The simplest devices are on or off. If they are grossly oversized they cycle a lot and make homes less comfortable by pushing a huge volume of hot or cold air briefly.
Variable speed compressors are better, but blowers may not also be variable speed. So you'll get better efficiency but may still suffer a feeling of draftiness.
A properly sized variable speed unit will operate within some optimal band of efficiency and constantly output nearly the minimum necessary air volume to achieve the target temperature.
When I was shopping for a mini split recently all the models I could find seemed to be variable speed compressor and blowers, unless I was reading something wrong. So maybe this isn't an issue anymore? I oversized mine but wish I went even bigger after a cold snap we just had in the pnw this last winter.
I’ve had one for three years. While I generally love it, it cools too fast, which means it sucks at removing humidity, actually increases it a little. The installer over-provisioned.
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?
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.
> 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.
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.
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.
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.
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.
Trying to run it longer means it gets downright frigid and wastes electricity. Currently we just run a couple of dehumidifiers. Not ideal, looking into other solutions but it is cheapest and most practical.
Yes. In some very humid climates like the Southeast US, a central dehumidifier might be necessary (although they are not yet common in those climates). But in northern climates which tend to be drier, a right-sized AC or heat pump is all you need.
Combustion furnaces deliver very hot air so they can cycle on once in a while and then turn off. Heat pumps deliver warm not hot air so they have to run longer cycles. The slower rise in room temperature can even mean that it makes more sense to set a constant temperature on your thermostat instead of letting the temperature fall when you're away during the day (the conventional logic to save energy).
Why is this an issue? We have a ducted/ductless system. Air handler for ducts on first floor and basement. Upstairs has a mini split in each room. Having air handler constantly push air at a reasonable temp makes for a very comfortable living condition. Compared to a furnace that’s on/off with wider temperature range and more divergent temperatures throughout the area.
