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)