A discussion on the "Inverter technology"

NJ, Designer · February 2015

I am now certified by the F and M brands on mini split heat pumps, Not sure were to find D's course....

I installed a 9K slim duct in my basement, (5 rooms) central return, Its rated @ 9k BTU for cooling and 12K BTU for heating, the inverter (compressor) is rated @-5 Deg. last night it was 0 deg, and it held out so far, So good... Manual J says I need about 9K heat and 8K cooling, waiting for the summer to see the cooling side of it......

My mistake so far was mounting the compressor on the outside wall, It makes to much noise for me especially on defrost mode, next time it goes on a floor stand,

The pro's: for sizing this is a no brainer, the smallest conventional forced air system is really 2 tons, way over sized for what was needed, space saving is good, operational cost so far not a big change in my electric bill. No need to run the gas line vs a gas furnace.

My concerns, for cooling are as follows is a inverter compressor really better that a 2 stage compressor? the street seems to think that yes it's variable, However from both of my training courses, the single head systems have a Delta T of 20-25, which would be great, however that's @ full power speed etc, what happens when your running half load, I was lead to believe that you lose that Delta T, then there goes your humidity control when it is needed most, and it gets worse the multi head systems, only have a Delta T Of 15 Deg, now maybe the "Professor" can help me here, but the smaller the Delta T the less humidity control, so I am concerned selling the multi head system, when there is any concern with humidity control, and the single head system, the smallest is 9K so again in a small room and part load what will happen with the humidity control?

Attached is my design for my basement it is a Town house,

Thanks for reading and let the party begin.....

bio_guy · February 2015

Delta T is probably greater at low output than at high because the blower slows down.

Running at partial speed is more efficient than full output. This is likely because the outdoor heat exchangers (and indoor in heating mode) have what might be called excess capacity in them at partial output.

I believe that the "multi-head" systems will not throttle back as well as the 1:1 systems since the lowest output available will be higher. Still, my 2:1 (two of them) and 3:1 (one) systems control humidity extremely well. One take-home there is that if you are installing a multi and one 1:1, install the latter in the bedroom or where you will see partial load a lot (night).

Oversizing them helps them heat more efficiently, but could hurt in humidity control, I suppose.

Another advantage of inverter systems, and as an independent factor, small systems, is that they run very well off of portable gensets. I could not get my mini splits to bog a fairly well-matched genset. when I turned on the minis but a refrig or freezer would.

ChrisJ · February 2015

bio_guy said:
Delta T is probably greater at low output than at high because the blower slows down.

Running at partial speed is more efficient than full output. This is likely because the outdoor heat exchangers (and indoor in heating mode) have what might be called excess capacity in them at partial output.

I believe that the "multi-head" systems will not throttle back as well as the 1:1 systems since the lowest output available will be higher. Still, my 2:1 (two of them) and 3:1 (one) systems control humidity extremely well. One take-home there is that if you are installing a multi and one 1:1, install the latter in the bedroom or where you will see partial load a lot (night).

Oversizing them helps them heat more efficiently, but could hurt in humidity control, I suppose.

Another advantage of inverter systems, and as an independent factor, small systems, is that they run very well off of portable gensets. I could not get my mini splits to bog a fairly well-matched genset. when I turned on the minis but a refrig or freezer would.

What bogged the generator, the compressor starting, or the defroster heater?

I don't know about a more modern fridge, but my early 1930s units draw around 12A for a split second and then drop down to around 2.5-3A. Actual wattage consumption is 150-170W.

I have a feeling even a modern fridge has similar consumption on startup but likely lower running amps due to run capacitors.

The defroster heaters on the other hand......my antique units have no such monster.

bio_guy · February 2015

The compressors starting on the refrig and the manual defrost chest freezer bogged the genset that started the mini splits without such nonsense.

ChrisJ · February 2015

bio_guy said:
The compressors starting on the refrig and the manual defrost chest freezer bogged the genset that started the mini splits without such nonsense.

I guess they also use some sort of soft-start setup?

Interesting.

SWEI · February 2015

ChrisJ said:
I guess they also use some sort of soft-start setup?

Standard feature on even a generic VFD, which you can use with pretty much any 3Ø motor. Conventional induction motors have somewhat limited range on a VFD due to the rotor geometry and its interaction with the field at different RPMs. Swap in some rare earth permanent magnets on the rotor and you get huge increases in both turndown and turnup.

bio_guy · February 2015

I am not an EE so take this with that warning: My interpretation of lowered inrush with VFD is that the inrush results from mismatch between motor speed and power frequency. Since the VFD starts up with a lower mismatch and maintains a good match until required motor speed is met, there is little inrush.

With a clamp-on meter, when I start up the systems, they dawdle around at a very low current draw for a short time, a few seconds, but noticeable. After that, they ramp up smoothly, over a few seconds, to a little over a constant amperage that they settle into very quickly.

Note that my meter is not an RMS meter. I wish I could justify the expense for my paltry needs (none in reality).

Observations were made in cooling mode only, not heat. I ran the indoor units set at just below the actual temp, and way below the actual temp. In addition, I ran indoor units singly and in combination. Finally, I ran them in emergency mode. That locks the compressor drives at fixed frequencies (48 or 32 Hz for cooling, compressor unit dependent.) The manuals give expected amperages and pressures in that mode.

Jack · February 2015

My understanding is that prior to a start the unit opens all valves and equalizes the pressure in the condensing unit so it is not having to overcome the remaining head pressure in the system.

My experience with cooling when I lived in MA, a humid environment, and fired up my unit was that it ran on a very low speed for quite a while. In the mean time the condensate was just pouring out of it. Once the humidity was reduced the unit would ramp up to address higher temps, if necessary. I can't give you the engineering behind it, unfortunately...for me.

I have 3 systems on my home. I do feel that care must be taken in locating the condensing unit. Two of mine are wall bracket mou.nted and not much of a problem. I think you need to be careful in isolating the line set from the building structure and use the sound isolation hangers or supports. Mythird system is roof mounted and occasionally I can hear it, but it is not offensive. I spoke to a guy recently who put a Mitsu system in his home and located the unit right outside his bedroom. That was a mistake.

Harvey Ramer · February 2015

I work with Daikin so I can not attest to the specific operation of other brands.

With Daikin;
The actual temp versus the set point accompanied with the speed of recovery is what dictates the evap fan speed. The condenser operates to maintain a specific super heat at the EVAP. That just the basics, I'm sure there's some behind the scenes stuff that I don't know about.

However;
With Daikin you cannot oversize the evaps by much and still expect decent dehumidification. I have tested it. You really have to size the evaps to design cooling loads in my climate.
This holds true on the Multisplit units and some of the single splits. There are some models that can address both temp and humidity simultaneously.

There are workarounds that perform exceptionally well. So you can't oversize the evap but you can jump a size up on the condenser. Say a 9k evap and a 12k condenser. This gives you increased heat output at low ambient while maintaining ac integrity. (Aka, Mit. Hyper Heat) Another thing you can do is size the condenser for heating design and stage evaps in the same space. This also allows for full capacity at low ambient while maintaining AC quality.

SWEI · February 2015

Harvey Ramer said:
With Daikin you cannot oversize the evaps by much and still expect decent dehumidification. I have tested it. You really have to size the evaps to design cooling loads in my climate.
This holds true on the Multisplit units and some of the single splits. There are some models that can address both temp and humidity simultaneously.

There are workarounds that perform exceptionally well. So you can't oversize the evap but you can jump a size up on the condenser. Say a 9k evap and a 12k condenser. This gives you increased heat output at low ambient while maintaining ac integrity. (Aka, Mit. Hyper Heat) Another thing you can do is size the condenser for heating design and stage evaps in the same space. This also allows for full capacity at low ambient while maintaining AC quality.

This holds true in our climate as well, which is quite different than PA. Size the indoor units to the load, size the outdoor to the design heating conditions. This almost always means an "oversized" outdoor unit, at least until you get into the commercial multi-head stuff where diversity factors start to really take hold.

A discussion on the "Inverter technology"

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