Insulated or Uninsulated Duct/Plenum

[soj]

I am installing a packaged unit system in my shop. I intend to run the return and supply ducts straight through the wall at the floor level. Those two ducts will be insulated since they will go outside to connect to the pac unit. I have some 16" round duct that I will use at ceiling level to carry the air to the other side of the shop. I am considering building a rectangular duct to get from floor level to the 16" pipe at the ceiling. I want to go rectangular so I can make it wider so it won't extend as far into the shop. Since all this duct work is in the conditioned space, it does not need insulation for heat/cooling loss. What I am unsure about is condensation on the outside surface. I am concerned that close to the floor where the cooled air is coming in and is still very cold the vertical section of duct will sweat. Especially when the unit is first turned on and before it has had time to de-humidify the air.

Any thoughts from the HVAC guys, or anybody who has had uninsulated ducts in the conditioned space?

 

[Milton Shaw]

Insulate it. You will sweat otherwise. Internal insulation will last a lot longer than external as far as damage from being in the shop. Looks neater if its is done right.

 

[Jamie V]

Will the space be conditioned all the time or will it be turned on and off?

 

[soj]

Will the space be conditioned all the time or will it be turned on and off?

An air conditioned shop is a new situation for me, so I am unsure. I am thinking that leaving it on all the time would be the best. I have a little understanding of heat soak and humidity buildup. I have a programmable thermostat and could set it to be warmer (or cooler in winter time) during the night. Since the ambient temp is lower at night, that would probably add very little to the run time, but it should run enough to keep the humidity from building back up overnight. I am retired, so when I am home I am in the shop every day, so it would make sense to leave it on. But from time to time we are gone anywhere from a couple of days to a couple of weeks. During those times is when I am not sure if it would pay to leave it on or not. When we are gone we turn the house thermostat up to 80.

School me.
-jp

 

[Jamie V]

JP, it's pretty simple actually. The reason ductwork will condensate is that the dew point of the air around it is higher then the temperature of the metal. So if you don't run the AC enough to get the humidity down (dew point) then when the metal gets real cold it will sweat.

If you can wrap it with insulation after the fact then you have the option to see if it will be a problem first.

There is no cut and dry answer. It all depends on your climate, how well your building is insulated and sealed up, and how you control the thermostat.

 

[soj]

JP, it's pretty simple actually. The reason ductwork will condensate is that the dew point of the air around it is higher then the temperature of the metal. So if you don't run the AC enough to get the humidity down (dew point) then when the metal gets real cold it will sweat.

If you can wrap it with insulation after the fact then you have the option to see if it will be a problem first.

There is no cut and dry answer. It all depends on your climate, how well your building is insulated and sealed up, and how you control the thermostat.

All that is pretty much what I was thinking.

For the horizontal pipe, the wait and see was the plan, since it will be accessible on all sides after installed.

But for the vertical section, which will be against the wall, I wanted to get it right to start with. I have some 3/4" foil lined foam board (R5), and I can make it out of that.

So, a question about sizing. Since the air will be moving vertically, does the area of the duct need to be larger than a horizontal run? Obviously, I know nothing about sizing duct work, just trying to apply what little common sense I have. It seems to me that it would be harder to push the air UP (overcoming gravity) than SIDEWAYS. And IF that is true, you could compensate by making the vertical duct a little larger in sq. inches than the rest of the run.

In actual numbers: The inlet and outlet on the pac unit are 14"W X 16"H, 224 sq in. The 16" round that was connected to this unit in its previous installation is 201.06 sq in. I was considering making the vertical run 24" X 12" (outside) which would be (with 3/4" wall thickness) 22.5" X 10.5" (inside), 236.25 sq in, larger than any other part of the system. Is that big enough, or should I go larger? The foam board is 4' X 8' sheets, so it would be easy enough to go to 24" X 16" or 24" X 24". 24X24 seems overkill at 506.25 sq in.

Also, does the turn the air has to make as it enters and leaves the vertical portion affect the required dimensions?
-jp

 

[Jamie V]

JP, it doesn't quite work like that as far as duct size goes.

Do you have any info on the unit besides the outlet size? Do you have any fan specs?

Ductwork sizing isn't as easy as just square inches. Air isn't like water where you put a gallon in and a gallon comes out. You can create enough turbulence that no air can come out (that's kinda hard but you get the idea).

Are you making the ductwork out of sheet metal then insulating it?

To be the most efficient when the ductwork turns it needs either radius elbows or turning vanes if square. Round ductwork (not flex) is the most efficient way to run it. And you also should reduce the size of the ductwork after every so many outlets.

Do you have a rough sketch of what your trying to do?

 

[soj]

JP, it doesn't quite work like that as far as duct size goes.

I knew it was not a simple matter, and that's why I am here to get expert/experienced help. Thanks Jamie V for your help so far, and for any more to come.

Do you have any info on the unit besides the outlet size? Do you have any fan specs?

From the installation manual: (BTW, this is a Carrier Infinity 15 SEER, model 48XT-A360903-TP) Fan Nominal Airflow (Cfm) Efficiency: 1050, Max: 1200, Gas Heat(Low Stage): 875, Gas Heat(High Stage): 1410. Fan size 11" X 10". Motor HP: 3/4. Required Filter Size: 24" X 30" X 1".

Ductwork sizing isn't as easy as just square inches. Air isn't like water where you put a gallon in and a gallon comes out. You can create enough turbulence that no air can come out (that's kinda hard but you get the idea).

Are you making the ductwork out of sheet metal then insulating it?

I was planing on using 34" thick foil faced foam board, rated R5. Gluing the joints and then sealing with foil tape. Tape is Nashua #322 HVAC Tape.

To be the most efficient when the ductwork turns it needs either radius elbows or turning vanes if square.

I knew about the turning vane, just didn't know what it was called. I planned a piece of curved sheet metal at the bottom to turn the air flow up, and another at the top opposite the 16" round takeoff. In my mind this vertical section is sorta like a plenum, but since it is much longer in one dimension, I feel like it is more like a duct.

Round ductwork (not flex) is the most efficient way to run it.

If round is the most efficient, should I just get a square to round transition (or use a round takeoff at the unit) and use two elbows?

And you also should reduce the size of the ductwork after every so many outlets.

See mental sketch below.

Do you have a rough sketch of what your trying to do?

I don't have a sketch, but I will get one.

Meanwhile, here is a mental sketch:

Pac unit supply and return ducts (rectangular, matching size of openings on the unit) coming through the wall just above floor level. Return air plenum (with filters) on the right, vertical duct (of whatever size and configuration we come up with) on the left. Goes up about 10-12' where it will connect to the horizontal 16" round pipe. I also have a 16" to 14" reducer and some 14" pipe. No outlets are planned for now, I just want to get the air to the far side of the shop from the return to create circulation. I have enough 16" pipe to get just a couple feet past half way. I have a walled off area with no ceiling for parts/tool storage, and a small bath room (with a ceiling). Plans call for at some point adding another small room for an office/break room. At that time I will put a insulated ceiling over that area, insulated walls to the roof of the building, creating a un-conditioned attic that will reduce the volume of conditioned space. I will also then add outlets in the ceiling of each of those rooms, and add the 14" pipe to the end of the main run, if necessary to push air into the rooms.

Most answers are above, except: Why am I running the supply duct up inside inside of outside, like I see most commercial installations? I feel like it is better to have all duct work in the conditioned space instead of out in the heat/cold. I have room for it inside, so why not? It also lets me cut the holes in the wall in one place, and enclose both through-the-wall, insulated ducts with one sheet metal enclosure.

Also, I have the 14" and 16" ducts because they were removed from the house where the unit was previously installed.
-jp

 

[soj]

Here is the rough sketch, keyword rough.

The dimension should read: "With 12' Sidewalls. It was cut off when I rotated it.

Curved lines inside the vertical duct are the turning vanes.
-jp

 

[DC73]

I am retired, so when I am home I am in the shop every day, so it would make sense to leave it on. But from time to time we are gone anywhere from a couple of days to a couple of weeks. During those times is when I am not sure if it would pay to leave it on or not. When we are gone we turn the house thermostat up to 80.

School me.
-jp

From an energy efficiency and cost of operation standpoint, it's better to turn off the AC any time you're not using the shop and especially when you won't be using it for several days. You'll find plenty of articles on the internet that say its cheaper to leave the unit on all of the time but those articles would be wrong. Valid studies show otherwise.

Humidity control is an issue we don't have to deal with as much out here. If humidity creates big problems for you, you could make a case to leave the unit on more often.

Human comfort can be a factor if you don't want to wait for the shop to cool down.

If you are in the shop on a regular schedule, one suggestion would be to use a programmable thermostat. Make it a Wi-Fi thermostat and you can control it from a smart phone or computer from wherever you have internet access. A programmable thermostat would also give you some humidity control and still allow you to leave the unit off the majority of the time when the shop is not in use.

DC

 

[LS6 Tommy]

Insulate it. You will sweat otherwise. Internal insulation will last a lot longer than external as far as damage from being in the shop. Looks neater if its is done right.

X2. Always insulate the supply side.

Tommy

 

[pseudorealityx]

I'd insulate the first couple feet of return duct too where it penetrates the wall. You can get some odd issues with colder mornings where the duct may be cold near the penetration, and it can actually fall below interior dew point if the shop is warmed up.

Just a note... the outlet and inlet of HVAC units are basically ALWAYS way undersized vs what you would want to do for actual duct work sizing.

 

[Jamie V]

Here are some ideas for you. I would NOT make any ductwork from duct board. I would use galvanized sheet metal then wrap it.

I would also use canvas connections at the unit on both supply and return.