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  1. #1
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    Oversizing with 2 stage equipment to save energy/run less

    I was hoping someone could direct me to some studies/tests done on the energy usage of different sized systems for the same load. I have never taken any classes on load calc or system design or anything like that but have tried to read on it a lot.

    Thinking about it in my head, it would seem that having a larger system that can cool your house down when you get home (or on the way home with a wifi tstat) instead of having to maintain a fairly low temp throughout the day would offer a lot of energy savings. The low stage operation would then allow it to maintain the temp without cycling on and off too much. Does anyone know why this would not be the case?

  2. #2
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    You would save more energy by tightening the envelope in your home and using a smaller unit to begin with.

    Sent from my SM-G965W using Tapatalk

  3. #3
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    I do not know of any such studies - although there may well be some - but I am interested in talking about it. And perhaps some of the far more knowledgable about these things than me will later add to the discussion.

    True Two Stage, to me, would have to have the indoor air flow adjusted to Exactly and Properly match the system's cooling capacity in each of the stages.

    1. I don't know if this is typical of residential-grade system designs.

    2. It would seem somewhat difficult as the air duct design and installation is such a large part of air flow - and the actual air ducts would not change in size to match the capacity of each stage. <g>

    I like your concept, as expressed, and it would seem to be 'the best of all worlds' - but I do think the operating costs would be somewhat higher as the reduction in cooling capacity is unlikely to track linearly with operating cost.

    Again; I do not 'know' this - but I suspect that two-stage systems are 'maximized' for full capacity operation, with the lower stage being treated as an also-ran kind of add-on.

    I hope the really smart guys chime in soon. <g>

    PHM
    -------------




    Quote Originally Posted by ats1986 View Post
    I was hoping someone could direct me to some studies/tests done on the energy usage of different sized systems for the same load. I have never taken any classes on load calc or system design or anything like that but have tried to read on it a lot.

    Thinking about it in my head, it would seem that having a larger system that can cool your house down when you get home (or on the way home with a wifi tstat) instead of having to maintain a fairly low temp throughout the day would offer a lot of energy savings. The low stage operation would then allow it to maintain the temp without cycling on and off too much. Does anyone know why this would not be the case?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  4. #4
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    All around bad idea IMO. Especially if the ducts arnt upgraded. At my last company the sales people had a bad habit of oversizing two stage equipment for whatever reason and one person told me his bills were actually higher despite the higher seer new equipment.


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  5. #5
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    I have seen systems similar to what the OP is asking about. The evaps are put in series, which takes care of the duct sizing. The application was always for 100% outside air. And you'd typically put a hot gas valve on the primary coil, which would be the second coil.

    It was of course done because it needed to be, had nothing to do with energy efficiencies.


    Quote Originally Posted by Poodle Head Mikey View Post
    I do not know of any such studies - although there may well be some - but I am interested in talking about it. And perhaps some of the far more knowledgable about these things than me will later add to the discussion.

    True Two Stage, to me, would have to have the indoor air flow adjusted to Exactly and Properly match the system's cooling capacity in each of the stages.

    1. I don't know if this is typical of residential-grade system designs.

    2. It would seem somewhat difficult as the air duct design and installation is such a large part of air flow - and the actual air ducts would not change in size to match the capacity of each stage. <g>

    I like your concept, as expressed, and it would seem to be 'the best of all worlds' - but I do think the operating costs would be somewhat higher as the reduction in cooling capacity is unlikely to track linearly with operating cost.

    Again; I do not 'know' this - but I suspect that two-stage systems are 'maximized' for full capacity operation, with the lower stage being treated as an also-ran kind of add-on.

    I hope the really smart guys chime in soon. <g>

    PHM
    -------------
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  6. #6
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    I would imagine you want to size the ducting to work with the full capacity of the system but match the low stage to the actual need of the house. I've heard varying opinions on "oversizing" ducting but im sure its better than undersizing.

    I can also see this being a good solution to situations similar to my parent's house where they have a large but well insulated house so they are fine with a 3 1/2 or 4 ton system most of the time but often have large family gatherings at their house with 20+ people there and hours of cooking so it's no fun if the a/c can't keep up with that extra load. I'm not asking this to help size their system, it's just another thought.

  7. #7
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    http://www.fsec.ucf.edu/en/publicati...-RR-383-12.pdf

    Fl Solar research has various studies that will answer your question.
    Just a sample of some of their work.
    When you come up with answer, let us all know.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  8. #8
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    Variable speed is probably the best compromise. I know it's done in mini-splits, but is it done in other residential or light commercial RTU's?

    If it isn't done yet, I'm sure it's just right around the corner.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  9. #9
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    Thank of it this way, the structures heat gain is what it is. If you remove all the heat in an hour or over the whole day it still takes the same amount of work.

    Granted there are subtle advantages that would promote a fast pull down, such as less starting amps and less cycling so the coil stays moisture laden also AC’s are more efficient when the indoor temps are high and heat gain to the space would slightly decrease because of less temperature differential.


    Despite all these subtle advantages you would gain, once you figure equipment cost and additional ducting and addition repair costs added by equipment with more components, more refrigerant it would be a wash at best and thats if it was designed this way at construction. Not even close to a wash if it were a retro job or “upgrade”

    With only the expense of the stat you could install a programmable stat with smart recovery and get the best of both worlds. You’ll just have to experiment with how far you can go with the setback.



    Sent from my iPhone using Tapatalk

  10. #10
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    Quote Originally Posted by ats1986 View Post
    I was hoping someone could direct me to some studies/tests done on the energy usage of different sized systems for the same load. I have never taken any classes on load calc or system design or anything like that but have tried to read on it a lot.

    Thinking about it in my head, it would seem that having a larger system that can cool your house down when you get home (or on the way home with a wifi tstat) instead of having to maintain a fairly low temp throughout the day would offer a lot of energy savings. The low stage operation would then allow it to maintain the temp without cycling on and off too much. Does anyone know why this would not be the case?
    My experience is that a properly sized a/c and setting the stat up or off when routinely unoccupied, is practical in many settings and with many occupants. I do this with a FL house, It is unoccpied from noon to 5:00 PM. By 500 PM the home is up to 80^F. Cooling down to 75^F takes a hour plus. on a 85^F day. We get a big amount of moisture removal, below 50%RH. On a design day, it takes longer, but feels good.
    I agree if the a/c is undersized, forget it.
    No matter during evenings and rainy days, supplemental dehumidification is needed to remove moisture from infiltration and occupants.
    When green grass homes are unoccupied for days, turn off the a/c and maintain <50%RH better and for a fraction of cost of attempting with a/c at any setting.
    This is a good discussion.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  11. #11
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    I picture that it takes less work, and has less operating cost, to run a 5 ton system where a 2 ton system would be the proper size. To the same indoor temperature. Especially once the heat-soak of the structure is gone. And I imagine this might be a decent way to go about things in places where the outdoor humidity is always low.

    But back to what I started out with - IF a two-stage system had BOTH of the stages optimized for dehumidification - then I think the advantages would outweigh the costs.

    My question is: Are They Ever set up that way? Or are they even capable of being set up that way? And I mean with the cooling coil always 30º below the RAT? In both stages?

    I do not know, but I picture that low-stage operation is just left to 'run wild', so the evap. temperature trends high and so the best advantages of the smaller capacity are never realized. Is that accurate?

    PHM
    ------------


    Quote Originally Posted by Core_d View Post
    Thank of it this way, the structures heat gain is what it is. If you remove all the heat in an hour or over the whole day it still takes the same amount of work.

    Granted there are subtle advantages that would promote a fast pull down, such as less starting amps and less cycling so the coil stays moisture laden also AC’s are more efficient when the indoor temps are high and heat gain to the space would slightly decrease because of less temperature differential.


    Despite all these subtle advantages you would gain, once you figure equipment cost and additional ducting and addition repair costs added by equipment with more components, more refrigerant it would be a wash at best and thats if it was designed this way at construction. Not even close to a wash if it were a retro job or “upgrade”

    With only the expense of the stat you could install a programmable stat with smart recovery and get the best of both worlds. You’ll just have to experiment with how far you can go with the setback.



    Sent from my iPhone using Tapatalk
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
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    I have a crazy doctor friend who expressed that exact concern to me. His house had 6 1/2 tons of cooling. 3.5 tons for the second floor and 3 tons for the first floor. He worked about 50 miles away and so bought a condo near that hospital so he could stay there at times and avoid the travel.

    He Could Not Bear my advice to just let his big-house AC maintain the temp while he was at the condo. So instead he would shut it off for days of summer-time roasting ambients and then come home to wail that it wasn't working because . . . . his house wouldn't cool. <g>

    So I installed 4 more tons of mini splits. And he's happy. To me it's laughably ridiculous that he has over 10 tons of AC in his house - but when he has a party . . . . it's always cool in the house. <g>

    PHM
    -------------


    Quote Originally Posted by ats1986 View Post
    I would imagine you want to size the ducting to work with the full capacity of the system but match the low stage to the actual need of the house. I've heard varying opinions on "oversizing" ducting but im sure its better than undersizing.

    I can also see this being a good solution to situations similar to my parent's house where they have a large but well insulated house so they are fine with a 3 1/2 or 4 ton system most of the time but often have large family gatherings at their house with 20+ people there and hours of cooking so it's no fun if the a/c can't keep up with that extra load. I'm not asking this to help size their system, it's just another thought.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  13. #13
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    Quote Originally Posted by Poodle Head Mikey View Post
    I have a crazy doctor friend who expressed that exact concern to me. His house had 6 1/2 tons of cooling. 3.5 tons for the second floor and 3 tons for the first floor. He worked about 50 miles away and so bought a condo near that hospital so he could stay there at times and avoid the travel.

    He Could Not Bear my advice to just let his big-house AC maintain the temp while he was at the condo. So instead he would shut it off for days of summer-time roasting ambients and then come home to wail that it wasn't working because . . . . his house wouldn't cool. <g>

    So I installed 4 more tons of mini splits. And he's happy. To me it's laughably ridiculous that he has over 10 tons of AC in his house - but when he has a party . . . . it's always cool in the house. <g>

    PHM
    -------------

    Hmm...that might be a better solution for most situations right there. Install a supplemental mini split in a living room or master bedroom or wherever the occupants would be most likely to stay once they get home. Then you aren't worried about having to set the system up for one stage vs the other or redo and ducting. It may not make sense for families that leave the house unoccupied for only 5 or 6 hours of a day; but people who have very irregular schedules or are gone for long periods it makes more sense

    I never would have thought that changing out a system that is reasonably efficient and going with an oversized 2 stage setup like this would have been practical. It would only make sense in situations where you are replacing everything in the first place.

    I'm making my way through that document that Teddy Bear posted. I'm a slow reader...

    Quote Originally Posted by Core_d View Post
    Thank of it this way, the structures heat gain is what it is. If you remove all the heat in an hour or over the whole day it still takes the same amount of work.
    Is that true though? I have to think that your house is gaining more BTUs per hour if you keep it at 75 than if it is at 85. Which means those extra BTUs are having to be removed as they are gained. If your house can just get up to 85 or whatever it will maintain with out a/c throughout the day then you will no have to remove several hours worth of heat gain.

    Again, I have almost zero real training on this topic so I could be totally wrong.

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