[ithos]

Originally Posted by jrref

Mostly correct.
With the latest Carrier Infinity Greenspeed system, 27VNA3, it will modulate from 20% to 100% in 1% increments.

Sizing can be a little complicated since once you determine the size you need from your calculations, these heat pump and furnace systems only come in fixed sizes. Every 12,000 btu equals 1 ton. So, if your calculations say a 3.25 ton system is needed for example, most will choose a 3 ton unit if the equipment is a single stage system so it doesn't short cycle. For a Variable speed system they would probably choose a 3.5 ton system since it will modulate and use only the "speed/capacity" it needs at any given point in time and there is no danger of short cycling.

Where sizing even gets more "tricky" is when an undersized system won't cool down to a specific set point no matter how long it runs. Since the sizing calculation is based on a max. temperature, say 95 degrees, on a very hot day an undersized system may run for a very long time or be on most of the day trying to lower the temperature to a low set point when no matter how long it runs, will never achieve that temperature. With an oversized system, on the same day it will get the home to it's target temperature but on more normal cooler days it may "short cycle" which you absolutely don't want. Besides not running long enough to remove the humidity, there is a lot of wear and tear on the system cycling On and Off like that all day.

With a variable speed system, you don't have to worry about any of this as long as the unit is sized properly or even a little oversized because on the hottest days, it will use all it's capacity say 90-100% to reach the target temperature and all other days it will use whatever portion of it's capacity that's needed at any given time. Also, with systems like the Carrier Infinity Greenspeed, once the target temperature is reached, if the humidity target is not reached, the unit will run in "dehumidify" mode which is cooling at a very low speed to remove the humidity without decreasing the temperature more than a degree or two.

Hope this helps.

Yes it does. So I guess there are problems if you oversize the compressor due to minimum speeds limits since it may make it more difficult at times to achieve the target SHR (Sensible Heat Ratio). I had to look that term up.
Sensible Heat Ratio (SHR) - HVAC School