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Air Con Temp All Over the Place in This Cooler Weather


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Posted (edited)

A question for the engineer members, out of interest.  I should be able to work this out for myself, but its a bit of a mystery to me. My Hitachi  aircons are just 4 years old, regularly serviced.   In very hot weather, set to 22 degrees, they can cool the room down to what feels like near freezing, certainly uncomfortably cold for sleeping. However, in the last few weeks, when the night time temperature has been around 26 degrees, lower than normal for this time of year,  they seem to struggle to get down to anything like that same 'cold' temperature, in fact they often feel like just a fan.  In addition,  set to 'dry', they seem to respond better as they suck the humidity out of the room, so the difference between  'dry' and 'cool' settings is negligible in terms of end result.  I'm sure that its all to do with the temperature gradient between out side and inside and the prevailing humidly  but does anyone have a more 'engineering/scientific' answer to why this is so? . As I say, just out of  interest. 

Edited by Pilotman
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At 22 meat locker temps you would have rain most days lately - the humidity is over 70 percent at normal temp.  Sorry can't provide science but have had no issues getting it too low for my liking (which is above 25 always). 

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Air con on 22 degrees seems a lot colder than central heating in UK on 22 feels warmer.

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5 hours ago, bankruatsteve said:

Are your AC's "inverter" or old style?

Inverter

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Inverter should hold the temperature, so that doesn't seem normal.  Maybe a difference in brand?  Are you citing actual temperature measurements or how it "feels" to you?  A 65% RH can leave you feeling much different than 45%.  

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When you say rain most days, do you mean water gushing out of the unit, like has happened to mine last couple of nights? I use the dry setting, usually set to 23-25°C.

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When it's cooler outside, I've noted our inverter units need to be set to a lower than normal temp to kick off the compressor. My normal for the downstairs is 30 during the day, which keeps it comfortable when the sun is blazing and heating up the walls....after sundown, I need to lower to 29 or even 28 depending on how cool it is outside to keep it from just blowing the fan. 

 

Upstairs normal is 27 at night...but when cool outside need to drop that to 26 to get the compressor to kick off.

 

I think that's normal for most units of this type.

 

22 is pretty low....wow. Maybe you have undersized units ?

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32 minutes ago, bankruatsteve said:

I happen to agree with that.  But, I think many would also agree that 28 is pretty high.  😎

I need to actually measure the inside temp...the unit is a bit oversized but at 28 it is vert cold

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5 hours ago, bankruatsteve said:

Inverter should hold the temperature, so that doesn't seem normal.  Maybe a difference in brand?  Are you citing actual temperature measurements or how it "feels" to you?  A 65% RH can leave you feeling much different than 45%.  

I am not monitoring the temp directly, It just hoe it feels . That must be the next step I guess. 

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2 hours ago, tonray said:

Maybe you have undersized units ?

possibly so, but its not such a serious issues that I would consider upgrading. Common sense would suggest that a smaller unit would cope better with lower outside temperatures? 

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Just now, Pilotman said:

possibly so, but its not such a serious issues that I would consider upgrading. Common sense would suggest that a smaller unit would cope better with lower outside temperatures? 

It depends...but 22 tells me either it's  undersized --or-- the internal thermostat may need calibration (if that's even possible). 22 may actually be 25 is my guess

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7 minutes ago, tonray said:

It depends...but 22 tells me either it's  undersized --or-- the internal thermostat may need calibration (if that's even possible). 22 may actually be 25 is my guess

I will try setting 16 degrees, the lowest possible setting and see what happens.  Thanks 

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That should work. Inverters in theory should use proportional control but they still probably use cheaper on/off control. 

You 22 might not be 22 and your 26 might not be 26. So the difference is probably too small for the compressor to kick in.  

If 16 works then an indicated 18 or 20 might also work. 

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The body is not a good temperature indicator because humidity levels can change the way we perceive ambient temperature.

 

For most single room AC installations, the longer the compressor runs the greater the reduction in humidity and subsequent comfort level. If the initial ambient temperature is much higher than the set point 22/34 for example, the compressor will run for longer than if it were only 22/28.

 

For inverter AC's the difference between set point and actual room temperature is the error used to calculate inverter control and how quickly it reacts. An already cool room that's only one or two degrees above the set point might not dehumidify well because control has the evaporator at lower level.

 

In theory, undersized systems should dehumidify better than those sized correctly when set point is close to outside ambient.

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Perhaps they need cleaning?  In general once a year would be sufficient, mine run far better after cleaning.

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5 minutes ago, Paul DS said:

Perhaps they need cleaning?  In general once a year would be sufficient, mine run far better after cleaning.

You should check the inside screens monthly to make sure they aren't gettng clogged up.  Easy to rinse them off.

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Posted (edited)

An air conditioner doesn't blow colder/warmer air at different room temperatures.  The setting will just determine how long/often the compressor runs.  Air conditioners don't make cold,  they move heat.  When the weather gets cooler your air conditioner shouldn't run as often.  That can make it feel more humid in the room which can make it feel warmer than it is.  The 'Dry' setting on air conditioners will make the compressor run every so often even if the room temperature doesn't reach the setting level.  That is to keep the room less humid.  The units I have looked at the dry setting defaults to 25 as the temperature level but as I said it will come every so often even if it doesn't drop that low.

 

Adding this:  Heat always tries to equalize, it moves from a hotter to a colder area until they are equal.  To move heat there has to be a temperature difference.  The compressor is not the same as a pump. It's purpose is to compress the refrigerant which causes it to become hotter before it enters the outdoor coil.  It has to be hotter at that point than the outside air to cause the heat to move from the refrigerant to the outdoor air.  On the indoors unit there is a thermal expansion valve that drops the refrigerant pressure thus temperature before it enters the coil.  The refrigerant at that point has to be colder than the room temperature to move the heat from the room to the refrigerant.  Now warm air can hold more moisture than cold air.  That is why it is called relative humidity.  80% humidity at 30° is not the same as 80% humidity at 20°.  When the air passes through the indoor coil the heat moves out of it causing it to drop temperature.  That also causes it to not hold as much moisture.  So you get condensation on the coil which must be drained to the outside(or run back into the room).  That is why air conditioners also dehumidify.

Edited by rwill
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18 minutes ago, Paul DS said:

Perhaps they need cleaning?  In general once a year would be sufficient, mine run far better after cleaning.

They are cleaned and serviced every 6 months. 

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9 hours ago, John435 said:

When you say rain most days, do you mean water gushing out of the unit, like has happened to mine last couple of nights? I use the dry setting, usually set to 23-25°C.

Ah. Sounds like what happened to me a couple weeks ago. Had the repair guy over, he cleaned the filter, and Bob's your Uncle. The signal light showing the filter needed cleaning had been on for a couple of months. I often set mine for 30° in this cooler weather, but I've lived here for so long I'm acclimated. Up until last year usually set it to 28°, but shut it off around midnight. Mine's an older Carrier model.

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I have a Daikin Inverter and older Mits non inverter and today we installed a new Carrier Inverter. We usually keep temps at 25, sometimes we drop to 24 but never lower. The house seems quite comfortable.

Must say, the new Carrier backlit remote is awesome.. Took me about an hour to figure out how to use it as it has so many different settings.

 

FYI.. I bought Carrier because it's the most efficient AC available. 

Daikin SEER is 17 and the Carrier is 24.. Huge difference.

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I've never been able to figure out why Thais, especially Bankers, Doctors, and Dentist like to keep their offices cold enough to store meat.  And now the odd-ball farang. 

If I run mine its bottom temp is 27C.  That's chilling when it's 35 to 40 outside.  Anything cooler is a waste of cold air and electricity.
 

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On 5/7/2021 at 7:33 PM, connda said:

I've never been able to figure out why Thais, especially Bankers, Doctors, and Dentist like to keep their offices cold enough to store meat.  And now the odd-ball farang. 

If I run mine its bottom temp is 27C.  That's chilling when it's 35 to 40 outside.  Anything cooler is a waste of cold air and electricity.
 

Doctors and dentists may do it slow bacterial growth

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Posted (edited)
On 5/7/2021 at 7:47 AM, John435 said:

When you say rain most days, do you mean water gushing out of the unit, like has happened to mine last couple of nights? I use the dry setting, usually set to 23-25°C.

Maybe you have a blocked condensate drain. The humidity condenses into a tray then runs away via a pipe. If the drainpipe gets blocked the tray fills with water and overflows.

Edited by phetphet
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