Is there a way for a novice to measure air flow through registers at home?

[SliderJeff]

Hey guys,

I live in Houston and own a two story home. Here's my setup:

A/C Model (2): Armstrong Concept 1200

Heater Model #1: G1N80AT075D12A Armstrong Ultra V Tech 80
Heater Model #2: GHJ075D16 Armstrong Ultra SX80

3.5 Ton Unit Downstairs
3 Ton Unit Upstairs

I've lived here for 8 years and we've always had issues with some rooms being hotter than others, including both of the kids bedrooms (upstairs) and our master bedroom (downstairs). I've asked my HVAC guy to look into balancing the house correctly, but he's always put me off as saying things like "it is balanced, it's just the nature of which rooms are located where in reference to the sun, as well as how far away they are from the supply". I'm not sure I believe him. It seems to me that I should be able to do SOMETHING by adjusting the cheap little dampers inside some of the vents, or at least partially close off some of the rooms that get cooler than other in order to give a little more cooling OOMPH to the rooms that are warmer. Please correct me if I am wrong here.

I'd like to do a qualitative comparison of each of the rooms by measuring the actual flow rates out of each of the vents. Is there a way that I can do this as a novice homeowner, or am I at the mercy of whomever I call out to do the measurement for me?

Thanks,
Jeff

[hj]

Unless you have an enclosed stairway with a door on it, the upstairs will ALWAYS be hotter than the downstairs. Warm air rises and it will accumulate upstairs, while the cooler air from the upstairs flows down to the first floor. It is simple physics and the only way to minimize it would be for the first floor's return duct to draw air from the second floor's ceiling where all the hot air is. It has little to do with register air flow. There is no way for the lower level's unit to circulate, or keep, its air away from the second floor.

[FloridaOrange]

If you have real issues that you think are due to balancing your diffusers look for a Test and Balance company that's local. We require contractors to get independent T&B work done on all our HVAC jobs.

[jimbo]

Actually, I have been looking for a simple air flow guage, a so far on the internet have not found anything under a few hundred bucks. If anyone finds something cheaper, let us know!

[Dana]

HVAC grille air flow measurement methods for the Compleat Cheapskate:

http://www.builditsolar.com/Referenc...lbnl-51550.pdf

http://www.builditsolar.com/Referenc...lbnl-51551.pdf

[jadnashua]

How about a small hand-held air speed indicator. The one pictured retails for $50. You'd need to look up some info on air density and the surface area of the register, but I think you could use this to figure it out.

[SliderJeff]

Unless you have an enclosed stairway with a door on it, the upstairs will ALWAYS be hotter than the downstairs. Warm air rises and it will accumulate upstairs, while the cooler air from the upstairs flows down to the first floor. It is simple physics and the only way to minimize it would be for the first floor's return duct to draw air from the second floor's ceiling where all the hot air is. It has little to do with register air flow. There is no way for the lower level's unit to circulate, or keep, its air away from the second floor.

HJ,

Just to clarify. I am well aware of the physics of hot air rising. The issue I am having is that one of the bedrooms on the upper floor is significantly warmer than all other rooms on this floor. The return air ducts are located in the ceiling on both floors. So it's hot air being returned back to the unit, regardless of which floor we are talking about. That's just how they do it in Texas from what I have seen. Up in Ohio where I grew up, the cold air returns were at floor level, as were the registers for heat and A/C. My best guess is that they plan on more cold than warm days in the North where it's just the opposite in the South.

Anyway, thanks for the replies, everyone.

[Runs with bison]

I've been working on some balance issues of my own so I can relate to your problems. I've got one corner of the house that runs quite warm in summer and cool in winter. It makes sense though: 1. It has only one register upstairs and downstairs on that corner. 2. The runs are long. 3. There are many windows in this corner. 4. There is likely some missing/damaged insulation.

An R3 window is going to result in a lot more heat flux than an insulated wall. In my case I'm considering tearing out some sheetrock to see if I can add another register in this corner on both levels. Eyeballing it I expect the losses in this corner to be about double that of other areas served by a single register.

Other issues are things like making sure the windows are tightly sealed, walls are actually insulated, etc. Often the ducts are leaky too, so if you can seal them you might get more air into and out of the end runs. If you can access them you might also consider insulating the supply ducts to these registers. I've been able to do this for some of my other runs--particularly one running through an uninsulated storage space.

Another thing that you can consider to improve room comfort in AC season is to convert to CFL's if you haven't already. Incandescents are like running a small space heater. PC's and TV's in rooms can add a lot of heat as well, I doubt they were factored into the original register placement/sizing.

I've experimented with an inexpensive handheld anemometer by La Crosse. It is tiny. The main problem is that it is hard to get it turning initially at low velocity. My solution was to remove the grate and stick my arm inside so that I could hold it in the center of the round duct where the velocity is highest. It's not a precise way to do things but it gives some scale when comparing the same duct sizes elsewhere the same way.

[nhmaster]

Why not start cranking down the register in the hot room until it's where you want?

[Runs with bison]

Why not start cranking down the register in the hot room until it's where you want?

Not sure what you mean if you are addressing me. His problem sounds similar to mine. With the AC running and the register full open in a given room it is not cooling well enough in that room. There's nothing left to adjust there. The only way to get more air in that room is to raise the whole system pressure by pinching down the other 90+% of the registers. There's only so much cranking down on other registers that makes sense to balance a problem section. At some point the whole system will be pinched down too much cutting total system capacity and hurting efficiency...and still without properly balancing the home. One of the problems with pinching the majority of the system back to balance it is that the overall duct pressure will be higher resulting in more losses via leaks in the ductwork.

Mine is a moderately large home with a lot of vertical half levels, a single unit and no zones. I manually zone it each season by shutting off nearly all the lower story registers in Summer (don't need any cold air down there to keep it comfortable.) In Winter I open the lower story registers up and pinch back on the upper story registers to balance each room. This works all except for the corner mentioned which simply cannot receive sufficient air (it actually is okay in the bottom corner in summer because the whole level stays cool even with the registers shut.)

If the problem section balance was only off by 50% or so then some rebalancing could get in the ballpark, but if one instead needs to double the flow in an end line, that requires about four times as much pressure drop. The proper solution would seem to be to add duct/register area where it is really needed. Otherwise it becomes a tail-wagging-the-dog problem.

It is possible that the OP has not been aggressive enough in pinching back some of the other dampers to try to self-balance the system.

[Thatguy]

Tape a large garbage bag over the register and time how long it takes to fill.
If you measure 100 CFM and the duct is 0.2 sq. ft. in area then the airflow is 500 FPM.

[Hube]

Tape a large garbage bag over the register and time how long it takes to fill.
If you measure 100 CFM and the duct is 0.2 sq. ft. in area then the airflow is 500 FPM.

WOW!!!!
That(snicker) method should give actual (accuracy)??? to perhaps anything between 30 to 80 %.
This is why they make anenometers, velometers, and other highly accurate air measuring instruments.And you don't have to buy them, you can rent them for a day or so at a very fair price.

[Thatguy]

WOW!!!!
That(snicker) method should give actual (accuracy)??? to perhaps anything between 30 to 80 %.
This is why they make anenometers, velometers, and other highly accurate air measuring instruments.And you don't have to buy them, you can rent them for a day or so at a very fair price.

I'd like to claim credit for this brilliant idea but I read it in a Guv. report somewhere on the Internet.

I've never tried it but my concern is that, at 7.5 gals/cu. ft. the bag would fill so fast you'd need a stopwatch to measure the filling time.
And, since air is compressible the accuracy of this method will definitely not be 1st class.

On the other hand, the average HO may not need much accuracy.


Ask and ye shall receive! \/ See page 9
http://www.builditsolar.com/Referenc...lbnl-51551.pdf

[Runs with bison]

The trash bag method should be sufficient for qualitative comparison. And, yes, it would require a stop watch. What is important in this is to determine the relative flow of vaious runs, not the precise flowrate. Bag tests are not one of my favorite ways to measure things, but sometimes it was the only way practical in an industrial environment with very low pressure systems similar to HVAC air handling--and those were for quantitative measurements.

Hube, it's not like the HVAC folks doing typical installs are precise in the first place. The duct layout obviously isn't well considered (if at all) in many applications. I've seen the most god-awful, half-assed work from these clowns. My most recent conversation with a vendor had them asking why I was sealing up all their leaky ductwork, they claimed it wasn't necessary for the ducts to be tight. Boy, did it every show in their install. The leaks around the air handler/AC/furnace discharge were blowing hard and had been for 14 years before I bought the place. It's amazing how much better the AC works just by getting the air to stay in the ductwork on the way to its destination.

[Thatguy]

The duct layout obviously isn't well considered (if at all) in many applications.

And I hear now that some HVAC units or models are more tolerant than others of a bad install so you can use this as a tie-breaker when choosing a new unit, assuming the manufacturer will tell you this info.

While I have y'alls attention do you have any HVAC lifespans to add to my list [below]? The bigger the sample size, the more accurate my inferences from the list.

HVAC life in years
1
7
7
10
10
12
12
13
15
15
15
18
18
20
20
20
20
20
20
25
30
30
35
50
56

How 'bout lifespans for elec. [not gas] water heaters?

BTW, according to this book
http://www.amazon.com/Mechanical-Eng...1573598&sr=8-4
for round ducts and laminar flow you only need to take one measurement in the duct center. This book is probably in your local library and is a lot easier to get than the ASHRAE or the ACCA books.