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Central Air Conditioners: Variable-Speed vs Soft Start Kits

Some time ago I detailed our 2021 decision to upgrade to a variable speed central air conditioner, the various features and benefits underlying that decision. That was the same year, after the Great Texas Freeze of 2021, we went about implementing a practical strategy for backup power.

We had been planning for these projects for quite some time, setting aside the necessary funds. Things might have been quite different if the old air conditioner had outright failed unexpectedly, requiring immediate replacement. We probably would have opted for a single-stage replacement purely on the basis of cost. After all, that’s what happened in 2002 when, as new homeowners, we were more-or-less forced into buying the American Standard Allegiance 12 system.

Single-stage systems certainly get the job done. They’re standard, builder-grade equipment. They simply don’t fit into our current strategy for backup power. At least, not without some fiddling. What follows in an exploration of that fiddling.

How Much Power?

Air conditioners have two major kinds of power ratings:

Locked Rotor Amps (LRA) is the amount of current required as the system starts. It takes a large amount of power to get the fans and compressor moving from an idle state. This often called the “in-rush” or “start-up surge.”

Rated Load Amps (RLA) is the amount of current required on an ongoing basis, once the system is running. This is typically much less than the start-up surge.

American Standard Allegiance 12 Label

Our old Allegiance 12 system needed a whopping 107A to start, but settled back down to 22A once running.

To successfully start it on backup power a generator would need to deliver: 107A @ 240 VAC = 25,680 Watts! That’s over 25 kW!

This explains why installed standby generators (Generac, Kohler, Cummins, etc.) tend to be in the 15-36 kW range. Our old Predator 8750 generator only delivered 8,750 Watts (peak) and 7,000 Watts continuous.

When running on generator (or inverter) power we’d simply not be able to use the air conditioner. The 9 kW generator could not provide enough power.

Variable Speed AC

Our new, variable speed AC system doesn’t have the massive LRA rating of the old one; LRA = 12A and RLA = 16A.

American Standard Platinum 18 Label

It never draws more than 16A @ 240 VAC = 3,840 Watts. That’s an easy load for our new Predator 9500 inverter. It’s worst case draw is about 50% of the inverter’s continuous rated power.

Incidentally, virtually all mini-split air conditioners (heat pumps, too!) are variable speed designs.

Specifications vs Measurements

It’s better to know with certainty than to guess. We can reference the specifications for LRA, but it’s better to take measurements. When we had the new AC unit installed I measured it’s peak surge current.

AS PL18 Surge Actual

On that warm day in July 2021 it drew only 10.82A. That’s 9% less than the 12A shown on the label. This tells us that the rated LRA is likely a worst case number. YMMV.

This is good to keep in mind as you make decisions based upon stated specifications. Measurements trump specifications in the real-world.

Soft Start Kits

If we’d not been able to install the VS system we would have purchased a single stage system, but added a soft start kit. A soft start kit modifies how a non-VS air conditioner starts. It ramps up the fan and the compressor slowly, staggering the startup load over a short time. To borrow a phrase, it “flattens the curve.”

These have been used in industrial applications for a long time. They allow running large compressors from a constrained power source.

In consumer applications they have been offered to allow people with RVs to run their air conditioner while off-the-grid. Many people with RVs carry small inverters to provide a quiet running source of power while travelling. Until relatively recently, the state-of-the-art in inverters delivered just 3-5 kW. By flattening the startup surge an RV owner is able to run their AC unit from a modest, relatively quiet inverter.

This origin story for soft start kits also explains why you’ll find low-cost models that only handle 120 VAC. This is what most of the RV market requires. RV AC systems are the 6-15k BTU range typically require 120 V power.

Eventually, a market for for residential soft starts emerged, driven by a homeowners desire to run air conditioning while on backup power. Residential applications require a soft start capable of operation at 240 VAC, and much higher power current.

Micro-Air EasyStart

Micro Air

Micro Air appears to be the leader in the space. They make a handful of soft start kits, some of which target residential applications.

Hyper Engineering Soft Starter

Hyper Engineering

Hyper Engineering also makes a SureStart kit for residential applications. Theirs can handle AC systems up to 7T.

RLA Reduction

Micro Air claims that their products deliver 40 – 70% reduction in RLA. Hyper Engineering offers the following nice little chart.

Typical Amperage Reduction with Sure-Start

Fishing For Information

Since I have no first hand experience with soft start kits, I thought I’d seek out guidance from those who do. I posed a question of claimed vs actual results in the r/Generator Subreddit.

A few people answered with real stats, indicating that 70% reduction of LRA is actually possible. One noted start-up surge of just 25A after installing soft start to a 2.5T AC unit. Another had just installed it to an older 2T AC unit and reported surge of 25A, down from 62A. That’s a reduction of 62%.

Others posed their own questions, which in part inspired me to write this post.

As I was working on this another thread was started, where someone was asking about experience different makes of soft starter. There’s some good info there as well.

Some Hands On

A neighbor recently bought a Pulsar 9500 invertor as part of their backup power solution. Once they had an inlet & interlock installed, I helped them with an initial test, transitioning their home to the newly unboxed invertor. Everything went smoothly. That is, until the air conditioner kicked in.

They have an existing Lennox XS16 2-stage air conditioner. It’s a reasonably new, mid-efficiency model with stated RLA of 96A. Doing a little math: 96A @ 240 VAC = 23,040 Watts. That’s 24 kW!

Without a soft start, the air conditioner simply trips the 50A breaker on the invertor. Reliably. Every single time it starts.

The Pulsar invertor has a very handy real-time display of voltage and load. It was pegged at 9.4 kW when the breaker tripped. I appreciated it acting in a “peak-hold” manner. It saved me fetching a clamp-on amp meter and repeating the experiment.

The fact that the breaker tripped is a good thing. The invertor protected itself. Nothing was damaged. Everything easily reset.

If we are to believe the table above, a Sure Start should reduce their RLA to around 29A. 29A @ 240 VAC = 6,960 Watts. That’s well within the 9,500 Watts their inverter is rated to handle.

This Week I Learned…

Our decision, taken in 2021, was to have a variable speed air conditioner installed, since we needed an entirely new system anyway. And like most things, I was attracted to what appeared to be an elegant, if costly, solution. It meets our various requirements. The house is comfortable. It integrates with Home Assistant. Our electric bill is a little smaller. And we can run the whole home on the new inverter should the need arise.

Yet, I sometimes wondered, “did I make the right choice?” At that time, I was not aware of soft start kits.

If you are crafting a strategy for backup power using a modest generator or inverter, and you have an existing central air conditioner that you want to keep, a soft start kit might be a way forward. They seem to be a viable solution. At least in some cases. They are inexpensive and do reduce the RLA substantially.

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