to increase the velocity to 2700 FPM across the elbow as per the engineers spec.:D


Wait a minute is that a slip and drive I see on the main truck? :(
I guess TDC was out of the question.

I bet they had to change the blower motor to a higher HP.

Core

[kuhnhunter]

Looks like the ductwork needs to be insulated, that is why it is "raining" everywhere.Was this ductwork in the conditioned space before and the dropped ceiling put in later? Anyway I would at least have the ductwork wrapped and have those bottlenecks taken out, have elbows put in with turning vanes.

[slimwoodie]

i guess they had to get "store bought" elbow's for that..

what a bunch-o-crap

[easybend]

Reminds me of World Trade Center for some reason.

[bobby m]

It may mean nothing, the ductwork out of the unit may be oversized,it all depends on the external static that the unit is capable of overcoming at design cfm.Depending on the run the dimension of that elbow may have been sufficient for entire run, but it is still a hacky looking job.

[ac/dc]

What size is the unit?? That 12x27 is actually 10x25
because of the isulation liner. That looks like a 7.5 ton
or maybe 10.My ductulator is in the truck but Im pretty
sure 10x25 wont even handle 5 ton (2000cfm)
Is it a plenum return ceiling? Maybe the sweating is due to
below normal supply air temps because of the airflow restrictions

[roc service]

There are 2 problems here.

The connections at the unit are too tight. The unit should be further away from the wall, with full radius ducts coming off both the supply and return openings. This is creating a static gain in the unit.

Secondly, the transitions at the top entering the building are another restriction. The installation is not going to work well. The unit is starving for return air and will not overcome the static in the reduced hoz duct.

This is a redo.

[hvacdoctor]

They likely reduced the duct size due the wall cut openings. It may have a limit on opening for structural integrity...Anyhow, this is common on larger ducted systems to reduce down then up again. Yes pressure drop is increased or squeezed if you know but as long as it opens back up again to correct size it should be Ok. If its a long run over 100' you may have air issues. Make sure the last take offs are at least 4-5' before end cap or termination point. This system needs to build up static pressure.

[master mechanic]

hvacdoctor

The supply and return elbows is causing a major duct restriction especially on a light commercial rooftop unit. It would be difficult to over come that excessive static even with a oversize drive kit. Structual problems should be solved before duct work is designed. A metal building is not that difficult to modify frame work to accomidate ductwork. Why install equipment when you know it will cause premature compressor failures due to poor duct design.I would like to traverse the supply duct to see just how short of air this unit is delivering.

[tinner73]

think of the rotation of the fan...the first elbow is opposite. this is bad in lots of ways. is the ductwork is insulated?

[shafeeqbasheer]

The duct sizes at the mouth seems to be fairly fine to accomodate 5600 CFM , but when the duct is reduced to 12x27 the head loss will be drastically raised which will result in a less efficient system and power consumption will shoot up...And as said the duct expands again..these contraction-expansion will bring more pressure losses due to foramtion of eddies in the flow....seems to be a unprofessional work all together

[r404a]

I put this unit in "on the side"

[markwolf]

Originally posted by roc service
There are 2 problems here.

The connections at the unit are too tight. The unit should be further away from the wall, with full radius ducts coming off both the supply and return openings. This is creating a static gain in the unit.

Secondly, the transitions at the top entering the building are another restriction. The installation is not going to work well. The unit is starving for return air and will not overcome the static in the reduced hoz duct.

This is a redo.

Full radius ells & moving the unit out from the building would look like pookie!There would be nothing wrong with square elbows providing they have turning vanes in them.Other than that I agree.

[hvacdoctor]

Did anyone actually test the system to see what is actually happening? Yes, the duct is crap but it should be tested. There is old saying "air don't care" meaning sometimes crazy bad installs do indeed work somehow...In this case it does look obviously bad and wrong...My Tim McCarverism of the day!