[RFI] HVAC Variable Speed Motors

W2RU - Bud Hippisley W2RU at frontiernet.net
Sat Apr 10 13:28:36 PDT 2010

On Apr 10, 2010, at 4:06 PM, kd4e wrote:

> Warning:  Many of the new HVAC systems are promoting
> variable-speed motors in their design - it is one way
> that they increase the "magic" SEER.

The York "Affinity 3S" heat pump system has just such a beast in it.  Wherever you see reference in their literature to a "variable speed air handler", you know you're dealing with a PWM RFI generator.

Recently I had occasion to check the RFI environment around a home with one of these in it.  The installation was between two and three years old and had been done by a reputable heating/cooling firm.

The RF noise from this system totally wiped out my mobile installation (TS-480 with a simple Hustler mast / resonator antenna) on at least 80, 40, and 20, both in the driveway and for at least a quarter mile in each direction on the road in front of the house.  Similarly, it totally "trashed" reception on my Radio Shack all-wave portable from the broadcast band up through 160 meters and beyond, and it was audible no matter where I carried that receiver on the 20-acre property.  By far the biggest amplitudes were found at the thermostat on an interior wall of the main floor and on the power / control wiring at the compressor outdoors, but there was more than enough hash near _all_ the power wiring in the house to mess up ham radio reception on any antenna on the property and for some distance beyond.  Interestingly, the air handler chassis itself, which was located in an attic area one floor above the thermostat and not more than 25 feet away, was relatively "quiet".  

We ran four cycles of turning the system on and off to be absolutely certain that it was the source of the noise.  As the fan powered down each time, you could hear the PWM waveform change on my mobile rig, just before the fan shut off each time.

Interestingly, the house had two heat pump systems -- the Affinity 3S, which handled the heating / cooling for a 2-year-old addition, and another one, pre-PWM, that handled the original (10-year-old) section of the house.  We cycled the old heat pump, too, and found it to be absolutely quiet except for a single RF "click" when it turned on, which had been my experience in the 1980s when I lived in a heat pump home of my own.

We contacted the dealer that installed the Affinity 3S system, and he knew nothing about RFI from variable speed motors but he said he would check with York.  I received a call back the very next morning; York knows all about the RFI problem and has an RFI Kit for their variable speed units.  Installed by the local dealer, it would have cost around $250, plus the cost of 8-conductor shielded wire (!) to run between the hi-tech thermostat and the air handler chassis in the attic.  Of course, we don't know how effective any retrofit kit might be, since it's clear to me (who used to design PWM power circuits for a living) that no particular effort appears to have been made to isolate the high transient currents in the motor drive circuitry from other circuitry and leads going in and out of the chassis, or to keep the PWM current loop area small.  

Give York credit for instantly responding with information about their RFI Kit (which, by the way, is a stock item in their Oklahoma facility), but wouldn't it be nicer if a little more time and pennies had been put into minimizing the loop areas and doing some other prudent avoidance engineering in the first place.  Locking the barn after the horse is out does not usually lead to a fully satisfactory outcome.

Bud, W2RU

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