Air flow Effects on Air Conditioning – Revisited

In my last post, we discussed the importance of keeping coils and filters clean and their effect on operating cost and longevity of the equipment.  We concentrated mostly on the outdoor condenser coil.  Now we will look at the effects of air flow on the indoor coil.  In one of my very early posts, I addressed proper indoor air flow for air conditioners. I have “revisited” this subject and added a little more information to that original post to hopefully help techs make proper diagnostics.  With this summer’s record-breaking heat, too often, techs just look at one “symptom” and don’t really find the problem because they know they have 10 more calls to make that  day and the boss has told them to, “get it going, get them while they are grateful, and get on to your next call.”

An air conditioner is not just “BTU’s IN A BOX” sitting out in the back yard.  An air conditioner is part of a SYSTEM, and that system consists of the outdoor unit, the line set, the indoor coil, the duct system, AND the indoor blower.  All of these need to be properly sized and operating for the air conditioning system to function properly.

Probably, one of the most overlooked operating components of an air conditioner is indoor air flow, yet this is critical to the proper operation of any air conditioner.

Ideally, air conditioners need 400 CFM (+/- 50 CFM) per ton of cooling to provide proper heat exchange and efficiencies of the system. Not enough air flow OR too much air flow can cause problems with the system.

When there is not enough air flow through the evaporator or a restriction in the air flow, the suction pressure is below normal because the refrigerant flowing through the evaporator picks up less heat than normal resulting in lower pressures.  Some of the most common air flow restrictions are dirty filters, dirty blower wheels, dirty coils, under-sized duct work, obstructed grills and registers, and duct leaks.  If this is not corrected, liquid could be returning to the compressor which will eventually cause compressor failure.

Too often, a technician will find an A-coil that has ice all over it.  They will “thaw” it out and then put refrigerant into the system without checking the air flow.  They add refrigerant to get their “suction pressure” above 35 degrees so the coils doesn’t freeze again. In mild weather, the unit will probably run but will be working harder than it has to because it is probably over-charged now.  When we get outdoor temperatures like we are seeing this summer, all of a sudden, the “over-charge” is now causing the unit to either short cycle on the high pressure switch, or lock-out if the unit has a manual reset pressure switch or a control board that will lock out the unit if the high pressure switch opens. There probably was nothing wrong with the charge in the first place but, too often, techs will add refrigerant to a system as a “band-aid” because they did not find the cause in the first place and felt they had to do something to justify their service call to the customer.

One thing to keep in mind, unless there is a leak in the system or a history of leaks, refrigerant does not disappear or wear out in a system.  If a unit has never had a history of leaks, the very last thing you should ever do is “adjust the charge” in an air conditioning system.  Most problems are caused by something external to the system — dirty coils, dirty filters, dirty indoor squirrel cages, motors failing, restricted driers, etc. There are only 3 things that can go wrong on the refrigerant side of a system, over-charge, under-charge, and non-condensibles — all other causes of refrigerant pressures being “out of whack” are usually caused by something external to the system.

Now, when there is too much air flow through the evaporator coil, the system will experience low sub-cooling, high discharge pressures/temperatures, and high saturated suction temperatures. The increased load on the coil transfers too much heat to the refrigerant. More refrigerant is vaporized, elevating the temperature and pressure.  The hotter refrigerant entering the condenser requires more of the condenser’s surface to reject the heat. More condenser is needed giving less room for sub-cooling and this equates to a lower sub-cooled refrigerant.

Here again, a tech will sometimes remove refrigerant from the system, thinking it is over charged due to the higher pressures gauge readings only. If the tech looked at super-heat and sub-cooling, along with the pressures, he would have realized that something else is going on with the system.

When sub-cooling is too low, flash gas can occur in the liquid line which and can cause noise in the system, improper metering at the TXV or orifice, fluctuations in the system capacity, performance issues, even higher discharge pressures causing even lower sub-cooling.  It becomes a “vicious circle”.

To properly check air flow, a magnahelic or manometer is needed to measure the E.S.P. (external static pressure) of the system.  Two reading are necessary – one after the filter in the return and one before the coil in the supply plenum.

  Once this value is known, the use of the blower performance charts in the installation instructions can help determine the amount of air flow through the system.  Most manufacturers size the CFM capacity of the blower based on one half (0.5 IWC) inches of water column.

Looking at the above chart, find the static reading you had on your magnahelic, find the model number of the furnace and you can see , based on what speed tap you are using, just how much CFM you are delivering.

As you can see, maintaining proper air flow in an air conditioning system is very important to the proper operation of that system.  Don’t overlook this when diagnosing problems.  Air flow is critical to the proper operation of an air conditioning system. And remember, adjusting the charge should be the absolute last thing you ever do to an air conditioning system.

After my first post on this subject, I received a comment and felt it was important enough to include here:

“cfm will make or break your system in the short-term and long-term. We need to get it right. As Ron Popiel use to say ‘set it and forget it ‘ only if it is right .” (Travis Keyser -UPG Tech Support)


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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18 Responses to Air flow Effects on Air Conditioning – Revisited

  1. Steve Oetker says:

    Congrats on your retirement, we usually joke that the guys on the corner with the cardboard signs are ex refrigeration/ac mechanics, maybe there is hope, looks like your doing great.

    • thanks for the comment. No “cardboard sign yet”. I still keep my hand in the business doing some consulting and training. We can never “retire” completely. It is nice to be able to pick and choose what I do and where I go though. :>)

  2. Shaun Hand says:

    I just found your site and found it very informative,thank you

  3. Maybe you can help me understand something about my apartment HVAC unit,. It is a decentralized split unit, with the indoor coil part mounted over the apartment’s water heater. It’s, shall we say, an elderly unit (the complex is over 20 years old). There has been a recurring problem with condensation from the cold coils dripping down onto the top of the water heater and accumulating in the drip pan under the water heater (thank goodness that is in place!). The apt. maintenance guy comes out when I call in a panic and wet vacs the water out of the drip pan and cleans the coils with that spray stuff. I live in Texas, so I have to call every other week or so. Anyhoo, the most recent “fix” has been to put a heavy cardboard rectangle to catch the drips and guide them into the drain pipe. Sigh. It doesn’t seem to be a sensible solution to me – wouldn’t the cardboard get too wet at some point and possible degrade into the drain? Doesn’t it block the air flow and make the unit work harder (and so I get to pay higher electric costs)? Keep in mind it’s still reaching 90° here during the day! Any information you can give me would be MUCH appreciated.

    • When an air handler is installed at hte ceiling or above the ceiling in the conditioned space, most codes require an auxiliary drain pan that is piped to a drain. This is to prevent damage in the case it does leak. What they should look for is where the water is coming from. Is the existing drain pan by the coils leaking? is there a problem with the drain?
      You are correct about the cardboard pan — it should be metal or plastic and still can be piped to a drain.

      • There IS a drain pan; it’s just that the cold coils are tilted at an unusual angle – like the right-hand down stroke of the capital letter A. So when the coils get a little bit dirty again, the condensation at the very top can’t run down to the drain pan but just drips straight down and misses the drain pan. I’m thinking the problem is that they are not REALLY cleaning the coils; just spraying until the side you can see is clean. As far as I know, they’ve never taken the coils off and cleaned the back side. There a few fins that are bent, too, which suggests to me that the refrigerant coil will malfunction at some point. Is my guess about the air flow being blocked correct, too? Will my electric costs be higher if the cardboard is reducing air flow?

      • all a/c coils have a drain pan. an auxiliary pan outside the unit is now code so if the coil does leak, it is there to catch it just like the one under you hot water tank. without knowing how the “cardboard” pan is installed, if it is blocking air flow, will not increase electric cost but could cause the coil to freeze up and cause potential damage.
        if the water on the coil is “falling” off the coil instead of “streaking down” the fins — yes the coil could stilll be very dirty or the blower is blowing too hard blowing the water off the coil or a combination of both.

  4. John Stead says:

    I do not have air conditioning but I have geothermal system which in the summer circulates cold water through coils with air forced through the coils to supply cold air. I understand that the cold water supply should enter the coils on the leaving air side of the coils opposite to the fan. My system is connected backwards the cold air enters the coils on the fan side. My question is how much more efficiency will I see if I reverse the connections and install the water input on the leaving air side? We have 32 units in our condo complex if half of these are installed backwards it seems it would be worth changing if we could increase efficiency for the overall system.

  5. G says:

    Hey there! I found this article searching google for ‘too much air flow causing a/c freezing outside unit’. Maybe you can lead me in the right direction? Two techs have come out with no luck on how to fix issue, but I feel it’s simply an oversized unit after reading your article…

    Home is in Orlando, Florida, starting to heat up now that summer is here.

    2,400 sq ft house build in 1989, built for what I’m guessing a 3-4 ton for 2,000 sq ft, and probably a 1.5-2ton for the two side bedrooms and bath totaling around 3-400 sq ft…since you can see in the attic where there were two air boxes and two returns…welllll

    Well after buying this foreclosure we see there is one 5ton plopped right in the middle where two a/c units used to be. Very low airflow to the two side bedrooms because the ducting in the attic was hacked together with flexible ducting twisting and turning all over. The side bedrooms have a hard time getting below 80. So basically a 5ton cooling the left side 2,000 sq ft of the house. Must be overkill…

    I have a feeling too much airflow is causing this issue. The outside unit freezes over at the pipes entering/exiting it. It stays running outside, but the air handler inside shuts off, and you just hear noise in the lines coming out of it. Also, very cold air radiating from the airbox/return area. This just occurred again today after I changed the air filter(allowing even more airflow).

    Does this sound accurate? Appreciate and tips/info

    • Units icing up is more likely to be caused by not enough air rather than too much air. From your description of the duct work, not enough sir movement is your problem.

      • G says:

        Yea this is tough, I opened up the restriction the bedrooms and it went away the past few days. So does lead to believing it was not enough flow, but it still occurred one night after I did all that. It’s very random and I can’t pinpoint when it will happen. What’s odd though is all the noise in the lines from the air handler. Also when it happens, the outside unit is still running, with the lines entering and exit frozen, but the inside handler is off and hissing pretty loud. There is very cold air radiating from the return by the floor…and seems like it would continue to stay like this for hours on end if I don’t turn the system completely off…Something just seems off, even if the flow isn’t restricted at all…and I can’t trust the system to work properly while I’m away. Any tech I have some out will say everything is fine, but I know it isn’t…any other thoughts?

      • If the outside unit is running, so should the inside unit. If the inside unit is off, you could have an intermittent blower motor, bad fan relay, loose thermostat wire connection for the fan, etc.. Your dealer needs to find out why the fan shuts off with the outside unit stays running.

  6. Manuel Lago says:

    Hello I found your article to be helpful thank you. I do have some questions that i hope you can help me with. My a/c cools properly but there is very little air coming out of the vents other than the one that is directly next to the inside unit. From what I have read here it seems that air flow is my culprit. Besides a leak in the dict work in the attic, could clutter in a home cause enough of a restriction to affect the a/c unit? Living in Miami and hoping for a cold(ish?) winter.

    • Chances are, if your a/c is cooling properly in Miami, there probably is nothing wrong with the air flow. Not enough air flow would cause problems like the coil icing or the unit shutting off on a safety.
      Just because you can’t feel a lot of air at the registers does not necessarily mean you have a problem.
      Just make sure the filter is clean and that you have the unit checked yearly to assure proper operation and fewer possible breakdowns.

  7. Billy Abella says:

    How does the cool coming from the air-conditioning unit is affected by a high stock of buffer stock in a grocery selling area?

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