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hydronicsmike Member Posts: 855

When it comes to cooling setpoints, what are your thoughts on temperature control based on outdoor condition?

I just came across a customer who suggested that it would be plenty to have a cooling setpoint of nearly 80°F when it is 95-100°F outside, where he would like to have a cooling setpoint of 73°F when it is 80-85°F outside. Would that make any sense to you, or, do you think that's nonsense? It sure could save money, no doubt.

It appears to me that this would not be a bad idea. Its a temperature difference that makes you comfortable, isn't it? It's 100-104°F out here today and to me, 85°F would feel more comfortable right about now.



  • Henry Q.
    Henry Q. Member Posts: 5

    Hi Mike:
    Cooling is very dependant on relative humidity. Most systems are not set up with a humidistat, but they should be. If you are in a very humid climate, a 15 degree delta T will be comfortable if you have a 20-25 % lower humidity. If you have ever been in a restaurant or facility with an oversized A/C system, the compressor does not run enough minutes per hour to remove the humidity and it feels like a meat locker as a result. In New England, cooling load calcs by ACCA Manual J always produce a lower sized load than you would expect, but the dehumidification only occurs when you have long run cycles. This is especially true with newer equipment designed for energy efficiency because the evaporator coil temp is running higher. If you have a hydro-air system, you can do dehumidification and reheat with a couple of simple controls to produce an ideal level of RH at a higher design temperature.Hope this helps.
  • hydronicsmike
    hydronicsmike Member Posts: 855
    This makes...

    ...an awful lot of sense to me, that humidity may be a factor. I am just wondering why I see so many forced air systems then with simple heating/cooling control without humidity sensors. I would suspect that further south in the US, you'd have to have humidity control as well, right?

    The parts I have seen in Canada and northern/central US didn't have those. That is including the coastal lines of WA, OR, NH, MA, CT, NY. But then, to be fair, I don't get shown around nearly as much anymore as I used to. Schedules are just too tight sometimes.

    What about the setpoint alone? Would you agree or disagree with changing cooling setpoints based on outdoor temp? I am just curious as to what the Pro's think about it.
    I would expect to control the humidity level as well.


  • Dale
    Dale Member Posts: 1,317

    I find that the system size is the key to what temp to set. I tell people that in the upper midwest it's the humidity that makes you feel hot. I find that if a system is set to 78 degrees with 30 -50 Percent RH most people are comfortable. To get that you need to undersize the system 10 percent or so since this area does't often see ashrae design temps for too long AND to SLOW down the fan from factory settings. Furnaces are made to match cooling load in Tenessee or some other mid south place so the factory set cooling fan setting is usually too high to get good dehumidification, remember the old 400 CFM per ton, ever look at the lowest cfm at even a high esp like .5? I often set the AC and heating on the same low speed, on my own house I have the AC on low and the heat on med low to be in the middle of the heat rise. This is a reason the modern variable speed blowers feel better. I think the Trane has a humidistat that slows the blower if it senses high humidity.
  • hydronicsmike
    hydronicsmike Member Posts: 855

    Very good information! Thanks.

    More feedback?
  • Wayco Wayne
    Wayco Wayne Member Posts: 615
    Hey Mike.

    Some very good answers. Humidity is the key down here in Washington DC where our forefathers got a deal on some swampland and said lets build the Capitol there. I have had great success with the 2 speed units made by Bryant/Carrier. They run at half capacity until the temp starts to rise a little too much and then switches to high capacity. Running at low speed gives long running time and great humidity removal. The difference in comfort is noticable. Even to a big lug like me.

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  • hydronicsmike
    hydronicsmike Member Posts: 855
    Thanks Wayne.

    Very good information.

    Too bad I missed you the last time I was out your way. Must have had enough the year before ;)

    Not coming to Wetstock?

  • flange
    flange Member Posts: 153

    in general, you dont want to be any cooler than 20 degrres below outdoor ambiant for true comfort. if a system were sized correctly this would be the outcome anyway. there are some very nice systems out there now that do indeed utilize outdoor compensation. carrier makes the thermidistat which works very well with the newer variable speed drive air handlers, as well as the signature stat by lennox. as stated, during extreme conditions, your ac should be runnning for long periods of time anyway, which will naturally dehumidify, otherwise you are oversized. there are all sort of rules of thumb used in sizing ac, most are terrible. a manual j is the only way to go, even though at first you will not believe the numbers.
  • hydronicsmike
    hydronicsmike Member Posts: 855
    You don't know...

    ...how valuable this information is to me. I was wondering about that since long ago. Thank you soo much!

This discussion has been closed.