Comments on: The 2 Primary Causes of Reduced Air Flow in Ducts

By: Joe

Joe — Thu, 26 Mar 2015 04:32:06 +0000

We bought a 3 bedroom model
We bought a 3 bedroom model condo recently (built around 2011)

I noticed that the smaller air registers (2 smaller bedrooms) and two registers in the kitchen and dining nook were not getting air flow or little air flow (tested with fan on only).

The AC company sent a tech to investigate. The tech immediately brought a a pair of tin pliers (used to bend sheet metal). He immediately bent the ar register diffusers from 45 degrees to 90 degrees (vertical).
This increased the air flow, but now the air flow only flows directly down instead of directing the air across the room.

I think that ductwork was improperly sizes or installed causing an increase of friction to the point the registers have little air flow.

I would appreciate comments concerning my thoughts on this

By: John Proctor

John Proctor — Fri, 22 Aug 2014 19:46:26 +0000

Bill, does the rooftop unit
Bill, does the rooftop unit have a BPM/ECM fan motor or a PSC?

By: John Proctor

John Proctor — Fri, 04 Apr 2014 01:32:00 +0000

A few clarifications: 1) All
A few clarifications: 1) All flex duct comes with an inner liner, it will not make a flex duct have the same pressure drop as a metal duct of the same size.
2)Anytime you make a change of direction with a flex duct, a metal elbow is strongly advised.
3)All this talk about a friction rate to design to makes me real nervous. Remember the friction rate is set by the available static pressure after all the non-duct elements pressure drops are subtracted. Then the available pressure is divided by the total effective length.
4)With a PSC motor increasing the static pressure will slow the flow and reduce the fan motor watt draw. However it will also reduce the sensible cooling and the compressor will run longer. See http://www.proctoreng.com/utilities/Airflow.html 
5)with a Brushless Permanent Magnet Motor (aka BPM/ECM) reducing the static pressure will allow the motor to provide the same airflow and sensible capacity with less watt draw.

By: David Butler

David Butler — Fri, 04 Apr 2014 00:26:47 +0000

@Conor, while its true that
@Conor, while its true that blower power represents a large portion of total system power as source equipment efficiencies rise (especially for PSC blowers), as MJ said, it's ludicrous to think that the blower will consume more power than the compressor.

Regarding 300 FPM return duct limit in CA... are you sure that's not for grille face velocity?

Manual D specifies 300 FPM max face velocity for filter grilles. I typically specify 250 or less in high performance homes when using filter grilles.

Regarding duct liner for flex... pls post a link. I don't have any idea what you're referring to.

By: M. Johnson

M. Johnson — Thu, 03 Apr 2014 20:21:33 +0000

Again, to CONOR: You reported
Again, to CONOR: You reported "... The operation of a centrifugal blower motor is the highest operating cost in most comfort systems, annually trumping the operation cost of a refrigerant compressor, outdoor axial fan, or geothermal closed loop water circulation pump."

Hmmm. As I said in an earlier post, the ECM air handler draws in the ballpark of 300W. The 3.0 ton AC (Trane 2TTX9036A1)that makes it blow, draws in the ballpark of 2800W. What must happen for the 300W item to use more energy than the 2800W item? They generally run at the same time -- no "fan on" thermostat setting for this house.

Although I live in the hot-humid Houston area, you might say I'm from Missouri.

By: M. Johnson

M. Johnson — Thu, 03 Apr 2014 20:06:35 +0000

CONOR -- You reported ".
CONOR -- You reported "...Centrifugal E.C.M. blower motors commonly installed in residential systems operate at their most energy efficient against an IWC PD/ 100 ft. of equivalent duct length of .4 to .6..."

I may not be understanding exactly what the definition of efficiency is here. Perhaps you can help break it down into simpler terms.

My furnace is American Standard ECM type (model AUD080R9V4K) and I was fortunate enough to snag a copy of the Service Facts document. It compares wattage vs ESP, External Static Pressure measured in IWC, inches of water column. From it, one measure of efficiency:

Nominal 1200 cfm air flow,
1) at 0.1 ESP, 170W, rated air flow 1163 cfm
2) at 0.3 ESP, 230W, 1178 cfm
3) at 0.5 ESP, 285W, 1188 cfm
4) at 0.7 ESP, 345W, 1183 cfm
5) at 0.9 ESP, 405W, 1178 cfm

First of all, am I speaking the same vocabulary as you? Second it would seem energy efficiency goes down as ESP goes up -- which argues against the existence of a sweet spot if I understand what you say.

By: Blake

Blake — Thu, 03 Apr 2014 19:22:23 +0000

Agree with need but it is
Agree with need but it is almost impossible and rare in houses over 40 years old and those with small crawl spaces.

By: Conor

Conor — Thu, 03 Apr 2014 19:21:20 +0000

It would be most effective if
It would be most effective if you politely asked the outside consultant for the performance curve of the blower he is specifying. The operation of a centrifugal blower motor is the highest operating cost in most comfort systems, annually trumping the operation cost of a refrigerant compressor, outdoor axial fan, or geothermal closed loop water circulation pump.

Centrifugal E.C.M. blower motors commonly installed in residential systems operate at their most energy efficient against an IWC PD/ 100 ft. of equivalent duct length of .4 to .6. Many persons employed in the residential market are not aware and design–or just install–comfort duct systems to an IWC PD of 1.0 or even 2.0.

Also, if you design a residential system to exactly .8 IWC PD, the ducts will most likely, finally be installed to perhaps 1.0 anyhow.

For residential buildings in California one particular code is becoming more stringent and will only allow a maximum of 300 fpm in return ducts or return duct registers (yes, that makes for one ginormous amount of return register, AK, or free area). This requirement solves many problems inherent to duct design and installation when performed by many residential installers; it lessens noise pollution, collects more particles due to a slower rate of flow at a filter, and 'hopefully' cuts down on the power draw of the blower motor.

That code does not mention turbulence mitigation or equivalent duct diameters before, or after, the blower, but I think this is an error on the code writers’ part.

A comfort system performance issue resulting from duct and duct register design and specification that many fail to pay attention to is the direction or characteristics of air-throw (an endearing term I use instead of “airflow”) from supply registers to indoor comfort zones. Proper indoor air-throw can be specified prior to construction by first determining air duct velocity just before any supply register and de-rating that velocity for any turbulence created by the upstream ducts or general system effect, then looking up the performance data from a good air supply register manufacturer and noting the tested and documented air-throw length, fall, or fpm. Good manufacturers will also note the temperature or temperature differences used in their labs during the determination of performance documentation. This is all explained extremely well in ACCA’s Manual T.

A lot of methods for duct system design employ safety factors or result in unbalanced systems before on site balancing. One of the most effective ways to install comfortable indoor airflow to and from occupants is to test velocity at the end of ducts, after duct installation, and before register specification or register installation. In my opinion this should be a step involved during nearly every system installation’s commissioning, testing, and permanent balancing stage.

(If you are not in a home from the 70’s and have no jumper ducts, close a door if you want to make testing, balancing, math, and real world application more chunky.)

For variable speed blower operation there is a method in Manual D section 9-15 that can be employed to kinda ‘fudge’ a compromise regarding air-throw, comfort, and noise when installing single speed supply registers (to do this you need to locate or determine through engineering assumptions the previously mentioned register performance data). But I think that testing and balancing on site is more effective than most any specification of supply registers without it.

In your commercial industry you have an advantage because almost all systems are designed by engineers and almost all equipment performance is properly documented; circumstances are often opposite in the residential industry. I have not yet designed a commercial system, but the mathematics in regards to operation of blowers against the resistance of finally installed duct systems never change.

Again regarding your first question0 about duct design and code or best practices, your IWC PD should be specified while paying attention to variables such as noise, the material cost to the client to upsize ducts, the expanse that you have where all of those ducts are installed, and final balanced air throw at all installed registers just to name a few. But an important variable that I was not aware of until taking classes form experienced service providers is the “sweet spot” in the curve of the blowers commonly being installed in new systems, and, although E.C.M.s motors are forgiving, the resultant operation cost and eventual, higher maintenance cost incurred when we do not specify and operate comfort system centrifugal blower motors within this beneficial parameter. “We should first sell comfort to a client” and then work our way backwards, but we must also keep in mind the operating costs that a client will pay for many, many years after our design and installation.

Regards,

Conor

P.S. Does anyone have experience with an entirely flex duct system WITH A LINER INSIDE while say installing to a tested, after commissioning and balancing, IWC PD/100 (may include airflow through house); noting manufacturer documented blower kW; and recording performance, by recording kW at the proper blower speed, against that tested IWC PD/ 100--with that lined flex duct?

By: M. Johnson

M. Johnson — Thu, 03 Apr 2014 17:45:00 +0000

Although I am in no way an
Although I am in no way an airflow expert, I did spend several years trying to troubleshoot a too-cold bedroom. In the process I bought Manual D (and Manual J) and did a lot of online talk with HVAC professionals.

Sizing. That is what I submit must be added to the list of causes of poor airflow. Return ducts are often under sized for the airflow required, and people often fail to recognize flex ducts need to be sized larger than hard duct for a given air flow. Filters are often installed with really high face velocities, making them both more restrictive when clean, and more vulnerable to air flow problems when in service a few months.

At the same time the industry is notorious for OVER sizing the AC equipment. Maybe heating too, but where I live AC dominates. For existing homes, this leads to an opportunity when replacing equipment: Choose smaller equipment, and the reduced air flow is more forgiving of the capabilities of the duct and filter system. Or add a new return to increase filter area and air flow capacity.

Of course it is superior to design right in the first place, which is a situation which only applies to new construction. Many more of us are left to clean up the problems created by poor decisions during that original construction.

By: Armando

Armando — Thu, 03 Apr 2014 16:58:07 +0000

Most HVAC guys wouldn't know
Most HVAC guys wouldn't know what a turning vanes is or what it's function is if it hit them in the face. Ha...