This past weekend saw the official start to summer. Of course, when we mix the 80+ to 90+ degree temperatures with a lot of rain, the humidity goes off the charts. One of the best things about air conditioning is that it removes humidity from the air to help keep us comfortable.
But the high humidity can also create possible problems with systems – namely, water leaks into the building. This is especially true on roof top units.
I just finished talking with a contractor who put new roof top units into a school over the winter but, now that summer is here, he has water in the base of the unit that drips down into a classroom.
He does have the proper trap on the drain pan so we could eliminate the possibility of that allowing water to be held in the pan and overflowing and the drain is clear. So, why would there be water in the base of the unit, outside the drain pan? Keep in mind that most roof top units have the evaporator mounted before the blower so it is a pull through coil. Does this give you any ideas now what the problem could be?
So, the next thing I ask him is how much CFM is he moving? Does he know what the return air and supply air static pressures are so we can calculate TSP (total external static pressure) on the duct system. His answer ( and I love it) was that the CFM was GOOD! He also mentioned that he already opened the adjustable pulley to slow the air down. He still could not give me any readings though and he never had a test and balance performed on the unit. He did tell me that he can SEE the water being pulled off the evaporator coil by the blower and landing in the base of the unit.
This last bit of information should give you a real clue as to what is happening on this site. He is definitely moving too much air and literally pulling water off the coil. It also turns out that he ordered a unit with a high static drive package figuring there was old duct work and never looked at the existing duct work. It turns out that the existing duct work is minimal and properly sized. So, his problem is way too much air across the coil.
I told him, that since he already has the adjustable pulley all the way open, the only way to cure this is to do a pulley re-sizing to reduce the air flow. I told him we needed the supply static, the return static, and the static pressure drop across the evaporator coil. Once we know what CFM he is actually moving, we can then calculate what size pulleys he needs to reduce the amount of air being delivered by the unit.
I know we all preach proper air flow for heating to assure there are no limit trip problems, but too often, the cooling air flow is ignored as long as the space is “cold”. Beside the fact that water blow off on a coil can be an issue, we all know too little air can cause a coil to freeze or allow liquid to slug back to the compressor, but TOO MUCH AIR creates some real problems — such as water blow off on pull through coils.
But one other thing to consider about too much air — If TOO Much air was flowing through the evaporator, what conditions would occur?
- Low Sub-Cooling
- High Discharge Pressure
- High Saturated Suction Temperature
The increase load on the coil transfer too much heat to the refrigerant. More refrigerant is vaporized, elevating the temperature & pressure. The hotter refrigerant entering the condenser requires more of the condenser surface to reject the heat. More condenser is needed, less room for sub-cooling. This equates to a lower sub-cooled refrigerant. This becomes a vicious cycle until the internal overload trips in the compressor, or in the worse case scenario, we kill the compressor from constantly overheating it.
So, my point with this post is regardless of heating or cooling or just ventilation, AIR FLOW / CFM is critical to a properly operating system. In this case, it will stop water leaks, but without realizing it, it will save the contractor possible compressor failures down the road because he had TOO MUCH air! This should always be part of a proper start-up on any piece of equipment, either residential of commercial.