Supply air duct 2

[pop1888]

Hello,

A contractor replaced a straight supply air duct in a condo with the air duct circled in red with multiple extra bends in the air duct. Can someone confirm that this change in configuration will also change the air flow negatively and what could be consequence of changing it to this configuration? The supply air duct is attached to a Fan coil.

Thanks.

 

[moideen]

Adding more bends or fittings to the existing duct will increase the pressure drop. Whether it reduces the CFM depends on the selected fan's ESP. If the fan's ESP is adequate to overcome the added static pressure, the supply airflow will not decrease. Verify the airflow at the outlet with an anemometer to ensure it meets the design flow.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[Snickster]

I don't think that adding a couple of 45 bends will effect air flow significantly.

 

[HVAC-Novice]

That will cause a small reduction in flow. if it is significant, we don't know. that depends on if that branch already was the critical path.

At least they used 45° and attempted to tape.

Looks like there was lack of coordination. Maybe didn't even have detailed plans for duct and plumbing. the HVAC contractor may not be at fault here.

 

[nuuvox000]

If it was already functioning just fine with the airflow before this, a couple of bends isn't going to change that.

 

[pop1888]

Hey, thanks for the replies.

Electrical engineer here so not familiar with some hvac terms. What is the fan's ESP and where would I find this info?

I think there's definitely some kind of reduction but just don't know how much. Are there tests that can be done to find out?

It is on the critical flow as only this duct supplis air to the whole unit and there is already a 90 degree bend coming out of the fan coil, and 90 degree bends before and after this section of the air duct before these new bends were introduced. With one 90 degree bend pretty much right after one of the 45 degree bend introduced.

I've attached the original damaged duct. There definitely was not any coordination and I don't even know if the pipe coming straight of of the concrete is fireproofed properly.

Any advice of suggestions on how to fix this properly or test to ensure that the change is insignificant would be much appreciated on how to fix this properly. Thanks!

 

[Snickster]

The damaged duct likely produced as much additional pressure drop as the 45 ells added. Two 45 deg ells add 10 feet (just approx. without looking up true values but can't be any more than this) of equivalent length of straight pipe which is insignificant considering the entire duct system may be a couple of hundred feet in equivalent length.

ESP is the static pressure that is measured between the inlet and outlet of the unit which it is rated for based on the rated flow. This is the pressure drop in the duct system external to the HVAC unit. Adding two 45 deg adds minimal amount to the ESP. For instance, most ducts are typically sized base on 0.1" w.g. pressure loss per 100 feet of length. If two 45 ells have an equivalent length of 10 feet total then the added ESP is only 0.01 inches water gage In a system that was designed for 0.2" w.g. if 200 ft. long this is insignificant. I would just observe if there is any change in cooling ability of the unit before I try to get contractor to make any more changes. Or if you are really concerned you can hire someone to get a measurement of the actual flowrate in the discharge duct and compare it to the rated flow of the unit.

 

[pop1888]

For sure the new duct will be working better than the old damaged duct, even a piece of duct tape taped to the puncture in the damaged duct would have worked better. But I don't think the original HVAC design was for the pipe to come out in that spot and it was designed to be a straight duct run. I think the pipe was not installed properly. So the question is whether or not this new installation will cause less performance to the HVAC unit compared to if everything was installed properly in the first place with the pipe that would have allowed for a straight run of the air duct.

 

[HVAC-Novice]

That looks like some residential unit. Trust me, nothing is designed in the engineering sense. They put a certain size furnace in each unit and run certain size of duct. One room gets less air than the other and in winter that may be good, in summer it will be bad. if the room being served now is really bad, it wasn't good to begin with.

FWIW, you can measure airflow with a hood or similar. And to add what Snickster said ESP, is EXTERNAL static pressure. That means the pressure EXTERNAL to a furnace or AHU and we ignore the pressure required for the coil, filter etc. That is already accounted for by the manufacturer. So ESP is what is EXTERNAL to the device and what is available for the duct system. So an HVAC designer only designs the duct work, but not the internal s of the unit. If you look at the spec sheet, they may give you the total pressure (inc. filters and coil) at a given flow. but ESP is what a duct designer needs.

 

[LittleInch]

What a botched up, hacked up mess of pipes and ducts that is.

Is this duct under positive or negative pressure?

Either way, those extra bends are negligible. The 90s are far worse plus the length of the duct makes a much bigger difference.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BronYrAur]

Is the room comfortable? That is the only thing that really matters. Trying to quantify the change in airflow before versus now is challenging. Bottom line, are there comfort issues now that you didn't have before? If not, don't worry about it.

 

[IanVG]

It's probably fine, as in, overall insignificant. Means and methods baby! Once you've seen 1990's era one-sheet chiller replacement jobs, and have seen the system running, then you understand the why you've got safety factors on everything! On my jobs (commercial), I sometimes include reference to specific SMACNA details, that show kinds of ductwork modifications and deviations that are allowed when other utilities are encountered in a space.