[Tarrosion]

I've heard this before -- that oversized cooling units (whether standalone AC or part of a heat pump) mean muggy interiors in the humid seasons. But...why? I'd think that a fixed amount of air compressed in the compressor means a fixed amount of condensation runoff from the unit, and it wouldn't matter much whether it's a big unit running occasionally or a small unit running frequently. Why is that wrong?

[throwup238]

It's not a fixed amount of condensation because the air around the AC only has so much humidity. It quickly condenses just a little bit of water and then shuts off before the humidity in the rest of the house can redistribute to replace the now dry air. By running for longer it allows the water in the rest of the house to actually make it to the compressor.

[danans]

> I'd think that a fixed amount of air compressed in the compressor means a fixed amount of condensation runoff from the unit

It's not compressing air (like in a car tire). It's compressing a refrigerant. That refrigerant goes through phase changes (liquid to gas).

One major issue is that for most ACs, the compressor is cycled on and off according to the target temperature (via a thermostat, usually at a single location), not humidity. That means humidity can rise without the AC kicking on to bring it down. Remember in most typical houses, temperature and humidity are not very uniformly distributed.

Furthermore, if the humidity rises high enough before the AC kicks on, and then the AC kicks on at high power, you can get sudden localized cooling and then condensation of humidity to liquid water inside the building, which leads to other problems, especially if it happens behind the walls.

[randcraw]

The volume of air that passes through the heat pump must be cooled (or warmed) at the same rate as it is dehumidified, unless humidity control can be done independent of the pump. If you oversize the pump, the house is cooled faster than it is dehumidified, and the air reaches the desired temp before it reaches the desired humidity, and the compressor turns off while the air is still humid.

It's possible to independently add humidity when heating -- using a mist gun -- but not to remove it during cooling. However, if the heat pump has a "dry mode" it can dehumidify without also cooling by switching back and forth between heat and cool mode. If not, to dry the air further, it must cool it further.

[throw0101c]

It takes time for the humidity to be removed out of the air: a 'particular' cubic foot (metre) of air that passes over the coils can be cooled quite quickly, but won't be dehumidified as quickly.

So when the unit runs it can drop the temperature by the necessary (e.g.) 5F (2C), but it may only drop the humidity by 5%, when it needs to drop by (say) 10%. So a 'too-short' run-time can adequately cool the air, but not necessarily remove moisture.

It's also easier to generate 'excess' humidity by bathing/shower than it is to generate excess heat (cooking could generate both). So the humidity can creep up in value while the temperature stays more steady.

[lbotos]

I don't think it's about duty cycle -- it's seriously about speed of temp change and I _think_ dew point.

But yes, I have a heat pump and in NYC Summer I cannot run it on anything but low otherwise it increases the humidity. It took me a few weeks of looking at the temp humidity graphs to understand that point.

[BenjiWiebe]

As it was explained to me, it is about duty cycle. The condensation doesn't instantly accumulate enough to make droplets and run all the way down the drain, so if the compressor only runs briefly the condensation is still on the fins and evaporates again. You need to keep the compressor going long enough that you actually have water running down the drain, instead of condensing/evaporating cyclically.

[lbotos]

Yes, I believe you are right!