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  1. #1
    I understand that for the most part, it's probably a $$ thing, but it seems like there should be at least SOME units that completely turn off the fan AND compressor when temp is reached ?

    What is the reason that they run constantly ? Just curious.

  2. #2
    Join Date
    May 2000
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    Indianapolis, IN, USA
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    Most have that capability, it's a DIP switch or some other kind of switch hidden somewhere.

  3. #3
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    Apr 2004
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    I hate having the fan run all the time when I am staying at a hotel. If they have a means of controlling the fan, its not on the main panel.

  4. #4
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    Apr 2005
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    ulster county new york
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    22
    We have some old Carrier Ptac's that have a cycle switch on the right hand side underneath the cover. But not all Ptac's have them. Usually the units with an energy management control package do.

  5. #5
    Join Date
    Nov 2004
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    83
    That is a very good question Chief, and one that very few people know. It is because they are the same as a window unit. By design most will make a noise that will be heard. The purpose of having the fan run all the time is so that one will be get used to the sound and will hardly notice the different when the compressor kicks in or out. This way you want be awaken during the night. If they were design so that every thing cuts off during the off cycle, the sudden noise will wake you every time it came on.

  6. #6
    Join Date
    Mar 2004
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    1,042
    I don't think that's really the reason.

    The thermostat in a PTAC is right up against the coil, which is sandwiched in an exterior wall and isn't insulated to speak of.

    If you don't run airflow across that thermostat all the time, it does a really sloppy job of sensing actual room temperature. It's too biased by outdoor temperature and coil temperature when the fan isn't running.

    With the fan running all the time, the thermostat works better but you have to listen to it, and it ruins your humidity control.

    The *best* PTAC setup, in my book, uses the "external thermostat" option that some manufacturers offer. They come with a totally blank control panel, and you wire them up to a traditional thermostat off toward the center of the space you're actually trying to condition. Then you can set the fan to not run between cycles and not impair temp control. With the thermostat moved that far away from the unit, you also keep the supply air from making a u-turn into the return and forcing the thermostat to short cycle.

    I stayed in a hotel in Houston with that setup, and was hugely impressed by how well it could regulate temperature and keep the humidity down, as compared to any regular PTAC setup with the internal thermostat. The cycling noise didn't bother me at all, and I'm a light sleeper. Maybe the answer is that only small capacity PTACs are quiet, and everybody almost always specs them way oversized. That room in Houston ran nice, long cycles, like you see in regular residential equipment, between having the thermostat relocated and not being oversized.

  7. #7
    Join Date
    Apr 2002
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    11,808
    Originally posted by tuesday
    That is a very good question Chief, and one that very few people know. It is because they are the same as a window unit. By design most will make a noise that will be heard. The purpose of having the fan run all the time is so that one will be get used to the sound and will hardly notice the different when the compressor kicks in or out. This way you want be awaken during the night. If they were design so that every thing cuts off during the off cycle, the sudden noise will wake you every time it came on.
    I thought they ran the fans steady to keep pulling air across the thermostat

    I should learn to read all the posts so I don't sound like a parrot.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

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  8. #8
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    Nov 2000
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    Waco, Texas, USA
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    tuesday is correct
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  9. #9
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Originally posted by Steve Wiggins
    tuesday is correct

    And your proff is??

  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    34,317
    I think it is all of the above plus price. Adding the cycle switch adds a little and you know how cheap everyone is. But pulling air over the stat constantly makes a world of difference in keeping the room an even temp. If you are on cycle, the stat built in has no way to know what the room temp is, just what the temp in the unit is. On an outside wall in cool weather, it may never detect a warming of the room. I like a steady hum. I've been known to turn the fan on of a unit set to cycle. Downside of course is higher humidity with the blowing over the warming coil during the off cycle.

  11. #11
    Join Date
    Feb 2005
    Location
    Richmond
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    480
    We stayed in a hotel in Houston once. The year before, we had a PTAC that had constant fan. But this year we had a different room and the fan would cycle. The fan came on every 10 min and ran for 3 min to check the temp. Very annoying. Ended up just running hi fan at night but was warm by morning.

  12. #12
    Join Date
    Apr 2002
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    11,808
    Originally posted by BaldLoonie
    I think it is all of the above plus price. Adding the cycle switch adds a little and you know how cheap everyone is. But pulling air over the stat constantly makes a world of difference in keeping the room an even temp. If you are on cycle, the stat built in has no way to know what the room temp is, just what the temp in the unit is. On an outside wall in cool weather, it may never detect a warming of the room. I like a steady hum. I've been known to turn the fan on of a unit set to cycle. Downside of course is higher humidity with the blowing over the warming coil during the off cycle.
    Plus with vent open they blow humid air in all the time. Usually fan pulls in fresh air direct from outside, it mixes with cold air coming off of coils. In a humid area they pull in more humidity than they can remove.

    You can also get them with wall mounted stats.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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