5 ms·

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

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.

ClumsyPilot 4y ago
In EU all AC systems must be variable by law
41b696ef1113 4y ago
>...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 4y ago
  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.
  - mcbishop 4y ago
    > and you also probably want a variable furnace Or just get a variable-speed heat pump to handle cooling and heating.
    - pandaman 4y ago
      Depending on where you live it could be much more expensive to use a heat pump for heating instead of a gas furnace.
pandaman 4y ago
Every major AC producer makes variable speed systems. They are called "invertors" sometimes.
- lostapathy 4y ago
  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.
  - pandaman 4y ago
    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).
    - criddell 4y ago
      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.
      - pandaman 4y ago
        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?

ClumsyPilot 4y ago

In EU all AC systems must be variable by law

41b696ef1113 4y ago

>...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 4y ago
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.
- mcbishop 4y ago
  > and you also probably want a variable furnace Or just get a variable-speed heat pump to handle cooling and heating.
  - pandaman 4y ago
    Depending on where you live it could be much more expensive to use a heat pump for heating instead of a gas furnace.

pandaman 4y ago

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

lostapathy 4y ago
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.
- pandaman 4y ago
  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).
  - criddell 4y ago
    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.
    - pandaman 4y ago
      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?