Maximum length of fresh air ductwork

[Scotmcg]

Simple question, the area I would like to draw my fresh air from ideally is about 50-60 feet away from where it enters the studio. Is that too far? Would it need to be rigid pipe rather than flex duct for that distance? Do I need to upsize it? It would only take 2-3 turns to get there.

Thanks,
Scotty

[John Steel]

Hello & howdo' Scotty?
Are you using a forced-air system or a ductless, mini-split system? The airflow requirements are different for both.

Is that too far?

Short answer is that I'm not sure without knowing more about your design.

. . . rigid pipe rather than flex duct . . .

I'd use rigid duct (for a whole host of reasons). The further your air has to travel, the greater the static pressure will become, so a rigid duct with a smooth surface would be preferable.

Do I need to upsize it?

The duct size should be determined by how much air you need in your room and there are there's formula for calculating that - what you almost certainly would have to do in these circs is to use a bigger fan. The fan has to be powerful enough to meet the airflow requirements of your room and also overcome the static pressure inherent in any HVAC system. Every component added to the path will add a small amount of resistance to airflow or static pressure. This can be cumulatively significant and is tricky to work out (although there is a very helpful app available from ASRHAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) which helps to detail static pressure for specific HVAC components.

It would only take 2-3 turns to get there.

The turns will also add to the overall static pressure of your system; each 90° turn adds the equivalent of the duct's diameter x 60 (assuming it were straight duct).
Is the reason you need to do this an air quality issue and will the duct be inside or outside? Best wishes, John.

[gullfo]

https://www.ashrae.org/technical-resour ... s/software

[Scotmcg]

Thanks for the replies. To clarify things, It's a single room, about 19 x 16 with a gabled ceiling that peaks at about 11 feet. The total volume is 2,708 cubic feet. There is a ductless mini split on the back wall.

According to my calculations, to achieve 6 air changes per hour, the mini split needs to move 271 CFM. My model does that easily. So for fresh air (271 * 30%) the fan needs to move 81 CFM. I was looking at the Fantech FG-4, which moves 110 CFM at 0.2” Ps.

The ducts will be in the attic space. I'm not doing the room within a room, but everything will be sealed airtight with spray foam insulation all around, so I will need the fresh return and stale exhaust. The stale air duct is short, maybe 6 feet total and straight with only 1 turn. The fresh is the issue. It will be in the ceiling at the back wall so it will turn from the ceiling 90 degrees, then run straight for about 30 feet, turn left 90 degrees, then run straight 20 feet, then turn down and terminate at the vent register.

I'm not supremely concerned about isolation, so I was not planning on building silencer boxes. I was planning on using 4" duct, same size as the fan.

Please let me know if I'm messing up here somewhere.

Thanks,
Scotty

[Paulus87]

Thanks for the replies. To clarify things, It's a single room, about 19 x 16 with a gabled ceiling that peaks at about 11 feet. The total volume is 2,708 cubic feet. There is a ductless mini split on the back wall.

According to my calculations, to achieve 6 air changes per hour, the mini split needs to move 271 CFM. My model does that easily. So for fresh air (271 * 30%) the fan needs to move 81 CFM. I was looking at the Fantech FG-4, which moves 110 CFM at 0.2” Ps.

The ducts will be in the attic space. I'm not doing the room within a room, but everything will be sealed airtight with spray foam insulation all around, so I will need the fresh return and stale exhaust. The stale air duct is short, maybe 6 feet total and straight with only 1 turn. The fresh is the issue. It will be in the ceiling at the back wall so it will turn from the ceiling 90 degrees, then run straight for about 30 feet, turn left 90 degrees, then run straight 20 feet, then turn down and terminate at the vent register.

I'm not supremely concerned about isolation, so I was not planning on building silencer boxes. I was planning on using 4" duct, same size as the fan.

Please let me know if I'm messing up here somewhere.



Thanks,
Scotty

I think calculating your static pressure would be worth while in your case since it is a particularly long run. I would also use a bigger diameter duct since in order to not hear the air rushing out of the duct you want the velocity to be below at least 300ft/m, ideally below 100ft/m. Currently yours is at 928.19ft/m. Going to an 8" will bring it down to 232ft/m and going to 10" takes it to 148.5ft/m.

[Scotmcg]

Thanks Paul, if I upsize the ducts what about the fan? Do i just use reducers back down from 8" to 4" to connect to the fan? Or do I need to upsize the fan to 8", which will move way more air than I need? If reducing is ok, can I leave the outgoing side of the fan at 4" as it will only be a foot or so from the end vent? Or do I need to increase the size again?

Appreciate the help,
Scotty

[Paulus87]

Thanks Paul, if I upsize the ducts what about the fan? Do i just use reducers back down from 8" to 4" to connect to the fan? Or do I need to upsize the fan to 8", which will move way more air than I need? If reducing is ok, can I leave the outgoing side of the fan at 4" as it will only be a foot or so from the end vent? Or do I need to increase the size again?

Appreciate the help,
Scotty

You can get reducers etc, but I think I would be tempted to just get a bigger fan and run it at a fraction of it's power so that the fan is quiet.

[gullfo]

agreed - larger duct, larger fan, slower speeds.

[Scotmcg]

One more question. If I go with 8" ducting, what size should the register grilles be?

Thanks

[gullfo]

if you're going right out of the duct - 8" if you're expanding via a plenum (or equiv), then whatever size that is. minimum expansion imho would be 12"x12" from the 8" duct.