The Danger of High Airflow in HVAC

I have a two stage single compressor that runs 70% in first stage and 100% in second. For these systems you wire Y 1 to Y and Y 2 to the BK terminal which runs the fan at 80% in first stage. This means you have more CFM per ton in first stage than second. My humidity was staying around 52% and then when it went into second stage it would go down to like 45%. It just wasn't consistently comfortable. So I wired it where the fan runs at 50% in first stage by using the Y low terminal. I then turned the fan speed up to 450 CFM per ton. It works great now keeping the humidity below 50% most of the time in both stages.

I learned this as a teen when I tried to run a car with no thermostat which also served as a flow restriction. It was always overheating. It is all about matching the heat transfer capacity to the mass available for transfer (simplified college level thermodynamics). If the available mass moves to fast, it is difficult for the heat to be transferred efficiently because it needs more saturation time. Moving air slower is also cheaper.

He's not just blowing air! Listen & Learn! Nice job Zack! 👍

too bad the smart guys on youtube are rarely reflected in the field and it's nearly impossible to find someone that will act like this without significant effort

Sizing the equipment properly is the first and most important step. If the size is incorrect, everything else will be a compromise when it comes to cooling with moderate to high latent loads.

You can tell he knows what he’s talking bout! That’s one sage beard!

Especially important on multi stage compressors. Lennox units almost always need to be dialed back.

I am turning down my fan speed to try to get more humidity out of the air. Thanks for the tip!

Thanks Zac. Keep on keepin on 🤘🏼

This is unbelievably valuable information. Thank you for sharing it. I (as a nonprofessional layman) have been running off of intuition and thought this might be correct but having it spelled out to me step by step makes this so much easier to conceptualize. Thanks dude seriously, its something I've been struggling with for a while.

Excellent information to know! Thanks

I had them tell me that slower fan speed will dehumidify the specific mass of air less but its a give and take balancing act. We sped up the furnace blower. I was hoping for medium speed but now its fast speed. I'm worried the p pipe is not deep enough and will splash back into the cooling coil area. But if that's not a problem then I'd rather have the fast speed. Now there's a Fridgidare dehumidifyer, and it makes a huge difference. I wait til about 5 pm to let the house go 80 degrees but dry. Then the AC comes on as the sun is shaded. The room is instantly cool. The objects in the room will have to slowly change their temperature. I'm in Waco where it's suicide to not have AC. The older technicians that charge a bit more to start are the honest fast best. I've seen many instances and i learn as we go every time. It takes twice as long to cool down a hot house when its 83 degrees vs 103 degrees, nothing is wrong.

I liked that. Keep up the good work, Zack.

Man, That's True Talk and educational 👍

I'm still puzzled that the HVAC gods decided CFM could mean either cubic feet/min or condenser fan motor. What happens when you order 1000 CFMs ?

Sad how many companies reward call backs, and eventually sales because the systems they touched become messed up especially when they are pushy Salesman with big muscle and clean white shirts

would you say 50 degree coil can cause mold and bacterial growth? depending on the seer rating coils of 40-45 degrees are perfect 🥴

We live in Central Florida. Lately we have had 95+ degree days. Our humidity in the house has been throughout the day between 35-45% and dry. We recently changed from Merv 8 filters to Merv 11 filters. We just cleaned both coils with a coil cleaner and just changed our filter like we do once a month. We keep our thermostat at 82 and sometimes to 84 during the day and at night we put it down to 77. So far so good but the electric bill will be higher obviously. Great video on this subject. We are looking into Mr Cool or EG4 Mini Splits to put in in the near future in each bedroom and in our living room to help supplement the Central Air Conditioner or keep it completely OFF. That way we can just cook the rooms we want without keeping on the entire system. Jeff -Sunny Central Florida ☀️

Hi Zack . Thank you for addressing this. My mother freaked out 2 winters ago when our 25 year old Trane furnace control module failed. I was not here and she had a Carrier infinity furnace installed. She had the evaporator coil and condensing unit replaced 5 years ago with American standard. It worked great and was very efficient. Since they are pretty much the same. After the carrier furnace was installed it is not as efficient. I believe its too much cfm for the coil. Only 15 degree differential . Should be 20 degree or better. I know there are several dip switches to change cfm. The guys who installed it said it's fine you have 10 degree subcooling. I have seen how to lower fan speed. Should I give it a try? I can't find any techs who know how to dial in a system. I'm pretty knowledgeable . What do you think. Thanks.

Almost missed this video. It's almost the end of the month!