Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit

A/C setback

Hi Brad,

First, I must admit I needed to look up Coanda effect :o). Always something new to learn.

So, based on what your saying and what I am observing, it seems my suspicions were correct. Setback is not worth it when less than a certain number of hours. 9 hours not being enough. Similar to what I said, lately I have been holding one temp indefinitely and it goes without saying the house is comfortable upon return home. But the temperature is also more stable because not only is the pocket of air in the house cool but the mass of the interior is too. And by default there is no recovery or pull down.

Sorry if I wasnt clear. When I said 'swelling' in my first post I meant (in other words) heat entrapment in the house after a hot day, not humidity. When the air comes on there is a cool pocket of air inside the house but the walls, etc are warm from direct sunlight.

At any rate, the paralell humidistat concept is very interesting. Yes, Hi-V removes more humidity but there are times when the temp is at set point but the humidity is a little too high and its a little uncomfortable. There are times I wish the system would come on just shortly to 'wring out' the interior air a little. A paralell humidistat would do that, correct?


  • Mark_46Mark_46 Member Posts: 312
    Cooling setback

    Looking for some input about thermostat setback during the cooling season. My system seems to take a little longer to recover but probably because it is high velocity with lower air flow. But more importantly, when the space reaches set point it seems this includes only the pocket of air in the space and the objects (eg large furniture) in the space are not. Even more pronounced and more important is the house swelling with radiant heat from direct sun light. For example siding, frame, attic space, roof, etc.

    So, is it worth it to set back the thermostat during the day? Typically my at-home comfort temp is 75* and I may set it back during the day from 8:00 am to 5:00 pm to 77-78*.

    Any thoughts?
  • Brad White_174Brad White_174 Member Posts: 1
    High Velocity Systems

    are typically known for long "pull-down" periods as you noted. The low mass-flow is the reason, even if the air is much cooler. It takes time to establish the "churn" (secondary air motion induced by the Coanda effect) with those lower air volumes.

    Once up and running, the overall air volume in motion is within reasonable limits and RH's tend to be less so you feel more comfortable.

    I was always advised to limit setbacks with HV systems to when you are gone for a long weekend but for a day or during the day, not so much if at all.

    If swelling is of concern, a secondary parallel humidistat might be in your future. Set the temperature for what reasonable setback you deem appropriate but set the RH for it's own high limit. The AC system will operate to the greater demand. Of course it is easier to maintain a lower RH the warmer it is, so go easy on the extremes.

    My $0.02

  • TechmanTechman Member Posts: 2,144
    Set back?

    Evening!I guess this depends on where you live,but,75* house temp seems a little hi,depending on what the house was sized for in the first place.Higher temps bring higher R.H.,also ,the setback is only 2*-3* which is not very much. And with a 5 o'clock restart I think you would be getting home around 7 or so ,in order to be back down to 75* air temp,not couch humidity or bed humidity or walls or floors.

  • Hi Techman. Thanks for the reply.

    I am located in very Northest NJ, outside of NYC.

    Yes, 75 is right on the comfort edge, we are still experimenting with what the right temps are. I try to run 75, and not 74, if it is comfortable. But sometimes we have to drop it one to 74. Perhaps 74 is where we should hold it. Time will tell.

    No, 4:30 - 5:00 start with a return home time of apprx 5:45. Based on what your saying, those settings are not enough?

  • Brad White_176Brad White_176 Member Posts: 1
    Coanda! Iko-Iko Iko Ikay....

    Hi Mark!

    The fact that your house is cool but still humid has me wondering if something else is not happening correctly. Dirty coil? Low refrigerant charge? Other things folks with more hands-on refrigerant experience can better address than I.

    High-V systems as you know use about half the air per ton and supply air to the space in the high 30's/low 40's. By that, say 40 degree saturated air, a straight-line psychrometric will get you to 33% RH at 75 degrees and 37% RH at 72 degrees.

    Yes, a parallel humidistat will help but keep in mind you may get chilly in the process unless you use reheat. As you can imagine, the AC will run until both temperature and RH are satisfied. The fact that you feel muggy tells me that you have other issues. The normal High V process should work better than that.

    My suggestion to use a parallel humidistat had more to do with setback/away times than normal occupied hours; just a way to keep the house more in-limits than it otherwise would be.

    In a perfect world, any AC system I would own in a humid climate would have a downstream hot gas (free energy) reheat coil for dehumidification. I hope someone does that in an off-the shelf application such as the pool unit folks do. Absent that, some solar for reheat would be pretty neat!

This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!