VFD's for home AC use.

[DASKAT]

I am in the middle of a discussion with a friend on the possible use of a VFD for the operation of a Home AC compressor. I think this would greatly enhance the longevity of the system and increase the efficiency also. Any thoughts on this?

 

[MikeHalloran]

Current high efficiency domestic A/C units already use a VFD on the air handler motor to provide variable speed and very quiet operation. The compressor would require a serious VFD because of its starting current, so it's typically cycled with a contactor.

My experience says the current limit to durability is that VFD controlling the air handler motor. Worse, the aftermarket is not supplying replacements. The VFD is built into the motor, and it's clearly more profitable to just supply replacement air handlers, which is all anyone will sell.

I looked at replacing the VFD in my penultimate air handler with an industrial unit. One, it doesn't have the backpressure sensing logic built in, and two, VFD manufacturers, in the fine print, suggest replacing all the capacitors after five years anyway, so they clearly haven't got a handle on what's expected for domestic A/C durability.


Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

Also, the load on a domestic AC unit is a function of the rate of heat transfer from the refrigerant to the air. By managing the air handler like Mike said you get virtually all of the benefit without the headaches. I just replaced my home heater with one that is 97% efficient. Being a thermo guy I didn't believe it so I got the data from Trane and their assumptions were even conservative and the real number is probably closer to 98% than 97%. The old one was 64% efficient (and that data was really optimistic).

As always, the big energy savings are in dynamic loads, not in positive displacement loads.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[DRWeig]

Well, I'm a controls guy and an energy simulator. So my home unit has a variable-speed drive on the indoor fan and the condenser fan that I added. Both are Johnson Controls S66 fan speed controllers, and I have a small, old Andover controller that does the thinking.

Rooms each have a small VAV damper. My controller varies indoor fan speed based on duct pressure, but I also have a supply-air temperature loop that compensates as the filters get dirty and insures minimum flow to prevent freeze-up.

Outdoor unit has head pressure control and helps keep the unit online in cold weather when needed. I didn't try anything with the compressor, because small single-stage units are pretty darned efficient at full speed and design suction / discharge pressure and the system psychrometrics will change if slowed down. Slow the flow of refrigerant down and suction pressure will rise. De-humidification will be reduced.

My system is complicated, but that's what happens when a good electrical engineer goes bad during a period with too much time on his hands. It doesn't really save enough energy to pay back the investment in a reasonable time period, but it was fun to do and I do get wonderful humidity control and a quieter home as side benefits. I keep wiring diagrams and control sequences in the unit in case something happens when I'm not there. The usual HVAC fix-it guy will have a hard time with it, so I also listed the local building automation companies.

My apologies to anyone who has to diagnose the unit when I'm out of the country or moved to another city or am otherwise unavailable.

Good on ya,

Goober Dave

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[DASKAT]

Thanks Dave
Great post, Kept us laughing for a while, and very informative as the other posts have been. Thanks to all.
Daskat