What CFM For Duct Design?

[DC73]

Planning for a future AC and natural gas furnace (updraft) in the new shop and I need some help with duct design. I've sized an AC unit and a furnace but duct design is one area I've not yet dabbled in.

Here are the airflow specs from one manufacturer's system that would meet my needs (both are a little overkill for my needs but are representative of standard and readily available systems in the size range needed):

Furnace (36,840 Btus - output):

Heating CFM @ Min Temp Rise = 1,137
Heating CFM @ Max Temp Rise = 569
Heating CFM @ Med Temp Rise = 758

AC (1.5 tons):

Low speed CFM @ 0.1 ESP (Max) = 816
Low speed CFM @ 0.8 ESP (Min) = 523
CFM/Fan Speed High @ 0.5 ESP = 1,227
CFM/Fan Speed Med @ 0.5 ESP = 1,051
CFM/Fan Speed Med-Low @ 0.5 ESP = 878
CFM/Fan Speed Low @ 0.5 ESP = 678
Max CFM @ Any ESP = 1,498
Min CFM @ Any ESP = 523

There is a note in the manufacturer's spec sheet that the ideal CFM for the AC is 400 CFM/ton so it would be 600 CFM for this unit.

Given all the above, what CFM does the duct work need to support? I found one source that suggested to design for the maximum furnace CFM which would be 1,137 but that is about twice the supposed ideal CFM for AC use so I thought I would get some additional advice before I go any further with the project.

Appreciate the help.

DC

 

[pseudorealityx]

If you're going through the trouble of doing real ductwork, you can get away with ~400-450 cfm/ton.

In general, for a shop, you want the higher CFM on the heating side... since it reduces the outlet temperature, and therefore you get less stratification.

On the cooling side, a lower cfm/ton gives you better humidity control, but given that you're not bringing outside air in, or trying to keep it at 65 in there, it shouldn't be an issue.

 

[DC73]

If you're going through the trouble of doing real ductwork, you can get away with ~400-450 cfm/ton.

In general, for a shop, you want the higher CFM on the heating side... since it reduces the outlet temperature, and therefore you get less stratification.

On the cooling side, a lower cfm/ton gives you better humidity control, but given that you're not bringing outside air in, or trying to keep it at 65 in there, it shouldn't be an issue.

We don't have a lot of humidity around here and so 78 feels very comfortable unless you are really exerting yourself.

So, you think designing for 1,137 CFM (the high heat number) is the way to go?

I am undecided on ductwork type. Around here, you mostly see ductboard used to make a plenum above the furnace into the attic and then insulated flexible duct attached directly to the plenum. This would probably be the easiest way to do it, except that I'm not sure where to source duct board since the box stores don't seem to carry it.

DC

 

[pseudorealityx]

We don't have a lot of humidity around here and so 78 feels very comfortable unless you are really exerting yourself.

So, you think designing for 1,137 CFM (the high heat number) is the way to go?

I am undecided on ductwork type. Around here, you mostly see ductboard used to make a plenum above the furnace into the attic and then insulated flexible duct attached directly to the plenum. This would probably be the easiest way to do it, except that I'm not sure where to source duct board since the box stores don't seem to carry it.

DC

I would run it ~675 total cfm. (450 cfm per ton)

The 1137 cfm number is based on 0.1" of static pressure. That's pretty close to just letting the unit blow open, with zero ductwork. The fan doesn't have enough power to keep 1137 cfm as air restriction from duct work adds up.

The quoted airflows for each fan speed are taken with some assumed static pressure restriction. What your final installation ends up with be different, and the flows will go up/down based on if you have more/less restriction in your duct system.

Duct board plenum + flex is simple and cheap, but it's also pretty restrictive.

Rigid metal round duct (not "spiral", which is a nicer looking, stiffer alternative) is typically the next least expensive, and is significantly better air restriction wise.

 

[DC73]

I would run it ~675 total cfm. (450 cfm per ton)

Duct board plenum + flex is simple and cheap, but it's also pretty restrictive.

If I go the duct board/flex route, should I plan for more than 675 CFM to make up for the added restriction? If so, how much more?

I appreciate the help.

DC

 

[DC73]

I found a couple of references online that suggest flex duct only flows about 80% of what metal duct does due to the inherent restrictions so I'll factor that in to the estimate.

DC

 

[pseudorealityx]

If I go the duct board/flex route, should I plan for more than 675 CFM to make up for the added restriction? If so, how much more?

I appreciate the help.

DC

That's not how it works. You determine the CFM you want, and then just run the fan higher/lower speed to match that.

Example... let's say you want 700 cfm.

With very restrictive ductwork, you may need to run the fan on high speed.

With very NONrestrictive ducwork, you may only need to run the fan on medium speed.

 

[DC73]

That's not how it works. You determine the CFM you want, and then just run the fan higher/lower speed to match that.

Example... let's say you want 700 cfm.

With very restrictive ductwork, you may need to run the fan on high speed.

With very NONrestrictive ducwork, you may only need to run the fan on medium speed.

Thanks. If you end up between duct sizes, is it better to choose the larger duct or the smaller one?

DC

 

[pseudorealityx]

All else being equal, I'd rather be conservative and go bigger. You can always damper down supply diffuser. Once the duct is too small, you can't make it bigger.

 

[LS6 Tommy]

If you're going through the trouble of doing real ductwork, you can get away with ~400-450 cfm/ton.

In general, for a shop, you want the higher CFM on the heating side... since it reduces the outlet temperature, and therefore you get less stratification.

On the cooling side, a lower cfm/ton gives you better humidity control, but given that you're not bringing outside air in, or trying to keep it at 65 in there, it shouldn't be an issue.

That's OK just in terms of airflow for distribution issues. In actual use, it's backwards. The low speed fan is always for the heat, the higher speed for cool. If you have stratification, it's because the supply ducts are too high. In a perfect world your heat should come from close to the floor and the cooling from overhead. You can skirt some of those issues if you have a low level return and a high level return. Pull from the ceiling in cooling season and from the floor in heating season.
I know it would get cost prohibitive really fast to fab out a complicated duct system in a larger area. Most likely the supply duct will be across the ceiling and there will be one return. In winter that's where ceiling fans come in.

Tommy

 

[pseudorealityx]

That's OK just in terms of airflow for distribution issues. In actual use, it's backwards. The low speed fan is always for the heat, the higher speed for cool. If you have stratification, it's because the supply ducts are too high. In a perfect world your heat should come from close to the floor and the cooling from overhead. You can skirt some of those issues if you have a low level return and a high level return. Pull from the ceiling in cooling season and from the floor in heating season.
I know it would get cost prohibitive really fast to fab out a complicated duct system in a larger area. Most likely the supply duct will be across the ceiling and there will be one return. In winter that's where ceiling fans come in.

Tommy

The return location doesn't provide much in the way of a "force" for the hot air to go one direction or another. The buoyant forces will be much stronger, especially with a high delta T.... 120 degree air is ~10% less dense than 60 degree air.

 

[LS6 Tommy]

The return location doesn't provide much in the way of a "force" for the hot air to go one direction or another. The buoyant forces will be much stronger, especially with a high delta T.... 120 degree air is ~10% less dense than 60 degree air.

Yes it does. Air is a fluid. If you remove the denser fluid at the bottom of a vessel as you add a less dense fluid from the top, the less dense fluid will extend down. If you don't remove the dense fluid at the bottom, but from the middle or near the top, the less dense fluid sits on top of it- stratification. That's why good duct designs for forced warm air systems use a floor level return, unless the supply is down at the floor, too. The denser air is being removed from the lower level so the less dense supply air doesn't stratify as much and it can fill the space better. Reducing delta T by increasing airflow acrossmthe heat exchanger is the weirdest way I have ever heard of reducing stratification. It's also a great way to increase cycling, burn time and energy usage.

Tommy

 

[pseudorealityx]

Yes it does. Air is a fluid. If you remove the denser fluid at the bottom of a vessel as you add a less dense fluid from the top, the less dense fluid will extend down. If you don't remove the dense fluid at the bottom, but from the middle or near the top, the less dense fluid sits on top of it- stratification. That's why good duct designs for forced warm air systems use a floor level return, unless the supply is down at the floor, too. The denser air is being removed from the lower level so the less dense supply air doesn't stratify as much and it can fill the space better.

I'm not saying a low return is a bad idea (assuming high supply). I'm just saying that in a space that might see 3 ACH per hour, it's just a very small pressure differential...to use a fluids term. The LARGER pressure differential will be based on temperature/density.

Reducing delta T by increasing airflow acrossmthe heat exchanger is the weirdest way I have ever heard of reducing stratification. It's also a great way to increase cycling, burn time and energy usage.

A BTU is a BTU. The burner can only output ~37k btu in this case. It doesn't care if it's running at a 20 degree delta T or 60 degree delta T. The only additional energy use is the fan speed. It will not change burn time or increase cycling at all. The increased airflow promotes both better mixing just based on velocity and throw, AND it will stratify less, because the density difference is less substantial.

I'm just saying that I would rather supply 105 degree air than 125 degree air.


Imagine the extremes.

Let's say we want to hold the temperature at 65 degrees. And the heat loss is 100000 btu.

If you want to AVOID stratification, which solution is better?

1000 cfm with a ~93 degree delta T?

or 10,000 cfm with a ~9.3 degree delta T?


Even using your earlier argument of a low return. Which airflow is going to get pulled down to that return faster? The one where we're only returning 1000 cfm or the one we're returning 10,000 cfm? Obviously the higher airflow option is better.

 

[LS6 Tommy]

A BTU is a BTU. The burner can only output ~37k btu in this case. It doesn't care if it's running at a 20 degree delta T or 60 degree delta T. The only additional energy use is the fan speed. It will not change burn time or increase cycling at all. The increased airflow promotes both better mixing just based on velocity and throw, AND it will stratify less, because the density difference is less substantial.

That's absolutely NOT true. The system DOES care about the Delta T. Airflow requirements are based on the max temp output of the heater and required temp rise, not the other way around. If you move the air across the heat exchanger too fast, you don't pick up any heat. Low Delta T = prolonged run times and possibly not enough temp rise, Period.

I'm just saying that I would rather supply 105 degree air than 125 degree air.

105 won't heat squat in a cold climate. That's why heat pumps that routinely make a 30* Delta T with a 70* return temp have to use strip heat to maintain in cold weather.

Imagine the extremes.

Let's say we want to hold the temperature at 65 degrees. And the heat loss is 100000 btu.

If you want to AVOID stratification, which solution is better?

1000 cfm with a ~93 degree delta T?

or 10,000 cfm with a ~9.3 degree delta T?


Even using your earlier argument of a low return. Which airflow is going to get pulled down to that return faster? The one where we're only returning 1000 cfm or the one we're returning 10,000 cfm? Obviously the higher airflow option is better.

Your 9.3 Delta T at 10k cfm will be like farting under the sheets to keep warm. The solution is designing in proper air distribution from the beginning, not massive air changes.

I'm not trying to start an argument. I see the concept you're going on and I understand how it would change things for a stratification problem to a certain extent. I just don't get what form of engineering logic you're using as I've never seen anyone else do what you're trying to do. It's just more or less backwards from any heat transfer, airflow or HVAC design philosophy I've ever been exposed to. Airflow is sized to the system BTU capabilities, not the other way around.

Tommy

 

[DC73]

That's why good duct designs for forced warm air systems use a floor level return,

I've decided against a floor level return. Although it might pull vapors in anyway being up high, I'd rather not have the return near the floor in a working garage where fumes and vapors tend to sink to floor level. I'll likely leave the system off when painting or doing any other work with solvents, etc. I'm just not comfortable with a floor level return which is too bad because it would certainly be the easiest way to go in this shop.

Given the information I've posted about the proposed system, what CFM would you recommend I design the ducts around?

Thanks,

DC

 

[LS6 Tommy]

I've decided against a floor level return. Although it might pull vapors in anyway being up high, I'd rather not have the return near the floor in a working garage where fumes and vapors tend to sink to floor level. I'll likely leave the system off when painting or doing any other work with solvents, etc. I'm just not comfortable with a floor level return which is too bad because it would certainly be the easiest way to go in this shop.

Given the information I've posted about the proposed system, what CFM would you recommend I design the ducts around?

Thanks,

DC

I really can't give a cut-and-dried answer. I'm not a design engineer, just a technician. I would think it wouldn't be too hard to find someone to help design the ductwork sizes according to the specified equipment you use.

Tommy

 

[Jackfre]

Depending upon lay-out of the building, which isn't described, wouldn't spiral be the way to go?

 

[pseudorealityx]

I'm not trying to start an argument. I see the concept you're going on and I understand how it would change things for a stratification problem to a certain extent. I just don't get what form of engineering logic you're using as I've never seen anyone else do what you're trying to do. It's just more or less backwards from any heat transfer, airflow or HVAC design philosophy I've ever been exposed to. Airflow is sized to the system BTU capabilities, not the other way around.

Tommy

1) We're talking about Lubbock Texas. Average low's in Dec/Jan are in the 20's. Average daytime temps in Dec/Jan are upper 40's to low 50's.

2) You're saying that if you run the air too fast, it won't pickup any heat. That's not true. Any HVAC performance data catalog will give a minimum cfm/ton on the heating side. I'm looking at a Trane Precendent catalog as I type and it says not to go below 320 cfm/ton on the heating side. As long as you aren't going crazy with the airflow (which I'm NOT advocating... 450cfm/ton is hardly extreme), then the heat gain to the airstream will match the unit rating, which is typically ~80-81% AFUE for standard efficiency equipment.

3) Using a heat pump example isn't really applicable. They lose capacity at lower outdoor temperatures because they use a refrigerant cycle. As most residential heat pumps are constant volume, as it gets colder outside, the discharge temp falls. At some point, the heat pump either goes into defrost; or the total heating capacity falls below the heat loss of the space, and the indoor temp starts to fall. In 'normal' climates, THAT is why you add supplemental electric heat.

4) IF you're trying to fight against stratification, and having fans isn't desired, then more airflow is a better choice than less airflow. Again, "more" airflow isn't trying to run the place with 50 air changes... it's just moving up the curve a bit.

5) I've used this strategy successfully in a big office building in downtown Buffalo NY that was have stratification issues in the exterior offices. We were getting extremes... 18-20 degree differences between air at floor level and ceiling level. The main driver of the stratification wasn't the discharge temp though... it was poorly insulated construction and unsatisfactory installation of the storefront glazing systems. While that also had to be remedied, increasing the air circulation also helped to improve the condition and comfort for the occupants.

 

[Zeke]

1) We're talking about Lubbock Texas. Average low's in Dec/Jan are in the 20's. Average daytime temps in Dec/Jan are upper 40's to low 50's.

2) You're saying that if you run the air too fast, it won't pickup any heat. That's not true. Any HVAC performance data catalog will give a minimum cfm/ton on the heating side. I'm looking at a Trane Precendent catalog as I type and it says not to go below 320 cfm/ton on the heating side. As long as you aren't going crazy with the airflow (which I'm NOT advocating... 450cfm/ton is hardly extreme), then the heat gain to the airstream will match the unit rating, which is typically ~80-81% AFUE for standard efficiency equipment.

3) Using a heat pump example isn't really applicable. They lose capacity at lower outdoor temperatures because they use a refrigerant cycle. As most residential heat pumps are constant volume, as it gets colder outside, the discharge temp falls. At some point, the heat pump either goes into defrost; or the total heating capacity falls below the heat loss of the space, and the indoor temp starts to fall. In 'normal' climates, THAT is why you add supplemental electric heat.

4) IF you're trying to fight against stratification, and having fans isn't desired, then more airflow is a better choice than less airflow. Again, "more" airflow isn't trying to run the place with 50 air changes... it's just moving up the curve a bit.

5) I've used this strategy successfully in a big office building in downtown Buffalo NY that was have stratification issues in the exterior offices. We were getting extremes... 18-20 degree differences between air at floor level and ceiling level. The main driver of the stratification wasn't the discharge temp though... it was poorly insulated construction and unsatisfactory installation of the storefront glazing systems. While that also had to be remedied, increasing the air circulation also helped to improve the condition and comfort for the occupants.

I think you wrapped it up right there.

Either you do a total load calc and design a duct system to optimally serve your needs or you just throw some pipes up there and blow the air around with fans.

Fans do help in the winter but so does a heated floor.

There really isn't enough info on the building for any discussion so you guys are just blowing hot air.

 

[LS6 Tommy]

1) We're talking about Lubbock Texas. Average low's in Dec/Jan are in the 20's. Average daytime temps in Dec/Jan are upper 40's to low 50's.

2) You're saying that if you run the air too fast, it won't pickup any heat. That's not true. Any HVAC performance data catalog will give a minimum cfm/ton on the heating side. I'm looking at a Trane Precendent catalog as I type and it says not to go below 320 cfm/ton on the heating side. As long as you aren't going crazy with the airflow (which I'm NOT advocating... 450cfm/ton is hardly extreme), then the heat gain to the airstream will match the unit rating, which is typically ~80-81% AFUE for standard efficiency equipment.

OK, we actually are on the same page here. I was assuming you didn't mean staying within the design parameters of the performance data.

3) Using a heat pump example isn't really applicable. They lose capacity at lower outdoor temperatures because they use a refrigerant cycle. As most residential heat pumps are constant volume, as it gets colder outside, the discharge temp falls. At some point, the heat pump either goes into defrost; or the total heating capacity falls below the heat loss of the space, and the indoor temp starts to fall. In 'normal' climates, THAT is why you add supplemental electric heat.

I understand that. Maybe using a heat pump as an example was a mistake. I was only trying to say a 30* temp rise wont give you much heat with a 70* return in cold weather.

4) IF you're trying to fight against stratification, and having fans isn't desired, then more airflow is a better choice than less airflow. Again, "more" airflow isn't trying to run the place with 50 air changes... it's just moving up the curve a bit.

Again, I agree. I thought you were talking about REALLY going nuts upsizing the blower.

5) I've used this strategy successfully in a big office building in downtown Buffalo NY that was have stratification issues in the exterior offices. We were getting extremes... 18-20 degree differences between air at floor level and ceiling level. The main driver of the stratification wasn't the discharge temp though... it was poorly insulated construction and unsatisfactory installation of the storefront glazing systems. While that also had to be remedied, increasing the air circulation also helped to improve the condition and comfort for the occupants.

Good discussion, pseudo!

Tommy

 

[uncle john]

you need a trane ductulator.it will tell you what size to make your duct work.

http://www.amazon.com/dp/B002LEI8F2/?tag=atomicindus08-20

 

[pseudorealityx]

Lots of companies make those. I'm partial to the ones from Cook Fans.