I’m installing HRV ducting in our house and would like to measure airflow at various points so that I can balance the flow. Wondering what instrumentation could be used for a DIYer to get reasonably accurate readings without breaking the bank? Any suggestions?
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Replies
http://www.dwyer-inst.com/Products/Product.cfm?Group_ID=173
That measures velocity ... not volume. But given equal air grills, equal velocity would be theoretically, more or less, equal volumes, I suppose.
CFM = A x V
The little Dwyer will get the job done for less than $50.
trouble is the 'A' of the equation ... maybe easy by just looking up the net free area of the manufacturer. Also not as easy as it sounds is the velocity since velocity will vary greatly across the grill/register. And you do need the velocity at the grill or it won't be accurate.
I'm not saying it may not be doable and a useful approach. Just that one needs to know the limitations to make good decisions and use the tools he is handed.
Why not keep it simple and measure the airflow in the duct before the register? Maybe not 100% accurate, but if the flow is in the pipe, it's gotta be close to the same at the register. Velocity may change, but the volume should remain fairly constant.
The register AK factors are readily available from the manufacturers.
This isn't a certified test and balance job.
I suppose so. Remove both grilles and measure velocity. Balance to the two exhaust streams and he'd be good to go.
a few ribbons tied to the grill... you'd be amazed how much you can do when you can "see" the air...
strips of tissue paper taped to the registers works also...
now before i'm blasted on this... i know it doesn't measure airflow... but you'd be amazed you think a register has good flow and you can feel it... but when you can "SEE" it it's amazing that you can see what one change in a damper over here does to this register over there....
I've spent hundreds of hours on a flow bench working on and testing air flow for automotive parts... so i kinda have a grasp about how air moves... pretty amazing how a small detail can affect much...
I use my method in my small office building all the time... i don't know about "balance" but i know i can direct the air where i want it... still leaving some in other places...
p
Normally done w/ an air flow hood by a Testing Adjusting and Balancing (TAB) person. Relatively an expensive, albeit simple device. But the other guys suggestion ... about the paper isn't too bad. Do you have some way of balancing if you find an imbalance? i.e. do you have balancing dampers in the system? Are all registers the same size/type/configuration? Do you know the cfm flow you would like at each outlet.
Use a "volumetric capture hood", which is foolproof and cheap in spite of the fancy name.
You rig up a way to cover the outlet with a 35-gal trash bag (or smaller, if needed), you smash the bag flat against the airflow with a friend standing by with a stopwatch. When he says GO, you let the air fill the bag, and you say STOP when it's full. The elapsed times with the various outlets will let you compare the flows.
With this method you can even calculate the exact CFM if you need to, but you seem to be looking only for balancing between the various outlets, so you don't really care what the CFM is.
Tip: Use the thinnest trash bag you can find so it doesn't impede the airflow. Make a simple frame to hold the mouth of bag in place over the register, and use a large piece of cardboard to smash the bag flat till you're ready to release it.
Lots of ideas, thanks.
What I am trying to balance is our HRV. I am drawing air from 2 bathrooms- the first is adjacent to the unit and the other is about 35-40 ft away. I need to achieve a certain CFM from both bathrooms (50 cfm min). Some have told me this is do-able, others have said not likely...so I want to rough in some ducting and see for myself.
Shawn
If you want to measure exhaust, the same setpup will work.
Just inflate tha bag first and record the time it takes to deflate.
BTW, it's almost certain that the farther intake will be weaker. You'll likely need some sort of damper at the inlet closest to the fan. You can make one by cutting a disk of sheetmetal slightly smaller than the ID of the duct, but leave a tab or ear of sheetmetal on one side. Use this tab to pop-rivet the disk to the inside of the duct near the register, and then simply bend the disk/tab to close off as much air as needed.
Edited 9/28/2009 10:09 am ET by rdesigns
Brilliant! A little more caffeine and I might've figured it out on my own :)
So, if I blow up a 35 gallon garbage bag, it should deflate in less than about 5.5 seconds if my duct flow is 50CFM, correct?
Shawn
yup
Now you are giving us more to work with! Since you have a huge disparity between the location of the two exhaust grilles, you should be concerned. Make sure you place a standard balancing damper in the exhaust duct closest to the HRV. You can't do anything w/ out that. When you do balance it, mark the damper handle position on the duct.
You are primarily concerned with the exhaust balance. What CFM of HRV are we moving on the exhaust side (at least the unit rated CFM)? If you just have the two outlets and you want to balance the CFM, you might calc the air flow using duct SP loss w/ the rough configuration you have and then make a good guess at setting the damper. Remember half damper shut (e.g. 45 degrees) is NOT 50% air flow ... it is not linear. 50% shut may still flow 70% air.
Technically, this is a tricky one given no real tools to measure. But if it were me, I'd just make a quasi semi educated WAG and set my damper based on the proportional size of the room to try to ensure good air flow in both. The one furthest from the HRV is going to have the most difficulty getting flow.
Hope you didn't use flex duct.
Wow! How simple sounding and logical. I'm still a bit skeptical. How do we know the volume? If the bag is made for e.g. a 30 gal trash can, that doesn't mean it is necessarily 30 gallons in our application.
Also on the exhaust side, you make it sound so simple. How do you fully inflate it before starting your test? I'm even more skeptical about that part of the test.
Very creative, though. (but I'm still a bit skeptical).
There is no method I know of to precisely measure CFM--even flow hoods are not perfect. Velometers and hot-wire anemometers are even less accurate. If you can get within 10%, you're doing well, and the intent of balancing is to do just that: balance the airflow so that each space receives its proportionate share of the air.
But the OP is looking for relative accuracy, I believe, along with reasonable balance. The method I described will do the job.
You can achieve really consistent supply-side accuracy with it if go to the trouble of making something like a laundry basket that contains the bag within a known volume--say, 2 cu. ft. When the deflated bag fills it, you know you have 2 cu.ft.
And, BTW, I like skeptical--we need more of it in the HVAC profession. Too much is taken for granted.
There are methods to measure ACCURATELY cfm air flow with a calibrated flow hood. Precision is generally not required, accuracy is often required. You are right, too that velometers and hot wire anemometers are fairly inaccurate and generally considered the wrong tool for the job. They give you an idea of relative air flow and that is about it. Much like a laser thermometer ... gives you an idea the general temperature, but NEVER take the reading 'to the bank'.
And you are right ... relative accuracy is all that is really necessary ... he just wants to avoid exhausting 15 cfm in one bath while the other is cranking 350.
Too many 'rules of thumb' used in HVAC ... and too many quasi half baked theories based on quantum leaps from solid science. Just read an interesting article in ASHRAE Journal ... VERY controversial and opposing views on the same science subject matter ... thought it unusual for that to happen w/ HVAC engineers.
But I'm digressing 'a little'.
Lotsa options.
Ponytl's suggestions are the simplest, and probably adequate for your use.
Are you going to do a full set of calculations to start with (e.g Manual J or your own spreadsheet, etc) to show the BTU per outlet needed, or just trying to get things comformtable.
If you want to do all the calculations, here is what a simple DIY accurate test set for what you should be able to cobble together out of the house, assuming you have a few electrical meters. If you dont have that type equipment, just 'go with the flow' as evidenced by tissue paper streamers, or go to the last paragraph.
1. You need to determine your blower performance curve. The website for your furnance or HP should have enough info to derive that curve. If not, you need to put an electric heater in your duct after the blower, measure blower power and heater power, measure inlet and outlet temperature, calculate the airflow based on thermal balance.
Lost already, then go back and use the tissue paper method.
2. Repeat 1 at at least 4 different flow rates (you will need to change the pulley or block some duct outlets or inlets. Now ploy your parabolic fan curve.
3. For each of the above tests, you should have already made yourself an anemometer out of an old toy propellor and motor. This is simply something like a battery powered toy airplane motor and prop. take the batteries out of the plane and install a resistor to dissipate the rated power of the plane (amps times battery voltage) and put a voltmeter across the resistor.
4. Now you have an anemometer that puts out a voltage proportional to the n-root of the airpeed. You will have 4 different voltages from the tests in #2. You will know the air volume (based on duct cross section area and 2 or 3 iteration of reynolds number) from the energy balance equations from the data of #1. With the set of 4 equations, you can now calcualate the n-root for the voltage and have a chart that pretty accurately plots voltage vs. air speed in the cneter of the duct.
The above is a variation of techniques using caibrated equipment to balance airflow to electronic equipment racks on airplanes.
If all the above is too much, just eyeball the speed of a pinwheel for comparisons or resort to the tissue paper example.
Have fun.
BTW, in my own house except for intial setup (using a pitot tube and manometer,which you dont want to hear about), I resort to something even simpler than tissue paper streamers --
--- simply wet your hand with a damp cloth and hold it in front to the airflow - cooling effect just like wind speed factor, easily readable off a wind chill chart , accurate if you use a wet bulb thermoometer.. (now, why didn't I just say that in the first place <G>