what is location?...these are high end systems with little field history on reliability....there have been noise issues but perhaps this issue has been worked out. I would get a price on a good 2 stage system from Trane or AmStd like the XR17 and XL18i just for comparison. You can bet though with high end systems any type of repair will be expensive if not covered by warranty.

IMO

I would be curious as to what prices you were quoted.

Thanks for the responses I live in Scottsdale AZ. The summers are very hot and I am looking to reduce my energy usage and reduce noise level when the unit is running.

With variable speed I have been told you can maintain a particular temperature without the unit running at full output, thereby being quieter and more energy efficient.

i am going to be staying in this home for a very long time so don't mind spending a little extra over a two stage unit if it is worth it long term.

There isn't anything quieter than full variable speed (meaning all motors are variable speed including the compressor). In low speeds you won't even hear it run unless you are right on top of it and even then the only way to really know it's on is seeing the fan spinning.

quiet yes when working correctly but there have been reports of noise issues on these true full var speed condensers.

IMO

Running at lower speed does not necessarily mean more efficient.

i am going to be staying in this home for a very long time so don't mind spending a little extra over a two stage unit if it is worth it long term.

Buying high end units may not be worth in the long run even if you plan to live in your house another 20+ years. You have to consider the life of the equipment, the amount of usage, and the cost of electricity. It may make sense in a hot climate like Arizona. The only way to tell is to do a cost analysis.

quiet yes when working correctly but there have been reports of noise issues on these true full var speed condensers.

I got a complaint for noise a few years ago on a Amana 2 speed unit that was installed by some other company. I had made a repair to a different unit same property that had failed due to a bad installation in which condensate got into and fried the circuit board and variable speed motor to the furnace.

These two units sat right next to one another so the customer thought the noise was coming from the system I worked on. Not so, another installation problem on the other system as well.

Installation is everything.

Running at lower speed does not necessarily mean more efficient.

In comparison to what?

I think the OP is under the assumption the operating efficiency of the equipment is a function of the operating speed. In other words running in a low stage is more efficient than running in the highest stage.

It is in terms of energy used. The highest stage would be least efficient because it would burn more energy.

That is true, but the highest stage also produces the most heating and cooling.

The load on the house constantly changes. The heat of the day is typically 3pm to 6pm. So doing a little math tells you 1/8th of the day is typically when you need highest stage... maybe as much as 1/6 of the day in some high heat times.

That is how multi stage equipment efficiencies are achieved. But these systems aren't just about saving on utilities they are about comfort, noise reduction etc.

You don't buy this type of equipment for what it will save you on utilities because they likely won't make up the difference in cost* (depending on utility rates where you live.)

But what about when the heat pump is heating in the winter? During the dead of winter the coldest part of the day is sunset to sunrise. In northern states that could be as long as 15 hours per day. That would mean the heat pump is likely to run in the its highest stage for long period of times.

But what about when the heat pump is heating in the winter?

The OP lives in Arizona. Northern states that get really cold like in the 20's and lower for long periods aren't really a good climate for a heat pump let alone a high end completely variable speed HVAC system. A heat pump can work in the northern climates but there are tradeoffs.

As always: Design, design, design.

I was questioning your statement that the highest stage (heat pump, AC, furnace, etc.) is the least efficient because it uses the most energy. If you have data that supports your statement then please provide it. There have been differences of opinion on this topic. So perhaps you can set the record straight for all of us.

If the only available heating alternatives are electric resistance and oil, I can assure you that a Heat Pump is a viable option in the North. Not everyone has access to natural gas and I haven't seen coal since I was a kid.

I am not really trying to debate whether the technology is worth it or not. I am getting the Trane 20i for about the same price as the 18i so will probably go with the 20i. I guess I will find out down the road how reliable the unit is, as not many seem to have experience with it and they haven't been out long enough to determine that. Trane has the better name recognition vs Rudd so you pay a bit more for that I guess with performance being the same.

I was questioning your statement that the highest stage (heat pump, AC, furnace, etc.) is the least efficient because it uses the most energy. If you have data that supports your statement then please provide it.

The word 'efficient' is most often used to determine operational cost. So when I use the word 'efficient' in this way it means that it will cost you more to 'run' the systemwhen it runs in the highest stage.

Efficiency in the sense of being able to cool 20 degrees from that of outside temperature (in my market 20 degrees is design, your location may differ.) Is related to 'performance' and not operational cost.

If the load is above and beyond what a system can handle... this has to do with outdoor ambient temperature, size of the system, size of the structure, insulated, building tightness, duct system sizing. Efficiency related to this is the ability to attain design temperature within the structure and maintain it.

Let me know what you mean by 'efficient' because this is a loaded word.

It's like saying a heat pump is more efficient than a gas furnace. But this statement would be true because a gas furnace spews heat from the flue while a heat pump does not. This doesn't mean a heat pump is 'CHEAPER' to operate than a gas furnace.

Saltidawg,

'This is off topic' - but shows things can be made to work regardless of climate due to DESIGN of the system.

In my early days I worked for an employer in a Chicago suburb in which they had data rooms that had to be 'cooled' year round. These rooms were cooled by small little residential split systems in which the condensers sat on the roof with an air handler inside the data room.

Most residential based systems instructions tell you not to run the system below 55 degree OAT. But yet we ran these systems in 0 degree temps and -40 degree below zero wind chill. More importantly, they worked and did what we wanted them to do.

How do you suppose we did it?

Let me know what you mean by 'efficient' because this is a loaded word.

I use the same definition as the US Government. Efficiency is the amount of heating or cooling produced compared to the amount of energy (electricity, gas, etc.) used to produce it. When you are comparing the same HVAC system (furnace, heat pump, AC, etc.) then efficiency will track costs. But if you are comparing a furnace to a heat pump, then it the comparison gets more complicated.

My original question was about the efficiency performance of the various stages of an AC in a hot climate. I was asking specifically about comparing the operating costs of the same piece of equipment in various states. You stated the high stage is less efficient because it uses more energy. The high stage does use more electricity, but it should be operating for a shorter period of time.

The question is does the shorter operating time at a higher output offset the higher use in energy?

I was asking specifically about comparing the operating costs of the same piece of equipment in various states. You stated the high stage is less efficient because it uses more energy. The high stage does use more electricity, but it should be operating for a shorter period of time.

This is a hard question to answer... because the answer depends on many different things.

The question is does the shorter operating time at a higher output offset the higher use in energy?

The main reason the higher output exists is 'typically' to maintain comfort of the structure during high load times. How you asked this question is misguided, because a two or more staged AC is designed in which it will engage the lower stage first which will cause the system to run longer at a reduced energy usage.

If the lower output doesn't maintain the load of the structure only then will the higher output engage... meaning stage 2 or higher.

The idea going forward in manufacturing processes of higher end HVAC systems is what is called 'Load Matching'.

In order to understand what 'load matching' is you must know what a load is. A load is ANY heat producing source placed on a structure that you are trying to cool via mechanical means. (Air Conditioning)

Realize the load... how ever much it amounts to be is constantly changing. The heat of the day, appliances in the structure, radiant heat, convection, people, the structures envelope, lighting, area or volume --- anything that produces heat in the structure is a load.

Now that you have a better grasp on what a heating load is... let's change the equation a little to hopefully convey this to you so you understand it.

Let's say you are a produce farmer and at one orchard you have 150 tons of apples that need to be hauled to market. 150 Tons of apples is your load. This load requires work to move the 150 tons to market. You could use a pick up truck to move this load, but it would be extremely inefficient it would take you many trips. The best way to move this load is via semi tractor trailer.

At a new smaller orchard you just started producing cherries and you have 500 pounds to move to market. 500 pounds is your load in this case. Now does it make any sense to you to pay for a semi tractor trailer to move this load?

The heat of the day is from 3-6pm. This is the only time a home needs a semi in the way of an air conditioner to move the heat from inside to outside. The rest of the day it only needs a pick up truck. 1/8 to 1/6 of a day --- semi --- 7/8 to 5/6 of the day --- pick up truck.

Short cycling does not equal efficiency... short cycling equals wear and tear and job security for me. Provided of course that the flaming rock in the sky does not burn out.