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>...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?

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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.

> and you also probably want a variable furnace

Or just get a variable-speed heat pump to handle cooling and heating.

Depending on where you live it could be much more expensive to use a heat pump for heating instead of a gas furnace.

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