Checking airflow

[Kymike]

What is the best way to check airflow for cfm? I'm considering an anemometer but not sure yet. I've read that you can check airflow with wet buld and dry bulb temps but not sure how that works.

[surenuff]

You can place the unit on heat and let it run about five minutes or so to reach a "steady state". Then use the formula CFM= BTU/1.08xTD
If it is electric heat, you can take voltage reading, measure resistance through the heat strips and use voltage/resistance to come up with amp draw. voltage times amp draw gives you watts. Watts x 3.41 gives you btuh. If it is gas, you need to know the effiecency of the furnace to get more accurate, but if it is a 100,000 btuh input 80% furnace, then you can just use 80,000 btu as the output rating and should get you in the ballpark. If you have the Mfg. info on the coil, you can measure pressure drop across the coil and get real close to by comparing the pressure drop to the literature on the coil. Increase pressure drop equals increase airflow. Hope this helps.

[54regcab]

How well does the pressure drop across the coil test work on older systems? I suspect the coil needs to be cleaned before an accurate coil pressure test can be ran?

[dsprice]

I would be curious to see more possibilities on this. Since Energy Star has become the 2 worst words in the English language this is what I have used to determine airflow...

First, I always use a transition between all components. Furnace, coil, media guard, etc...and set my blower speed according to manufacturer performance chart to deliver as close to 350 CFM/ton at .5 ESP as I can get. When system is up and running I compare actual ESP to chart to see what I'm getting and allow a max on gas fired furnace ESP of .6 and air handlers ESP of .8 Anything above I start checking at boxes and returns. I also like to see the static on the return to be 1/3 of the ESP.

[surenuff]

How well does the pressure drop across the coil test work on older systems? I suspect the coil needs to be cleaned before an accurate coil pressure test can be ran?

Yes, a coil would need to be relatively clean to get an accurate reading.

[heatingman]

For using pressure drop, to know actual airflow, you need to have a performance chart for something. Either the blower on the unit, or pressure drop chart for the coil both wet and dry.

Without either one of those, all TESP testing will tell you is .5 or less = good, .5 or more, maybe not good.

Temp rise is a simple test that gives you a real # to look at. (see surenuff)

But when/if you use the test, make sure the blower is set to cooling speed, otherwise you will be seeing the airflow in heat mode and not for cooling.

Performance charts are great, but on service calls, they don't exist.

A anemometer would be a good tool to have, especially fixing hot/cold areas caused by lacking flow to a particular area.

[Senior1]

Heat rise is good for ele heat fan coils except in the summer. If its 95 outside and you are troubleshooting, last I want to do is heat the customers already hot house up more. I have a electronic anemometer that measures FPM and then use a chart for R/A type. Its not perfect, but better than not checking it.

Like anything, its gets easier the more you do it. I remember not checking SH on electrical problem no cools, but now its second nature and shows a dirty coil quickly.l

I also like using product data and ESP when I can get it,, but usually don't have it.

As far as wet/dry blub, you got me there, sensible and latent capacities, yes, CFM? not that I know of. Of course a large TD will show you low airflow is a problem.

[talbot3]

When the ac is operating a change in enthalpy of 6.67 btu/lb across the evaporator coil indicates 400 cfm/ton.


Sent from my iPad using Tapatalk HD

[surenuff]

For using pressure drop, to know actual airflow, you need to have a performance chart for something. Either the blower on the unit, or pressure drop chart for the coil both wet and dry.

Without either one of those, all TESP testing will tell you is .5 or less = good, .5 or more, maybe not good.

Temp rise is a simple test that gives you a real # to look at. (see surenuff)

But when/if you use the test, make sure the blower is set to cooling speed, otherwise you will be seeing the airflow in heat mode and not for cooling.

Performance charts are great, but on service calls, they don't exist.

A anemometer would be a good tool to have, especially fixing hot/cold areas caused by lacking flow to a particular area.

That is sorta the reason I said he would need the literatue on that specific coil.

[surenuff]

Heat rise is good for ele heat fan coils except in the summer. If its 95 outside and you are troubleshooting, last I want to do is heat the customers already hot house up more. I have a electronic anemometer that measures FPM and then use a chart for R/A type. Its not perfect, but better than not checking it.

Like anything, its gets easier the more you do it. I remember not checking SH on electrical problem no cools, but now its second nature and shows a dirty coil quickly.l

I also like using product data and ESP when I can get it,, but usually don't have it.

As far as wet/dry blub, you got me there, sensible and latent capacities, yes, CFM? not that I know of. Of course a large TD will show you low airflow is a problem.

He was asking for ways to check without anemometer, as he has not purchased one yet.

[surenuff]

When the ac is operating a change in enthalpy of 6.67 btu/lb across the evaporator coil indicates 400 cfm/ton.


Sent from my iPad using Tapatalk HD

Good answer!

[energy star]

http://www.youtube.com/watch?v=f9wuKxsrQ7A&feature=g-user-a&list=PL396B06D5978D602F&context=G232d153UCGXQYbcTJ33Y_DTngkZbKQUtuj2PbROzS za0FUSUbYE0

[david912]

When the ac is operating a change in enthalpy of 6.67 btu/lb across the evaporator coil indicates 400 cfm/ton.


Sent from my iPad using Tapatalk HD

I would like to understand this a little bit better, would you mind explaining this, i guess i havent ever fully understood enthalpy, and how would you go about figuring the 6.67 btu/lb across the evap

[talbot3]

Enthalpy is a measurement of heat energy of the air. It can be calculated using
The dry bulb temp and relative humidity or looked up on the saturation curve. There are a number of apps for smart phones that will tell you the enthalpy if you know the dry bulb temp and RH.

The formula for calculating the capacity is capacity=4.5 * (enthalpy in - enthalpy out) * air flow. The 4.5 constant changes depending on elevation.

The capacity can be estimated by measuring the dry bulb air temperature change at the condenser coils using only the sensible heat equation sense no latent transfer takes place... Condenser fan flow rate should be in mfr literature ....1.08 * ( temp out - temp in ) * air flow.

Now all you need is dry bulb and RH at return before coil and supply just after coil to calculate change in enthalpy. This can be used with the capacity calculated at the condenser to solve for flow rate across coil. Flow rate = capacity / ( 4.5 * change in enthalpy).

Sent from my iPad using Tapatalk HD

[energy star]

He's measuring the wet bulb at the supply and return, then subtracting the supply wet bulb from the return wet bulb. His target number may be that 6.67 enthalpy difference between the two. I don't think it is the most accurate way to accomplish this task. Look at the video I posted, then look up the same guy showing the first two he made. You may understand it bit better. The first two are not perfect, but you will grasp it better. Getting the correct BTU will assure you the system is working as a whole properly.

[talbot3]

It's a ballpark estimate because enthalpy is very sensitive to moisture in the air and most Hygrometers measure RH +- 3 to 5 percent. Nothing is going to beat a flow hood or anemometer for measuring flow rate. But the enthalpy method can be used to see if there is an air flow problem.


Sent from my iPad using Tapatalk HD

[hvacrmedic]

I would like to understand this a little bit better, would you mind explaining this, i guess i havent ever fully understood enthalpy, and how would you go about figuring the 6.67 btu/lb across the evap

It's based on the formula

Btu/hr = 4.5 x cfm x ΔH

If we have 400 cfm and 1 ton of cooling, then

ΔH = 12,000/(4.5 x 400) = 6.666...

This isn't a very good way to estimate air flow. The factor 4.5 is in reality a variable, and it is rounded up from a (usually) smaller value, for simplicity. In addition the actual capacity at that moment in time must be known precisely in order to use the formula to determine cfm. The capacity is in turn typically estimated by first measuring air flow, which of course is what you're trying to determine. So in effect, you cannot determine air flow with this formula.

[54regcab]

Add that you also need to know airflow of the condenser to know capacity. If condenser is dirty, capacity will drop considerably.

[hvacrmedic]

Add that you also need to know airflow of the condenser to know capacity. If condenser is dirty, capacity will drop considerably.

There are a lot of factors that affect the system capacity. A dirty condenser coil has the same effect on capacity as a higher ambient temperature. Both will cause capacity to drop. Conversely, excessive condenser airflow, a damp condenser coil, or a lower ambient, can all increase the capacity. Capacity is also affected by the indoor wet bulb temperature.

It's easy to make the mistake of thinking that a 3 ton unit ought to be producing, oh, about 36,000 btu/hr. Under test conditions, and with a fairly new system with proper charge and proper airflow, this assumption would be ok even if not exact, but how often do we encounter test conditions and a new system together? Some 3 ton units don't even produce 36,000 btu/hr at test conditions, further complicating this process. The best thing to do is to consult the performance data for the unit.

A properly functioning 10 SEER 3 ton unit might produce fully 3.5 tons of cooling at 75° ambient with an indoor entering air wb temp of 70°. The same unit might produce only 2.5 tons of cooling at 95° ambient with an entering wb temp of 57°. On an older system the capacity might run lower overall than its specs call for.

I've attached a performace table so you can see these changes in capacity with indoor/outdoor conditions for yourself.

While a ΔH of 6.73 (this is the actual value if we use scfm and more precise psychrometric data) would be an ideal ΔH when 400 cfm per ton are desired, there are some complications that prevent us from attaining that goal. For starters, the system cannot actually maintain a given scfm per ton except under very specific sets of conditions. When the system is running at a higher capacity, the ΔH will be higher, when its running at a lower capacity the ΔH will be lower. The cfm per ton cannot therefore be constant. It can actually vary quite a bit after you leave it. The scfm will also change as the indoor conditions change.

What it boils down to, is that even with the testo 435 used in the video, you are not going to be able to attain a great deal of precision in your airflow adjustments. Not without consulting the performance data for the system, which the guy in the video didn't do. Even then don't expect the system to maintain 400 cfm per ton after you leave, cause it won't. This is the type of thing, I'm sure, that led to the coining of the expression "close enough for governement work." :) In a nutshell, you can't use ΔH as an indicator of airflow. It isn't reliable. 400 cfm per ton is a rule of thumb, and it is intended that you interpet it to mean 400 cfm per nameplate ton. The engineers who designed it understand that the actual capacity isn't a constant. 400 cfm per nominal ton is a compromise that will provide optimal performance under varying conditions. On systems with larger evap coils the value has been lowered in order to provide adequate dehumidification, but the rated cfm is still no less a compromise on those systems. The cfm must be measured directly and adjusted for the nominal tonnage, without relying upon the systems actual capacity in any way.


http://www.youtube.com/watch?v=f9wuKxsrQ7A&feature=g-user-a&list=PL396B06D5978D602F&context=G232d153UCGXQYbcTJ33Y_DTngkZbKQUtuj2PbROzS za0FUSUbYE0


http://hvac-talk.com/vbb/attachment.php?attachmentid=257431&d=1334728487

[dsprice]

I don't see how you can determine airflow without a magnahelix. I've never heard of checking across a coil as described above but will start looking at it in the field to see what I get and compare to ESP.

Sent from my ADR6350 using Tapatalk 2

[energy star]

The best way is a Flow Hood, Alnor or similar. Duct Blaster has a new attachment out also.

[surenuff]

The sad thing is, is that even the manufacturers of flow hoods and anemometers, and even duct blasters etc.... will all tell you that in the field due to varying factors your reading on airflow could be off up to 15%.

[david912]

Soo enthalpy is not a good way to check air flow. Either an anometer or magnahelic to test static psi would be better

[energy star]

I do not agree. They now take back pressure into account.

[surenuff]

I do not agree. They now take back pressure into account.

I read an artical recently that was talking about the percentage of error because of back pressure etc.. and the tests done to see if powered flow hoods were more accurate, and the tests showed that they both still had about a 15% error rate in the field. I was also at a recent HVAC seminar where a major manufacturer of air flow testing , and even he was talking about how their equipment is not as accurate as he wished it was. What equipment do you guys hold as the most accuarate. What do you use, or what would you say is the best?

[energy star]

http://www.aikencolon.com/Alnor-EBT721-EBT-721-Air-Balancing-Balometer-Flow-Capture-Hood_p_2549.html

Some specs on bottom of above link.