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Comment by andrewmg | Hacker News Reader

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.

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

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.

I don't think you can get a variable speed system at the same price as a single speed one or even close to that: it's different, more complicated hardware. And, as it is already more expensive, they don't make variable speed versions of the cheapest designs hence they use more expensive materials and processes in the production (sound insulation, fancy finned coils, weather protection and such).

Do you know if a variable speed system will let you oversize and avoid the typical problems from doing so (short cycling, failure to dehumidify, etc...)? I haven't been able to find out a definitive answer. It seems like it wouldn't take a very smart thermostat to measure humidity and cycle times and throttle cooling to ensure that both factors are accounted for.

I don't know, sorry. You better to talk with an HVAC installer. Perhaps your issue is that your system is not sized right from the beginning, did you get a schedule J or got the tonnage from the previous install/square footage?

I have talked to two different HVAC companies and our system is sized correctly. When I complain that when it's 105 out I'm lucky if it can hold 80 they tell me it's working as expected. They say no system can do more than about a 25F drop but I have a hard time believing that. People in hotter climates somehow are able to cool their homes more than 25 degrees and commercial spaces (like stores and offices) somehow manage to cool more than that.

On the other hand, I'm asking them to sell me something that's more expensive and the fact that they don't want to tells me that they have some reason to believe I wouldn't be happy with the result.

It costs money to do a schedule J measurements so it's unlikely you've got it from just talking. What you describe sounds like an undersized system. Max 25 drop is obviously false, I have solid 75 with 115 outside with a residential AC, for example.

We've paid $225 for the consultation.

The max 25 drop does seem to be a widely reported thing. For example, my first search result for "max ac drop" is this:

https://www.jerrykelly.com/blog/how-many-degrees-cooler-can-...

The explanation in the article does not make sense to me unless it's talking about window units. Ducted systems do not cool the outside air so the amount of temperature drop in the air passing through is not relevant. As long as it can cool faster than the air is being heated inside it can drop the inside temperature to the temperature of the coil, which is unlikely to be set at 80 degree in your case.

I can't find a single article saying anything else. The 20-25 degree maximum temperature drop seems to be everywhere which leads me to think it must be true but I don't understand why. I'm trying to find out what happens if you go oversized on a variable speed system and can't find any information.

In EU all AC systems must be variable by law

>...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?

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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