Copeland 2-stage compressor is around 70% capacity on "low". Most, if not all, manufacturers only make the two stage products in even tonnages. If I use a 2-stage in a 3.5 ton house, I need to use a 4 ton unit, which would be about 2.8 tons on low. Hardly much difference.
Two stage - what's the point?
prec_mktg
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Is a 2 stage condenser required to run @ lower airflow? I’m getting estimates on replacing AC in the Florida Keys. So far the 2 companies I’ve talked to recommend a 1 stage condenser to keep cost down because the condenser only lasts about 6 years when you live on salt water. But I’m also recommended a variable speed AH. Why a variable AH with a 1 stage condenser? Can you use a thermostat with a humidity sensor connected to the dehumidification terminal on the AH to reduce airflow with the 1 stage condenser or is a 2 stage required?
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25,203 Posts
With a VS blower and the right thermostat you can slow the blower if the humidity is too high in the house with adjustments in 5% increments.
First, 2 stage units are only made in whole ton sizes. So there is no 2 stage 3.5 ton units.
Second, what you propose would opposite of what a 2 stage A/C is suppose to do.
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833 Posts
I'm not sure you fully grasp how a 2 stage should work. By design they run alot.
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61,972 Posts
Might want to be careful with that...
My experience (about a dozen of them installed)...
Says the ductwork has to be a certain way (not just manual D... specifics)...
LONG story.
Yeah, I definitely agree. Not every application is right for 2 stage or VS. I was saying for my own home I would upgrade to a VS if my systems crapped out. Come to think of it almost all of the VS systems I've installed are full zoning. I can only think of 2 that aren't zoned.
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Sounds like you have had more experience with the zoning side than I have...
I have only 2 zoned inverter drive split systems out there... out of a dozen...
The others... had to do some duct mods on each home... to make them work properly.
And I could have put in a dozen more... except the required mods to the house... would have been to expensive.
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505 Posts
"When i upgraded to a 3 ton 2 stage from a single stage 3 ton HP, i have noticed that except on mild days, it would take forever for first stage to satisfy the thermostat."
Which is exactly how it is supposed to work.
"I should have had them move up to a 3.5 ton unit"
No, you shouldn't have.
"so that first stage would work better and then on temp extreme days 2nd stage would work more efficiently and not run as long."
Running non-stop gives you the best possible efficiency, cycling reduces it, so it wouldn't "work better".
"I will have to make sure to do that for my next unit!"
Your funeral.
Design conditions means the temperature that is exceeded just 1% of the time on a 30 year average.
For me for cooling that's 90° F outside. Designing for 75° inside means a 15° difference. I personally prefer closer to 70°, but we'll go with 75°, it makes the math easier. A 2 stage has say a 66% capacity in first stage so a 10° difference.
So theoretically assuming no other fixed heat loads in the house and no solar gain, IE just an empty shaded box with an A/C that's exactly the right size:
75° or lower outside, system never runs.
76-84°, system cycles between off and first stage.
85°, system runs non stop in first stage
86-89°, system dithers between first and second stage
90°, system runs non-stop in second stage
91° and up, system can't maintain set point. This is the <1% of the time conditions.
With the solar gain and interior heat loads the system will start to run when it is cooler outside than inside, and of course system capacity decreases with increasing outside temp and hence head pressure but it just skews the span of the numbers, not the conclusion.
Intentionally oversizing so that you are covered that <1% of the time will cost you comfort and efficiency the other 99% of the time, which seems foolish to me.
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In a perfect world... that is a great arrangement...![Image]()
"When i upgraded to a 3 ton 2 stage from a single stage 3 ton HP, i have noticed that except on mild days, it would take forever for first stage to satisfy the thermostat."
Which is exactly how it is supposed to work.
"I should have had them move up to a 3.5 ton unit"
No, you shouldn't have.
"so that first stage would work better and then on temp extreme days 2nd stage would work more efficiently and not run as long."
Running non-stop gives you the best possible efficiency, cycling reduces it, so it wouldn't "work better".
"I will have to make sure to do that for my next unit!"
Your funeral.
Design conditions means the temperature that is exceeded just 1% of the time on a 30 year average.
For me for cooling that's 90° F outside. Designing for 75° inside means a 15° difference. I personally prefer closer to 70°, but we'll go with 75°, it makes the math easier. A 2 stage has say a 66% capacity in first stage so a 10° difference.
So theoretically assuming no other fixed heat loads in the house and no solar gain, IE just an empty shaded box with an A/C that's exactly the right size:
75° or lower outside, system never runs.
76-84°, system cycles between off and first stage.
85°, system runs non stop in first stage
86-89°, system dithers between first and second stage
90°, system runs non-stop in second stage
91° and up, system can't maintain set point. This is the <1% of the time conditions.
With the solar gain and interior heat loads the system will start to run when it is cooler outside than inside, and of course system capacity decreases with increasing outside temp and hence head pressure but it just skews the span of the numbers, not the conclusion.
Intentionally oversizing so that you are covered that <1% of the time will cost you comfort and efficiency the other 99% of the time, which seems foolish to me.
However...
I can tell you this: HO's down here... in a sticky AC market... will NOT tolerate being hot or sticky... regardless of ideal ideas...
They want to feel comfortable... and they do not care about whether it is as it is supposed to be.
We size to design... by a point between low and high... giving us a reserve of about 4-5*F above design.
The result is improved humidity control... reserve capacity...
Which makes for a happy customer!
Yeah, it may not be as efficient...
However not everyone thinks ONLY in efficiency.
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I want 50%RH when I am in my home with my family. My home is well built and air tight enough to require fresh air ventilation at a fresh air change in 4-5 hours. It is green grass climate.
At the design condition of 93^F, 75^F dew point. Evenings are 75^F, 75^F dew points. Six occupants.
Tell me about how I am going to provide 75^F, 50%RH, a 55^F dew point with 90 cfm of fresh air ventilation.
On a design day at peak, I need 30,000 btus sensible cooling and 7,000 btus latent cooling per hour.
The next 3 days are raining and I need 5,000 btus of latent only without any sensible per hour.
The rest of the three mild seasons, the loan is somewhere in between.
Multi-speed all you want, there is no way to deal with this extreme variable load with muilti-speen a/c.
Yet a properly setup conventional a/c will deal with designed a/c and ducts will maintain 75^F, 50%RH. As the sensible load declines, the a/c cycles to maintain sensible temperature and the extra latent capacity will maintain 50%RH.
As the length of the off cycles get longer as the sensible cooling declines, the %RH rises. With multi-speed a/c, we could increase the latent capacity and decrease the sensible capacity to some degree. By evening when very little or no sensible cooling is needed, multi-speed a/c is useless. On a rainy day forget any chance unless we use reheat.
Consider, use a simple single speed a/c with a well designed small whole house dehumidifier sized to handle the latent cooling load of a day without any sensible cooling load.
Simple and effective temperature and %RH during all of the possible weather conditions.
What is difficult about this.
We are doing thousand of fixes year. More and more are choosing this as the first choice for space conditioning.
Most a/c manufactures getting past the difficulty of admitting that the a/c can not deal with the wide range of sensible/latent ranges.
Fresh air is a must for good indoor air quality and supplemental dehumidification is needed to provide temperature and %RH during all of various weather conditions.
Your comments please.
Regard Teddy Bear
At the design condition of 93^F, 75^F dew point. Evenings are 75^F, 75^F dew points. Six occupants.
Tell me about how I am going to provide 75^F, 50%RH, a 55^F dew point with 90 cfm of fresh air ventilation.
On a design day at peak, I need 30,000 btus sensible cooling and 7,000 btus latent cooling per hour.
The next 3 days are raining and I need 5,000 btus of latent only without any sensible per hour.
The rest of the three mild seasons, the loan is somewhere in between.
Multi-speed all you want, there is no way to deal with this extreme variable load with muilti-speen a/c.
Yet a properly setup conventional a/c will deal with designed a/c and ducts will maintain 75^F, 50%RH. As the sensible load declines, the a/c cycles to maintain sensible temperature and the extra latent capacity will maintain 50%RH.
As the length of the off cycles get longer as the sensible cooling declines, the %RH rises. With multi-speed a/c, we could increase the latent capacity and decrease the sensible capacity to some degree. By evening when very little or no sensible cooling is needed, multi-speed a/c is useless. On a rainy day forget any chance unless we use reheat.
Consider, use a simple single speed a/c with a well designed small whole house dehumidifier sized to handle the latent cooling load of a day without any sensible cooling load.
Simple and effective temperature and %RH during all of the possible weather conditions.
What is difficult about this.
We are doing thousand of fixes year. More and more are choosing this as the first choice for space conditioning.
Most a/c manufactures getting past the difficulty of admitting that the a/c can not deal with the wide range of sensible/latent ranges.
Fresh air is a must for good indoor air quality and supplemental dehumidification is needed to provide temperature and %RH during all of various weather conditions.
Your comments please.
Regard Teddy Bear
If you look at the Heat Gain for a property and compare to your average temp hours there is a strong chance they correlate around that 70%. In My AO 70 % of the cooling hours per day are 86*F or less. 97.5 % is 93*F or less. why run 100% capacity and over cool the structure with short run cycles?
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5,226 Posts
in my case, it happens that about 90% of run time is first stage, the tstat saves run times and number of cycles in history. I do have a whole house dehum, but if I bump it down a dgreee or two, which the tstat can also do on its own, it will drop humidity in 30-40 minutes pretty well.
Trane used to make a 2 stage 16 seer, then made it communicating, which complicated it. now its a single stage. I liked that the 2 stage comp just needed 24 vdc to a solenoid to switch it, not too complicated.
Trane used to make a 2 stage 16 seer, then made it communicating, which complicated it. now its a single stage. I liked that the 2 stage comp just needed 24 vdc to a solenoid to switch it, not too complicated.
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2,206 Posts
My only gripe with the unloading scrolls is that the solenoid is not replaceable, but very simple/effective design
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