[andrewmg]

I’m no expert, but my understanding is that most residential systems don’t run at variable speeds. In effect, the AC is either on or off. And that’s what makes an oversized system a poor choice: there no way to slow it down.

One exception is Carrier’s Infinity system, which is variable speed. It’s relatively more expensive, still not all that expensive, and may be overkill if you don’t have multiple zones, which it can also handle. Also, you’re stuck with Carrier’s matching thermostats. But it may be a good choice if you’re looking for flexibility.

[pandaman]

Every major AC producer makes variable speed systems. They are called "invertors" sometimes.

[lostapathy]

Depending on where you live, though, actually finding an installer who can do one at a reasonable price can be a serious problem. Like I was quoted about double the price to go from their "standard" system to a variable speed unit when I replaced mine a couple years ago.

Most HVAC companies like to have a "standard" line they do and then charge dearly to deviate from that, because that's where callbacks and problems come from.

[ClumsyPilot]

In EU all AC systems must be variable by law

[41b696ef1113]

>...most residential systems don’t run at variable speeds.

Any reason why? I would naively think that putting in a variable speed motor with a dial would be peanuts on the total cost?

[pandaman]

It's not quite peanuts: when I had been replacing my AC system I was quoted the single speed system at $X, dual speed at $1.5X and variable speed at $2X. You need two variable speed motors: in the compressor (which is the most expensive part) and in the air handler (it's low power compared to compressor) and you also probably want a variable furnace, which is can be way more expensive than a fixed power furnace as you cannot just throttle your gas line to get less heat: that will cause condensate in the heat exchange and quick end of the furnace.