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jeff5may · 09-24-16, 02:33 PM

I am curious what a two stage inverter compressor has inside it. Does the thing revolve slower at stage one, using less energy in low stage? Or does some mechanism inside change the displacement or volumetric efficiency somehow? The answer seems elusive, to say the least.

Regarding low outdoor temperature operation, there are not a whole lot of heat pumps that do well near or below frost conditions. Most of the more economical units are designed with primarily cooling in mind, and are meant to provide supplemental heating only. Lots of these units go into "standby" or "backup heat" mode when outdoor temperatures approach freezing. Below this outdoor temperature, if they provide heat, it is done with low efficiency resistive heat strips, just like nearly every fullsize split heat pump on the market.

If you are looking for a mini split heat pump that does relatively well below 0 degF, you have your work cut out for you. Up until the last decade, only a handful of units were available at all. Of these few, the jury was split as to the effective heating ability versus efficiency at frigid temperatures. Prospective buyers had to choose between capacity loss at decent efficiency or efficiency loss at decent capacity. The two that come to mind are the Fujitsu halcyon, which had a linear capacity loss as outdoor temperature dropped, and the Mitsubishi hyper-heat, which attempted to maintain BTU output by running the compressor as fast as it could. The Mitsubishi also had supplemental heat strips to keep the discharge air temperature high when the compressor could only supply "warmed" air.

Looking at today's offerings, the units that are designed for ultra-low ambient temperature heating mode operation nearly all boast SEER ratings close to 30. They are not your everyday, general purpose mini-split units (like the NEO series). Most customers still aren't installing a mini-split as a primary system, and aren't trying to pay double for a unit is stout enough to act like one. I would be surprised to see an ASHP that had a COP of 3 at such low outdoor temperatures.

09-24-16, 02:33 PM	#8
jeff5may Supreme EcoRenovator Join Date: Jan 2010 Location: elizabethtown, ky, USA Posts: 2,428 Thanks: 431 Thanked 619 Times in 517 Posts	I am curious what a two stage inverter compressor has inside it. Does the thing revolve slower at stage one, using less energy in low stage? Or does some mechanism inside change the displacement or volumetric efficiency somehow? The answer seems elusive, to say the least. Regarding low outdoor temperature operation, there are not a whole lot of heat pumps that do well near or below frost conditions. Most of the more economical units are designed with primarily cooling in mind, and are meant to provide supplemental heating only. Lots of these units go into "standby" or "backup heat" mode when outdoor temperatures approach freezing. Below this outdoor temperature, if they provide heat, it is done with low efficiency resistive heat strips, just like nearly every fullsize split heat pump on the market. If you are looking for a mini split heat pump that does relatively well below 0 degF, you have your work cut out for you. Up until the last decade, only a handful of units were available at all. Of these few, the jury was split as to the effective heating ability versus efficiency at frigid temperatures. Prospective buyers had to choose between capacity loss at decent efficiency or efficiency loss at decent capacity. The two that come to mind are the Fujitsu halcyon, which had a linear capacity loss as outdoor temperature dropped, and the Mitsubishi hyper-heat, which attempted to maintain BTU output by running the compressor as fast as it could. The Mitsubishi also had supplemental heat strips to keep the discharge air temperature high when the compressor could only supply "warmed" air. Looking at today's offerings, the units that are designed for ultra-low ambient temperature heating mode operation nearly all boast SEER ratings close to 30. They are not your everyday, general purpose mini-split units (like the NEO series). Most customers still aren't installing a mini-split as a primary system, and aren't trying to pay double for a unit is stout enough to act like one. I would be surprised to see an ASHP that had a COP of 3 at such low outdoor temperatures. Last edited by jeff5may; 09-24-16 at 09:08 PM..