Air Conditioner Thermostate Not Stable

[gonzobrains]

Has anyone else here had an issue with air conditioners not maintaining a constant temperature? My place has an older Trane air conditioning unit. No matter what setting I put it at it never seems to stay at a stable temperature. I've had the thermostat replaced but no luck. It seems to run colder during the day and runs hotter at night, despite not touching the thermostat at all. Is this a fault of the a/c unit or maybe the thermostats just aren't very good quality and fluctuate naturally because of the ambient temperature outside? I'm really not sure what to do about this.

Thanks,

gb

[InterestedObserver]

Ambient air temperature is effecting the condenser efficiency.

[Crossy]

Are you actually MEASURING the temperature attained? Humans are awful at judging temperatures as there are so many other variables that affect our senses (humidity for one).

What temperature are you setting the thermostat to? If very low IO (post above) may have something.

[trogers]

It seems to run colder during the day and runs hotter at night' date=' despite not touching the thermostat at all.

[/quote']

I will venture a guess.

You have probably set it to 26C. During the day, ambient temperature is much higher - 30 to 32C and the compressor runs normally.

During the night, ambient temperature falls - 28 to 25C.

Compressor kicks in at set temp +1C, thus compressor will not be running often nor for long period. Humidity in the room increases, from human respiration, and a feeling that the air is hot and stuffy.

[stgrhe]

First of all the temperature you see on the remote display is not likely to be the true temperature. Just because the resolution of the display is good (=shows many digits) it doesn't mean that it accurately shows the correct value.

Secondly, domestic air conditioners are using a simple on-off control and such control is only accurate in one point on the scale. Hence, the turning on and turning off points are not the same as there is a hysteresis. With a change in the load, e.g. a door is opened and some people enter the room, the load condition changes and if that triggers the air conditioning unit to turn on the compressor the temperature will not be the same in the room after the load is stable again compared to what it was before the condition changed. Fluctuations are thus quite normal.

To overcome this problem, if that is what one want, one needs to insert a more sophisticated PI-control (P=proportional; I=integration) used in industrial applications.

[gonzobrains]

First of all, thanks for all the responses, everybody.

Second, yes, I am measuring the temperature. If I set the thermostat to 26 C, throughout the day the A/C will progressively get colder and colder, sometimes reaching 22 C!! I have to turn the thermostat up as high as 28 C or so before it will stop freezing my butt off!

But then at night the A/C stops running the compressor for long periods of time, and the temperature climbs up to 28 C. I thought it was a thermostat issue so I had it replaced, but the problem still exists.

I've never had a digital thermostat. It's always been the mechanical dial type (including the new one).

I realize they are not exact, but can I really expect such a hugh temperature difference? I mean, we're talking six degrees here. I can live with a degree or so difference, but it's really annoying having to reset the thermostat throughout the day. After all, what's the point of a thermostat if you have to do that?

Thanks,

gb

First of all the temperature you see on the remote display is not likely to be the true temperature. Just because the resolution of the display is good (=shows many digits) it doesn't mean that it accurately shows the correct value.

Secondly, domestic air conditioners are using a simple on-off control and such control is only accurate in one point on the scale. Hence, the turning on and turning off points are not the same as there is a hysteresis. With a change in the load, e.g. a door is opened and some people enter the room, the load condition changes and if that triggers the air conditioning unit to turn on the compressor the temperature will not be the same in the room after the load is stable again compared to what it was before the condition changed. Fluctuations are thus quite normal.

To overcome this problem, if that is what one want, one needs to insert a more sophisticated PI-control (P=proportional; I=integration) used in industrial applications.

[trogers]

First of all, thanks for all the responses, everybody.

Second, yes, I am measuring the temperature. If I set the thermostat to 26 C, throughout the day the A/C will progressively get colder and colder, sometimes reaching 22 C!! I have to turn the thermostat up as high as 28 C or so before it will stop freezing my butt off!

But then at night the A/C stops running the compressor for long periods of time, and the temperature climbs up to 28 C. I thought it was a thermostat issue so I had it replaced, but the problem still exists.

I've never had a digital thermostat. It's always been the mechanical dial type (including the new one).

I realize they are not exact, but can I really expect such a hugh temperature difference? I mean, we're talking six degrees here. I can live with a degree or so difference, but it's really annoying having to reset the thermostat throughout the day. After all, what's the point of a thermostat if you have to do that?

Thanks,

The thermal sensor and compressor on-off control are not on the remote control, but are built into the fan coil unit. If the fan coil unit is installed high up due to the room having a high ceiling, the temperature at fan coil level will probably be higher than the temperature shown on the remote control.

[thaipod]

I had a similar situation where the temperature was working o k for a few hours then it would get warm . I had two lot's of technicians check the thermostat and then re gas . Finally yesterday had another company come out and they removed the circuit board and showed me there was condensation on the board and deteriorated circuits , so replaced the board and remote and now have a constant temp again . cost around 2000 baht.

[Darrel]

I've never had a digital thermostat. It's always been the mechanical dial type (including the new one).

I realize they are not exact, but can I really expect such a hugh temperature difference?

I had this problem constantly in Europe, though in the opposite sense with central heating.

Those wall-mounted mechanical thermostats are hopelessly inaccurate. A spread of 4°C between on and off is good going for them! They are useless. Not helped of course by residual heat in walls etc.

Get a proper digital thermostat. And look carefully at the instructions/spec to ensure that you buy the most sensitive one that you can. I went through four or five in Europe before finding one that cut in and out within 0.5° variation. Once I installed that one I had no problems at all.

My modern split room airconditioners there and here all had built-in digital thermostats of course, and those worked correctly except for one which has a problem with the sensor. In the end they changed the main circuit board and it worked fine afterwards. I set my units at 25°, day and night.

[stgrhe]

Attached is a document that explains the principles of on / off control. It is written for a larger A/C plant during which the compressor(s) run continously, but the principles is nevertheless the same as for a domestic plant in which the compressor starts and stops.

It is paramount to understand that the law of physics has its limitations and thus controlling an A/C plant with on / off control will never lead to a fully constant room temperature due to the hysteresis.

Reason for editing: Forgot the attachment.

On - Off Control.doc

Edited July 7, 2011 by stgrhe

[gonzobrains]

My AC unit doesn't have a remote control. There is a mechanical thermostat on the wall. I'm pretty sure the sensor is in there because I hear a "click" originating within it every time the AC goes on or off. This is a Trane brand AC that's at least 20 years old, according to the technician.

I'll probably switch it out to a digital one, and move the thermostat into the center of the room instead of near the glass door.

The thermal sensor and compressor on-off control are not on the remote control, but are built into the fan coil unit. If the fan coil unit is installed high up due to the room having a high ceiling, the temperature at fan coil level will probably be higher than the temperature shown on the remote control.