Adjustable variance thermostat?

Discussion in 'HVAC Heating & Cooling' started by kylepeavy, Jan 5, 2009.

  1. kylepeavy

    kylepeavy New Member

    Messages:
    6
    I like to leave my upstairs a/c set at 85 during the day in the summer (that's the max it will go)....
    I noticed that during the heat of summer, the upstairs a/c cycles on every 15 minutes for about 2 minutes or less.
    I guess that the thermostat is +/- 1 degree and it's pretty quick to cool the small upstairs down from 85 to 84 and also quick to warm back up to 85 again. This is putting a lot of cycles on my a/c system and my only intent is to keep it from getting too hot and humid.

    I have googled like crazy and can't seem to find a thermostat that:
    - I can set higher than 85 (maybe 90 or 95)
    - and has an adjustable variance or at least a way to set the minimum cycle time

    Anybody have any ideas?

    Thanks, Kyle
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,820
    Location:
    New England
    Only running 2-minutes at a time isn't very efficient and won't remove much of any moisture. It also sounds like the a/c may be over-sized. If you still live there when it needs to be replaced, you may find a two stage system may work better and on low, it might be more closely matched to the needs when the thermostat is at 85. Some of those can use the smaller compressor, or the larger one, or both to give you three levels of cooling capability to match the load. The most comfortable and best at removing moisture is when it runs constantly, but this only happens when the load is matched to the weather conditions.

    Not up on thermostats that have a user settable differential or if they are actually available.
  3. kylepeavy

    kylepeavy New Member

    Messages:
    6
    Thanks, you bring up a good point that is probably is oversized. I would like to replace the two systems with a single two zone, multi-stage system. Don't you think that would be the best? But, since these units are only 7 years old, I'm trying to make do with what I have for the time being... overall they don't seem terribly inefficient.

    I had another idea last night - to try wrapping the thermostat in some kind of a very light insulation so the temperature in the thermostat would change more slowly than in the room. Maybe that will help me for a few years until I feel it's worth it to swap out the system.
  4. MaintenanceMan

    MaintenanceMan In the Trades

    Messages:
    54
    Location:
    Indiana
    Why don't you just turn the A/C off if you don't want it to run?
  5. kylepeavy

    kylepeavy New Member

    Messages:
    6
    I live in Dallas, and it gets incredibly hot upstairs (over 100) if I leave it completely off in the middle of the summer. I'm worried about the effect that might have on wood furniture, pictures, etc...
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,820
    Location:
    New England
    The changes in humidity when it can't dehumidify while set at 85 may be worse than the temperature changes.

    An old add from Trane makes a very interesting point: identical house, identical a/c unit, one with the variable speed fan, one with the fixed speed fan for the air handler/furnace. The one with the variable speed fan over the course of a long, humid summer extracted enough extra moisture to fill an average backyard swimming pool! It does that by stlarting out with much slower fan speeds where the coils get cold, and the air runs over it slowly, extracting a lot of moisture because the air is in contact longer and the coil is colder. If it can't satisfy the need after running for a prescribed time, it slowly ramps up the fan speed until it reaches maximum (it's a 16-speed fan) then runs until it has satisfied the thermostat. But, instead of turning off, it ramps down to a low speed to extract all of the cold you paid for and as much extra moisture as it can. It really makes a difference.

    Short cycling caused by either running the thing with the thermostat set high, or it being oversized, is the worst thing as you end up with a cold, but damp environment. An undersized unit running all of the time and the temp higher is likely to have you feeling better and more comfortable. Combining a variable speed air handler and a multi-stage a/c compressor system can let it match the load and provide the highest comfort and efficiency. It will cost more to install - you have to decide whether it is worth it in operating costs and purchase price verses the old standby.
  7. kylepeavy

    kylepeavy New Member

    Messages:
    6
    Thanks for the advice Jim. I will definitely replace with a variable speed fan & compressor when I replace.

    Is there any reason not to replace the two units (upstairs & downstairs) with a single multi-zone unit? The units are close to each other in the attic, so the extra duct-work needed would be minimal.
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,820
    Location:
    New England
    Zoning is done all the time in commercial, but not anywhere near that for residential. Best to ask a pro that can evaluate your loads and desires to provide the answer for that.
  9. gdog

    gdog write diagnostic firmware for embedded industrial

    Messages:
    45
    Location:
    Pac NW
    In response to your original question, when you say "adjustable variance", what you're referring to is hysteresis or often termed the "dead band" of the thermostat; it is how far (in degrees temperature) that the hvac will stay on/off above or below (i.e. overshoot) the setpoint. I think most commercially available thermostats have a hysteresis of around 1 degree F.

    This is an interesting question because a thermostat with an adjustable hysteresis would certainly be the easiest and least expensive fix for your situation.

    But like you, when I googled it, I could not find any commercially available thermostats with this feature. The only hits I found on the topic were patent proposals for such a thing.

    I don't understand why such a thing is not marketed; my guess is the manufacturers would be worried they would get more complaints from customers who didn't know how to use it. But I would certainly pay a little extra for such a feature. If anyone knows of such a product, please shout out.
  10. cattledog

    cattledog New Member

    Messages:
    42
    Location:
    Portland, Oregon
    Kyle--

    It's not easy to tame an oversized system which is short cycling.

    There are basically two ways in which thermostats run, and you need to determine how your existing control runs and what the settings are in order to work for longer run times.

    There are stats based on cycles per hour (cph) and variable time on/off during the cycle depending upon the deviation from set point. It sounds like you are effectively at 4 cycles (i.e. runs every 15 minutes) and two minutes of operation satisfies the setpoint. Some thermostats can be set for one or two cycles per hour. At two cycles per hour, you would be likely to double your current run time.

    Other thermostats are based on a differential between on/off, and there are many units with user differential settings from 1 degree on upwards. Typically these controllers will also contain a user setting for a delay or lockout between operations. For example, with a cooling differential of say 6 degrees, and a setpoint of 90, some thermostats would turn on at 90 and off at 84 and then back on again at 90 again after the delay period. Other controllers might run +/- 3 degrees around setpoint for a 6 degree differential. How long you would run to drop the six degrees and how long it would take to heat back up and turn on again is something you will need to determine. With high differential settings and long lock outs, you will be giving up more uniform temperature (comfort) for longer run times.

    You didn't say if the same thermostat controls heating as well as cooling and that would add another level of consideration in choosing a new thermostat.

    Check out thermostats and setpoint controllers at Tekmar.

    Regards,

    Richard

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