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  1. #1
    Join Date
    Jun 2006
    Posts
    110
    Research so far indicates the "garbage bag" method of measuring system airflow at registers is actually comparable to the high-tech devices that Joe HO doesn't have access to.

    I'm looking for four bits of feedback:

    1) Any tricks on how to implement the "garbage bag" method

    2) How to know what the total CFM out of a blower should be. I have a Bryant 355AAV042060 FA5A furnace.

    3) My understanding at this point: Under the ideal conditions (theoretical) if a blower is rated at 'x' CFM then if I add up all the CFM from each register when blower is at highest speed I will get 'x'.

    4) With my variable speed furnace( I also have a 3Ton AC condensor/coil) and it being the Evolution Plus90I model (but with a standard single stage stat, not Evolution stat), if the control board is set up to 400CFM/Ton (for AC) and I am running a 3Ton system, then the blower speed is going to be controlled to attain 1,200 CFM. If this is correct, then my 'x' (from #3 above) is '1,200' and my garbage bag measurements at all the registers should add up to that.

    Is this correct? My intent being to "calibrate" my garbage bag method in order to then perform a crude yet theoretically close "balance" of my system myself (I can't get the contractor to do this for me). I.E. if I know I shold be getting 'x' total CFM but my garbage bag only measures 'x-y%' then I can adjust my expected flows (from Manual J) by that same 'y%' in order to balance.

  2. #2
    Join Date
    May 2004
    Posts
    1,462
    Sorry but I have to say this, The Evolution control would give you the cfm.

    That said, measuring cfm at the registers is not the preferred way to do this...what about duct loss? This is a problem that is difficult to deal with, even for most pros. The best way is to take a good static reading at the inlet and outlet of the furnace coil and compare to the manufacturers pressure drop chart. Even this is a good estimate, not necessarily a absolute true measurement.

    You are too easy a customer. What do you mean your contractor will not balance the duct......make him do it.
    If all else fails....Try reading the directions!

    Tell it like it is and let the chips fall where they may.

    Any views or opinions stated here are strictly my own.


  3. #3
    If using a garbage were an accurate way to measure airflow then all of the professional's would not be spending thousands of dollars for a flow hood and instruments to do that.

  4. #4
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    >>Research so far indicates the "garbage bag" method of measuring system airflow at registers is actually comparable...

    I've gotta ask you what is this research. I have heard of this method from a Canadian source but have never heard anything good supporting it from this forum. And I think whenever you have a concensus on this board, they tend to be right. For one thing you must measure the seconds to fill, and the precision of your stopwatch will limit the precision (and therefore accuracy) of your CFM figure. If you go ahead and do some measurements this way, I am sincerely eager to hear how it turns out! But in the meantime I have to wonder if there could be better methods.

    The one super-amateur method I know of, would use a cardboard box placed over the supply vent, with a measured hole for air to exit. You need the hole large enough that it won't disrupt total airflow much, and small enough that you have a measurable pressure difference between the box and atmospheric. Then you might use a manometer to measure that pressure, and there are equations relating that to airflow. Everything has its drawbacks and this method would certainly be more complicated than the fill-a-bag method. And I have not done it myself, so who knows whether your method might be more practical.

    Dwyer makes a pitot tube for use with its manometers, which could be used to measure air speed. By taking various measurements you could get a semi-good measurement of CFM, the pros have a method for this and I better let them describe it. If you are interested of course.

    But to get really *good* measurements, the pros use flow hoods costing thousands and those must be calibrated frequently. I really don't think they would do this if a gas-bag method could be developed that would give good enough results.

    Just expressing my questions on this. By all means I ask you to share your experiences and results on this board if it does not run afoul of the DIY rules.

    Best of luck -- Pstu

  5. #5
    Join Date
    Aug 2003
    Location
    Central Kentucky
    Posts
    6,247
    Originally posted by pstu


    But to get really *good* measurements, the pros use flow hoods costing thousands and those must be calibrated frequently. I really don't think they would do this if a gas-bag method could be developed that would give good enough results.

    Exactly.
    Have you set up a Google alert for Carbon Monoxide yet?
    Click here to find out how.

  6. #6
    Join Date
    Sep 2006
    Posts
    91

    Are we talking Gas or Heat pump?

    Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

    Cfm = BTU out / 1.08 * delta T

    To help populate the equation...

    Watts = Amps * Volts
    Electric strips put out 3.413 BTUs per Watt.

    Same thing works on a gas furnace but you have to clock the gas meter and know the BTU value of the fuel gas. Or you can get close by making sure the gas pressure is correct and then going Input Btu's * AFUE but that's your call.

    -Brent

  7. #7
    Join Date
    Apr 2004
    Posts
    7,680
    Reading what comes out of the registers and what goes through the coil are generally two different numbers as a result of duct leakage.

    The feedback from the ECM is based on blower perfromance curves, torque and RPM, it doesnt read CFM, but it doesnt need to either. As with any ECM, assuming it is working properly, as long as you are below an inch, you should get what you ask of it.

    You can often calculate airflow with a PSC provided you knew the watts and had the blower performance charts to match. You can use the temperature rise method and get close. The static pressures will also tell you airflow if you have the charts. If you wanted a run by run calculation, then accurate velocity pressure and duct size will give you what you need. There are many ways to get close mesurements, and no good reason not to.


  8. #8
    Join Date
    May 2004
    Posts
    1,462
    Originally posted by docholiday

    The feedback from the ECM is based on blower perfromance curves, torque and RPM, it doesnt read CFM, but it doesnt need to either.
    .

    While this is technically true, the Infinity/Evolution control can provide a CFM reading from this information. We have checked both the CFM reading and the static reading many times and have always found them to be very accurate.

    The OPs furnace is compatible with this control and if he had it installed he could see the CFM output. Of course if he had the control it would automatically set the proper airflow settings for his system and he would not have to be worried about proper settings of the dip switches. Now duct balance is a different problem, so far I do not believe there is a control for that.
    If all else fails....Try reading the directions!

    Tell it like it is and let the chips fall where they may.

    Any views or opinions stated here are strictly my own.


  9. #9
    Join Date
    May 2004
    Posts
    1,462

    Re: Are we talking Gas or Heat pump?

    Originally posted by stafford
    Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

    Cfm = BTU out / 1.08 * delta T

    To help populate the equation...

    Watts = Amps * Volts
    Electric strips put out 3.413 BTUs per Watt.

    Same thing works on a gas furnace but you have to clock the gas meter and know the BTU value of the fuel gas. Or you can get close by making sure the gas pressure is correct and then going Input Btu's * AFUE but that's your call.

    -Brent
    This is a fairly accurate method for calculating CFM on PSC motors, but not on variable speed motors.
    If all else fails....Try reading the directions!

    Tell it like it is and let the chips fall where they may.

    Any views or opinions stated here are strictly my own.


  10. #10
    Join Date
    Sep 2006
    Posts
    91

    Re: Re: Are we talking Gas or Heat pump?

    Originally posted by plain spoken
    Originally posted by stafford
    Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

    Cfm = BTU out / 1.08 * delta T

    This is a fairly accurate method for calculating CFM on PSC motors, but not on variable speed motors.
    I don't know a thing about carrier stuff but I do know that
    physical laws don't change just because you have a variable speed drive. The equation always works because it is based on thermodynamics and independent of motor type unless you're leaving the door off the equipmet while you measure or some other obvious procedural mistake.

    DocHoliday is exactly right about how variable speed drives work, they just look at a fan curve based on torque and rpm programmed into the black module sitting on the end of the motor. They're really great, but not magic.

  11. #11
    Join Date
    Jun 2006
    Posts
    110
    Contractor keeps telling me he did balance the system, that holding his hand up to the register and feeling the air flow is the standard methodology used by all HVAC people in this area and I won't find anyone who would do it any differently. I simply have to wait until it gets real cold then run the heater then go sit in each room. If a room is cold I increase flow to that room. If a room is hot I decrease flow to that room.

    Interestingly, when I then ask him why the end result of his "balancing" was that all dampers on all ducts are wide open his answer is that the installation of the ducting was obviously perfect. The ducting was perfectly sized and perfectly run. That argument then tells me that either the system is perfectly balanced or I'm screwed because I'll not be able to increase flow to one room without decreasing flow to another.

    So I appear to only have one option, to figure out as close an estimate of the current state of balancing (what each register is putting out) based on Manual J and then show this to the contractor (at which time of course I will have balanced the system myself).

  12. #12
    Join Date
    Apr 2004
    Location
    where you at?
    Posts
    3,359

    Re: Are we talking Gas or Heat pump?

    Originally posted by stafford
    Easiest way to calculate air flow is to run the airhandler at the fan speed (e.g. cooling speed, etc.) you are curious about with the electric heat on. Measure the delta T across the equipment out of line of sight of the strips, the volts and amp draw of the electric heat only (not the blower).

    Cfm = BTU out / 1.08 * delta T

    To help populate the equation...

    Watts = Amps * Volts
    Electric strips put out 3.413 BTUs per Watt.

    Same thing works on a gas furnace but you have to clock the gas meter and know the BTU value of the fuel gas. Or you can get close by making sure the gas pressure is correct and then going Input Btu's * AFUE but that's your call.

    -Brent

    wouldn't it be easier to get a flow hood or pitot-traverse of the duct?
    what is the % error of the temperature gradient 'method'?


  13. #13
    Join Date
    Jun 2006
    Posts
    110
    First, some links for anyone else interested in violating the covenant and playing around with the "garbage bag" method of air flow measurements:

    Here's a very good study as to the accuracy of slow measurements in general, including the method under discussion here. Interestingly, it proposes that all the many thousands of dollars being spent on high-tech flow hoods and such may well be little more than show-boating with little net worth (sorry for the long link, but it is worth it for those truly interested):
    http://eetd.lbl.gov/ie/pdf/LBNL-5155...omeowner%22%22

    This one is simply a useful place for when you want to figure out the volume of your garbage bag, convert cubics, etc.:
    http://www.metric-conversions.org/volume

    This is the "source" of the method:
    http://www.cmhc.ca/en/co/maho/yohoyo...inaiqu_003.cfm



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