Energy Savings of 2-Stage Compressors?

iamweaselFebruary 4, 2014

Trying to decide between a Bryant Legacy and Preferred series gas pack for our new house. (Gas pack is all I can get on the first floor - not a split system which we will have on the 2nd floor.)
The Preferred series has a 2-stage compressor and that is the primary advantage over the basic system. (As well as offering a thermostat with humidity control on it that I can pair with the whole house humidifier we are getting.)
I am in North Carolina and the A/C runs quite a bit. Can anyone give me a ballpark estimate on how much energy the Preferred series will save over the Legacy because of the 2-stage compressor? (i.e., it should use about 10% less, etc.) Trying to decide if the upgrade cost is worth it. Any other tidbits of advice would be welcomed, too....Thank you!

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tigerdunes

Post Mdl numbers of each package system...

    Bookmark   February 4, 2014 at 12:24PM
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iamweasel

The preferred is a model 557E. I know this for sure.

I am not 100% sure of the Legacy "base" system that normally comes with the house, but I think it would be a 574D or 574C but not exactly sure which.

    Bookmark   February 4, 2014 at 12:46PM
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tigerdunes

What is price difference?

    Bookmark   February 4, 2014 at 1:13PM
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iamweasel

Price difference is about $3,000. (Got some builder markup in there so keep that in mind.)

The two main things that have peaked my interest in this is not only the energy savings but the upgraded thermostat with humidity controls on it, too. We are getting a whole house humidifier because we hate the dryness in the winter.

    Bookmark   February 4, 2014 at 1:35PM
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tigerdunes

The 577 is a much nicer unit, quieter, better cabinet, and I believe the warranty is better.

$3 k difference? No way . I guess your builder is putting 2 k in his pocket.

In my neck of the woods,we call that highway robbery.

I would tell builder I will handle the HVAC.

You do want a st steel heat exchanger and the tin plating option on the evap coil. I see nothing to indicate the 577 has a two stage condenser.

IMO

This post was edited by tigerdunes on Tue, Feb 4, 14 at 13:54

    Bookmark   February 4, 2014 at 1:42PM
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iamweasel

I know that is more than it should be but I would hate to put the basic unit in and be unhappy with it. With the new energy codes in NC i was concerned about the base unit cooling enough.

    Bookmark   February 4, 2014 at 2:29PM
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tigerdunes

These units are rated.

BTU cooling is cooling. Now the efficiency is a different story as far as operating cost. 14 SEER is better than 13 SEER.

I have no idea about NC code or standards.

Pay attn though. With either model, you want the st steel heat exchanger option and the tin plated coil option. Did dealer include those? If these are not included, you would be getting a POJ....

And again, I saw nothing in the website to indicate the 577 has a two stage condenser. Where did you get the idea it did?

IMO

    Bookmark   February 4, 2014 at 4:13PM
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iamweasel

See the attached brochure where it mentions the 2-stage compressor.

I do not know about the stainless steel heat exchanger or tin plated coil. I will ask about that.

NC codes dictate the units are rated for an 18-degree spread, meaning if it gets to be 95 outside then inside the basic legacy unit is rated to reach 77 inside, etc. I was afraid the house would be too hot in the summer here and that is why the interest in the 2-stage unit. I was told it can handle the extreme hot better and will make the humidity better so it "feels" cooler than the basic legacy unit.

    Bookmark   February 4, 2014 at 4:30PM
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iamweasel

Lets try this again...here is the brochure. Here is the last page of the brochure....look at item #1.

    Bookmark   February 4, 2014 at 4:36PM
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mike_home

I did a little research to satisfy my curiosity. I think both of you guys may be right.

The 577E (see attached product data) is a 2-stage furnace and AC. It even comes in half ton sizes which is unusual.

The 577C model is only a single stage furnace and AC. I am surprised Bryant has used this naming convention. It causes a lot of confusion. You also have to be really careful as to which model the builder installs.

As to your original question, a 2-stage AC does not save much money assuming it is about the same SEER/EER rating as the single stage. There is a small savings in that when running on the low stage it is not stopping and starting as often. It can provide better humidity control to a point where you may raise the temperature 1-2 degrees. If you do that then there is some modest amount of savings, but not enough to justify an extra $3000. With your electric rates you will never see a pay back. The 2-stage AC is intended to provide more comfort than energy savings.

Here is a link that might be useful: Bryant 577E product data

    Bookmark   February 5, 2014 at 8:56AM
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iamweasel

Yeah, I was confused on the model #'s, too. They said it definitely will be the 577E with the two-stage compressor.

I know what they are charging me is high but frankly I have no other option there. I have to use the contractor they have already signed up with so I can't shop this around.

I know I probably would not recoup that cost upcharge from energy savings alone but I am willing to pay extra for better comfort. How much, though, is what I'm trying to figure out. I was also hoping this upgraded model would do better on maintenance.

    Bookmark   February 5, 2014 at 9:10AM
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SaltiDawg

For the OP - with no disrespect to Mike - "The 2-stage AC is intended to provide more comfort than energy savings."

This is not correct. The cost per BTU of cooling with the 1st Stage only is considerably less than the cost per BTU when operating the 2nd Stage. (Parenthetically, virtually all manufacturers say something to the effect, "Two-stage scroll compressor�"operates at low stage most of the time for quieter, more efficient performance " They do not say that the 2 stage reduces humidity and thus may give the feeling of cooler air and allow you to raise the setpoint. )

I came under what I took to be a some personal attack a few months ago for attempting to support this based on my practical experience in compressor operation and also my educational background.

I mean no disrespect to ANYONE here...

PS I have absolutely no experience - nada, zip, zilch - in installing or maintaining home heating and A/C. Just a homeowner.

This post was edited by saltidawg on Wed, Feb 5, 14 at 10:37

    Bookmark   February 5, 2014 at 9:27AM
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mike_home

"The cost per BTU of cooling with the 1st Stage only is considerably less than the cost per BTU when operating the 2nd Stage."

Saltidawg,

We had this debate previously, but I never saw any conclusive evidence to support your position. I actually would be very happy it were true since I own 2-stage AC condensers. However I have yet to see a product data sheet from a manufacturer which states SEER/EER ratings for the low and high stages. You would think the manufacturers would the low stage efficiency as a selling point.

I would agree the efficiency may be a little higher in the low stage since the condenser is dissipating less heat inside an enclosure designed to handle the high stage. But I would not describe the energy usage as "considerably less".

Again I would be happy if you can prove me wrong. I respect your knowledge about compressors, but can you show me the proof?

    Bookmark   February 5, 2014 at 11:44AM
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SaltiDawg

mike,

I did not recall it was you that exchanged posts with me. In that exchange I referenced one White Paper discussing the very issue of efficiency versus 1/2 stages.

Here is another general discussion:

"When you consider that the part load output on a two-stage compressor operates at 67 percent, the result is a 22 percent decrease in cooling and heating capacity. At part load, the amount of kilowatts consumed is less with a two-stage unit. Because the unit can operate at either full or part load capacity, the kilowatts fall off faster than the capacity. The end result is a 40 percent increase in efficiency." (This is for a air-water HP.)

As I have tried to explain, mass flow rate in a given pump or compressor varies with the cube of the power required for operation... this is an accepted relationship and all of the home heating and cooling folks make the claim of increased efficiency with 2 stages compared to 1 stage.

With just a few keystrokes i found some number of sites that present efficiency data for 2 vs 1 stage applications - and yes, "considerably less" energy is an apt descriptor.

Some of the comparisons are more difficult to follow than we might like ....

As I said before, I accept my inability to adequately convince you of a fundamental operating parameter as my shortcoming and I an disappointed in my failure to be able to apply my background to this task.

    Bookmark   February 5, 2014 at 1:10PM
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mike_home

Saltidawg,

Can you post the link where you got the quote you posted? I would like to read the whole article. I would be interested in any other links which present efficiency data.

You had posted the cube law relationship in the past. I have not been able to find this equation and how it relates to efficiency.

We in danger of hi-jacking this thread. If you have good information then start a new thread with a new topic.

There is no need to apologize, we have provided good information. This would be a boring forum if we were all in complete agreement. :)

    Bookmark   February 5, 2014 at 1:24PM
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iamweasel

Good stuff guys, no worry about hijacking the thread as all of this relates to my initial question. :)

    Bookmark   February 5, 2014 at 1:28PM
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SaltiDawg

Final post by me.

Link below to some VERY simple tools that apply to fans/compressors/pumps.

As you read down and get to the section re wheel velocity changing, you'll see very clearly the cubed relationship to power.

ALSO: Ask to see a performance curve for a two stage A/C compressor that shows two stage operation to be more efficient than single stage. Ain't gonna happen. lol

.

Here is a link that might be useful: Do The Math

This post was edited by saltidawg on Wed, Feb 5, 14 at 15:51

    Bookmark   February 5, 2014 at 1:45PM
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