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My HVAC unit in the bedroom seems to increase humidity when turned on. I live in the tropics so humidity is 50-80% on average. Even if its 55% outside turning aircon on it seems to reach a level around 70% and stay there. We don't lower the temp that much. from 30C down to 26C.

The unit is oversized but my understanding is that might make it ineffective at dehumidifying but shouldn't increase humidity right?

The unit does have a cool mode vs a dry mode. Cool mode the fan doesn't turn off even if the fan is set too low. Dry mode can reduce humidity < 60% but it reduced the temp to 22C regardless of the temp setting which is too cold for us.

The outside compressor is 10M+ from the bedroom so I worry the pipes aren't sealed as they are old. The compressor and aircon are new though.

I did recently buy a seperate dehumidifier (rated at 15L). When the dehumidifier is turned on when the aircon is it will produce a lot of water (about 5L from 9pm-7am) but the humidity doesn't go much below 70. In fact it might start at 60 and end up at 70 in the morning.

We also have a air-filter which is effective at removing outside PM2.5 during the night at a low airflow setting which makes me think the room is reasonably sealed from the outside. We do have an ensuite bathroom but we've blocked the ventilation holes between in the door inbetween.

Whats going on? Is there anything I can fix on the aircon unit, for example is it problems with leaks in the pipes?

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  • Do you get much condensate draining from the A/C when it is running? Condensate is humidity condensing on the cold A/C pipe. If the A/C is not running cold enough, it is possible that it could cool the air (you are only talking about 4C, which isn't much) but not condense water out of the air. Check the temperature of the air coming out of the A/C ductwork. If you can (usually more complicated, but maybe easy with an infrared thermometer) check the temperature of the pipes coming in & going out of the heat exchanger. That may give us some clues.
    – manassehkatz-Moving 2 Codidact
    Commented Nov 9, 2020 at 16:13
  • Why is humidity stated as a percentage and not as absolute unit like "grams of water per cubic metre of air"? How much water can air hold? Does that change with temperature?
    – Harper - Reinstate Monica
    Commented Nov 9, 2020 at 17:30
  • @manassehkatz-Moving2Codidact I'm new to this so if you could explain in more detail where I should be looking and measuring? There is a cassette until in my ceiling which is hard to access. This connects to some pipes that go perhaps 15M to compressor on the balcony. The pipes seem to be insulated so I'm not sure if there is condensation on those and I can't see where water goes out of the outside compressor. There doesn't seem to be any kind of overflow pipe from the indoor cassette unit that I can see. If the only way is to try getting a measurement by going in the ceiling I will try.
    – djay
    Commented Nov 10, 2020 at 3:50
  • I can't say for sure with your system. But typically air conditioning works with a closed (sealed) loop of refrigerant. It comes out of the compressor cold and is insulated on the way to the heat exchanger. In the heat exchanger (presumably in your ceiling), air flows past the cold pipes. That warms the pipes (and the refrigerant inside them) and cools the air, and the air is then blown into your room to cool it. Because the pipes are so cold, the air cools so much as it passes the pipes that humidity from the air condenses on the pipes and drips down - this is called condensate.
    – manassehkatz-Moving 2 Codidact
    Commented Nov 10, 2020 at 4:00
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    @manassehkatz I'm encouraging OP to inquire about such things, as it will explain a great deal about the issue.
    – Harper - Reinstate Monica
    Commented Nov 10, 2020 at 4:39

2 Answers 2

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For an A/C unit to reduce the % of relative humidity in your home a properly sized A/C unit coupled with a dehumidifier must run a long time to decrease that humidity. When the A/C unit is first turned on, it begins to cool the room temperature which may actually increase the relative humidity. Just because the room temperature is reduced does not mean that the humidity has also been reduced. It usually takes a long time to reduce the % of relative humidity, that is why you should set the temperature and let the unit run 24/7. Remember, every thing in that room is at the same approximate relative humidity and temperature, (floor surfaces, furniture, etc).If you look up or search for the explanation of relative humidity you will get a good understanding of the term and how it is derived. After the units have run a couple days the % of relative humidity is still excessive then I would look for sources of the excessive humidity, say a leaking pipe, a wall opening, or a source of moisture.

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  • We generally run it overnight while we sleep. We aren't in the bedroom during the day so there isn't a need to waste electricity on it. I'm not sure the reasoning behind running it 24/7. Are you saying there is some source of water in my room that takes days to extract? It's already extracting perhaps 5 litres a night (15L throughput machine) so I'm not sure what that source could be that is providing that much water.
    – djay
    Commented Nov 10, 2020 at 3:44
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    The water is coming into your room all the time in the air from the outside. It is actually worse during the day (in most places), so if you only run the A/C at night it can be very hard for it to catch up.
    – manassehkatz-Moving 2 Codidact
    Commented Nov 10, 2020 at 4:45
  • @djay Try letting it run 24/7 for a few days and see if it not only cools, but dries out the house. Once you've confirmed that it's doing that and the bedroom has dried out, try turning it off during the day/on at night and see what happens. You'll probably discover that the room gets humid again fairly quickly and doesn't get dry enough. You'll probably need to run it at your overnight temp at night, then maybe allow it to warm 5-10°F during the day, then cool it again in the evening before going to bed, in order to keep the humidity in check.
    – FreeMan
    Commented Nov 10, 2020 at 12:44
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In my limited experience, most homes in the tropics are not particularly well air sealed. So, air moves from inside to outside easily, even with the doors and windows closed.

As such, when your A/C is running, it may well be that you are getting a great deal of outside air inside, and a great deal of inside air is moving outside - so when you bring in outside air and cool it, the humidity rises (since the water content remains the same, but the capacity of the air to hold water is reduced.) If the building/room is not sealed well enough to slow that air exchange enough that dehumidifying the air in the room is meaningful, you're just dehumidifying the air passing through, and more humidity is coming in with the fresh air flowing in.

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  • I think its reasonably sealed. One thing I forgot to mention is that we have an air filter running in addition due to pollution some days. We don't run it on max but auto (which is pretty low airflow. a lot lower than the dehumidifier) and yet it will bring down the PM2.5 levels a lot. Not sure how to compare leaked PM2.5 vs leaked humidity but it feels like if one had a problem the other would too?
    – djay
    Commented Nov 10, 2020 at 3:37

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