Oversizing with 2 stage equipment to save energy/run less

[ats1986]

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?

[BALloyd]

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

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[Poodle Head Mikey]

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
-------------

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?

[Core_d]

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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[BBeerme]

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.

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
-------------

[ats1986]

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.

[teddy bear]

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

[BBeerme]

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.

[Core_d]

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.



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[teddy bear]

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

[Poodle Head Mikey]

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
------------

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.



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[Poodle Head Mikey]

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
-------------

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.

[ats1986]

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...

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.

[Core_d]

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
------------

Yea, if set up properly most all the 2stage systems slow down the indoor fan to match the cooling capacity on first stage.


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[Core_d]

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.

Your not totally wrong, the amount of heat transfer is based on the heat difference but, The difference is probably negligible.

What I didnt think of last night is say you have a 5ton 2stage in a 2.5ton home. The 5t will actually be doing less work to quickly pull the house down because its actually doing less to get the sensible temperature to desired set point. I.E its not getting the moisture out of the air so less total enthalpy is actually removed.


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[BBeerme]

For what it is worth, it does take less energy to pull a hot house down late in the day, than it does to run the A/C all day. And exactly for the reason you stated in your first sentence here. Wanted to comment on your prior post on the topic in this thread, just been waiting for the time.

If the TD of the outside air versus the inside air decreases at some point, then that would be the beginning of your savings. On a bit of a tangent, here's an interesting scenario that happened several years back in California, where I live. It was a hot spell. So the electric company (PG&E) told everyone to leave their AC's off until they got home from work.

Well shiver my timbers, there was such a huge demand for electricity late in the day, it almost shut the system down state wide. So they came out with a revised recommendation. Set your stat at 78*F during the day, then lower it when you get home. That way it did not overload the electrical grid at 5-6 PM.

Personally, in the summer, with no one in the house, I leave my stat at 81*F. When I get home, I crank it down to 70*F and it usually doesn't shut off. I mean, if it is at 81*F when I get home. Works for me. Pisses me off when it turns off; being in this trade has made me acutely aware of temps and humidities. I just love having that cold low humidity air falling on me.

Problem is, at least where I live, if the system is put in per code, you cannot oversize the system. Which mine isn't. Just saying, you gonna have to do it on the sly.

I personally would have no problem with a perfectly designed system that was super efficient and would run continuous at design and meet every ridiculous code that was out there. Then have a secondary system in series with the primary that would kick in for hot pull downs.

All that said, improvements to the structure can go a LONG LONG ways. My house was built in 1958 with no insulation. Previous owner had some of that cellulose insulation blown in, so at least there's something besides just air in the walls now.

After being here for several years, the slider to the patio cracked, so I had to replace that. What a freakin' difference!

And just 'one' piece of glass.

If I were to yank all of the single pain encasement windows and replace with high efficiency windows, there would be a pay back. But I just haven't found the motivation. Point being, keep the heat transfer down, that's where you can really save some long term monies.

Your not totally wrong, the amount of heat transfer is based on the heat difference but, The difference is probably negligible.

What I didnt think of last night is say you have a 5ton 2stage in a 2.5ton home. The 5t will actually be doing less work to quickly pull the house down because its actually doing less to get the sensible temperature to desired set point. I.E its not getting the moisture out of the air so less total enthalpy is actually removed.


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[Core_d]

For what it is worth, it does take less energy to pull a hot house down late in the day, than it does to run the A/C all day. And exactly for the reason you stated in your first sentence here. Wanted to comment on your prior post on the topic in this thread, just been waiting for the time.

If the TD of the outside air versus the inside air decreases at some point, then that would be the beginning of your savings. On a bit of a tangent, here's an interesting scenario that happened several years back in California, where I live. It was a hot spell. So the electric company (PG&E) told everyone to leave their AC's off until they got home from work.

Well shiver my timbers, there was such a huge demand for electricity late in the day, it almost shut the system down state wide. So they came out with a revised recommendation. Set your stat at 78*F during the day, then lower it when you get home. That way it did not overload the electrical grid at 5-6 PM.

Personally, in the summer, with no one in the house, I leave my stat at 81*F. When I get home, I crank it down to 70*F and it usually doesn't shut off. I mean, if it is at 81*F when I get home. Works for me. Pisses me off when it turns off; being in this trade has made me acutely aware of temps and humidities. I just love having that cold low humidity air falling on me.

Problem is, at least where I live, if the system is put in per code, you cannot oversize the system. Which mine isn't. Just saying, you gonna have to do it on the sly.

I personally would have no problem with a perfectly designed system that was super efficient and would run continuous at design and meet every ridiculous code that was out there. Then have a secondary system in series with the primary that would kick in for hot pull downs.

All that said, improvements to the structure can go a LONG LONG ways. My house was built in 1958 with no insulation. Previous owner had some of that cellulose insulation blown in, so at least there's something besides just air in the walls now.

After being here for several years, the slider to the patio cracked, so I had to replace that. What a freakin' difference!

And just 'one' piece of glass.

If I were to yank all of the single pain encasement windows and replace with high efficiency windows, there would be a pay back. But I just haven't found the motivation. Point being, keep the heat transfer down, that's where you can really save some long term monies.

Yeah but your system is more-less properly sized. I dont think youd be as comfortable if you banged on a way oversized system when you got home and the saving you would reap if any would be pennies per the ton.


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[BBeerme]

If I had the choice, I'd throw on a hot gas valve on that oversized system.

I'd be so comfortable. The compressor could run at max. And it could run with almost no load and not freeze.

The real question in this thread is two fold. Efficiency or immediate comfort.

Each of us needs to decide what we want. And what we want to pay for.

Yeah but your system is more-less properly sized. I dont think youd be as comfortable if you banged on a way oversized system when you got home and the saving you would reap if any would be pennies per the ton.


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[Core_d]

If I had the choice, I'd throw on a hot gas valve on that oversized system.

I'd be so comfortable. The compressor could run at max. And it could run with almost no load and not freeze.

The real question in this thread is two fold. Efficiency or immediate comfort.

Each of us needs to decide what we want. And what we want to pay for.

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[Faceline]

The best pick is always to go with the variable speed motor air conditioning unit. It uses a good amount of power initially as compared to the fixed speed motor allowing your room to cool down faster. Also, a variable speed motor will adjust the compressor workload as per thermostats requirement.
However, a larger unit can cool better but what is the point here. We are talking efficiency here that simply means a regular unit will be fine.