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Ketia0

One unit or two? That is is the question (Air Conditioner)

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I'm debating on something and would like others opnion. Bonus if you have experince in Air Conditioning. ;)

We currently have a 1600 SQFT house with a nice Maytag 4 Ton unit. We have plans to add on to the house an extra 950 ish sqft.

Our original plan was to just add a whole extra unit, 2 ton, to cool and heat the add on, since it is on the other side of the house.

Would it be better to upgrade the whole system to a 6 ton unit that could cool the whole thing or two different units?

Thoughts?

Attached is a ROUGH drawing that will give you a basic idea of the layout.
"In this game you can't predict the future. You just have to play the odds. "-JohnMitchell

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I suppose it depends on whether you are using the entire house equally.

Frankly, (and I'm no HVAC expert), but I would have two zones. Why would you want to cool/heat all parts the same? They seem separated "geographically" so why tie them together? It sounds like you have a working unit that services the existing house, so why replace it when adding a additional unit to cover the new square footage would make more financial sense?
lisa
WSCR 594
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Well my thinking is this..

1. I will want the house to be the same temperature throughout anyway. So if one thermostat is on 74 the other one will be too.

2. it wouldn't be much more, a few grand, to upgrade the old one to 6 ton. The 4 ton is about 6 years old, so I could sell it. Plus, I think there is a freon leak somewhere because I just had to have it charged.

3. All parts will be at cost as my wifes grandfather has a small AC business (And here is where people ask why I'm not asking him... He is 75 and is not that well "into the game" if ya know what I mean. So I am getting different opinions before I figure out the route I want to go)
"In this game you can't predict the future. You just have to play the odds. "-JohnMitchell

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HVAC guy in the house, use two units, period

the addition can easily be handled with a 1.5 ton unit, it actually shouldn't need that much but it's hard to find a decent unit in a smaller size, just have the installer set the fan speed on the lowest setting and it will work great, this assumes that they design and install the duct work properly for the lower air flow

but why the heck do you have 4 tons, that size house properly insulated should have 2-2.5 tons, the extra tonnage causes humidity problems and increases the power bill

over sized units are the largest problem we see in houses, the dealers get paid by the ton, therefore more is better - NOT

cooling is about temperature AND humidity removal, an oversized unit can't effectively remove humidity, so you have to set your t-stat lower to be comfortable

my upstairs 1.5 ton unit in hot humid SC handles 1400 sq ft just fine, humidity is around 40-50%, temp is 77 and it feels real cool because of the lower humidity
Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

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I have a high efficiency 5 ton 14+SEER unit running my whole 1900 sq ft house. Not sure why you would buy a second. When our old system packed it in, the new system cost $6K, used the existing ducting and my electric bill dropped by $50/month.

Unless you have some poorly designed ducting system, I think a single unit should do it all. And all the newer systems are far more efficient each couple years.

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I agree with Billeisele, the tonnage you have sounds high. A couple years ago I had to replace my old 3 ton unit that was worn out. I have a 2400 sf house that includes a 900 sf addition. I learned that the the HVAC unit was not upgraded when the addition was built, so that new area was always warmer in the summer and colder in the winter compared with the rest of the house. So I replaced the old unit with a 4 ton, and had a new air intake vent added in the add-on space and tied that into the duct system. Result was MUCH better air circulation and temperature management.
"Mediocre people don't like high achievers, and high achievers don't like mediocre people." - SIX TIME National Champion coach Nick Saban

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Ketia0

I'm debating on something and would like others opnion. Bonus if you have experince in Air Conditioning. ;)

We currently have a 1600 SQFT house with a nice Maytag 4 Ton unit. We have plans to add on to the house an extra 950 ish sqft.

Our original plan was to just add a whole extra unit, 2 ton, to cool and heat the add on, since it is on the other side of the house.

Would it be better to upgrade the whole system to a 6 ton unit that could cool the whole thing or two different units?

Thoughts?

Attached is a ROUGH drawing that will give you a basic idea of the layout.



Two words. Mini Split

done.
I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

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normiss

Or into the man cave.

Maybe I need to get on that this weekend.....sure would perform better than the portable unit in there now.

It'll never be my home brew area if I don't get my ass in gear!



Invite me down when you're done. :)
"Mediocre people don't like high achievers, and high achievers don't like mediocre people." - SIX TIME National Champion coach Nick Saban

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Iago

I had a 200 ft split level with 1.5 upstairs and 2 ton downstairs.

If you've got enough capacity, perhaps you can have some control in the duct work so you're not cooling the place when its not being used?

Needing a top off every season or two is no reason to replace an entire system. The really small leaks can be difficult and costly to sniff out, so Id just keep giving it a shot when it needs one.



2 story outhouse?:o

You prolly meant 1200 . . . right?
I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

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billvon

Consider a two-zone minisplit. They are generally more efficient and they are easier to retrofit into existing construction.



Depends on the layout of the space. If it is 2 rooms then this is a great idea. If more than 2 and the ones without a blower have exterior walls then it won't work well.
Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

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tkhayes

I have a high efficiency 5 ton 14+SEER unit running my whole 1900 sq ft house. Not sure why you would buy a second. When our old system packed it in, the new system cost $6K, used the existing ducting and my electric bill dropped by $50/month.

Unless you have some poorly designed ducting system, I think a single unit should do it all. And all the newer systems are far more efficient each couple years.



And if you had put in a 3-3.5 ton the bill would have dropped a lot more. The trick is in the duct system, it must be properly sized for the unit.

I have 2800 sq feet, 2 story. Monthly bill is $129. The units are properly sized. The neighbors house is almost identical. Their bill is $300+. The units were "upsized" because more is better BS. Normalize these 2 houses to account for variables, thermostat setting, lifestyle, etc., and his bill is about 70% higher.
Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

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Thanks for the advice. We have a 2000 sq ft home in Olympia, WA we may install AC in. We'll be using the existing F/A Gas ducts, of course. What kind of tonnage will we need with our milder, although humid, summers and how do I know the AC is optimized to my ductwork?

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JohnMitchell

Thanks for the advice. We have a 2000 sq ft home in Olympia, WA we may install AC in. We'll be using the existing F/A Gas ducts, of course. What kind of tonnage will we need with our milder, although humid, summers and how do I know the AC is optimized to my ductwork?



What is your glass/insulated wall ratio?

You prolly need to send him a floor plan, including how the walls were insulated, and all your ceiling heights, and window locations and whether they are insulated, or single pane glass.
I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

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Quote


What kind of tonnage will we need with our milder, although humid, summers and how do I know the AC is optimized to my ductwork?



I had the same kind of questions when I installed central A/C in my house. I'm guessing your existing ducts are for forced heat? Mine are, and I've had central A/C for a couple of years now. Since the density of cold air is significantly different from hot air, you'll soon know where all the leaks are in your duct work. I had the guy install a pretty powerful system to handle pushing the cold air up from the basement. Right now, my basement is a freezer when the system is up, but not so much for the rest of my house. Next step is to install some kind of boosters in my duct work to help push the cold air to the end of my house. FYI...
We are all engines of karma

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StreetScooby

Quote


What kind of tonnage will we need with our milder, although humid, summers and how do I know the AC is optimized to my ductwork?



I had the same kind of questions when I installed central A/C in my house. I'm guessing your existing ducts are for forced heat? Mine are, and I've had central A/C for a couple of years now. Since the density of cold air is significantly different from hot air, you'll soon know where all the leaks are in your duct work. I had the guy install a pretty powerful system to handle pushing the cold air up from the basement. Right now, my basement is a freezer when the system is up, but not so much for the rest of my house. Next step is to install some kind of boosters in my duct work to help push the cold air to the end of my house. FYI...



This is a quick fix, but you will want to replace it with a better unit later on.
I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

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JohnMitchell

Thanks for the advice. We have a 2000 sq ft home in Olympia, WA we may install AC in. We'll be using the existing F/A Gas ducts, of course. What kind of tonnage will we need with our milder, although humid, summers and how do I know the AC is optimized to my ductwork?


One Jump Wonder

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StreetScooby

That's what my guy is talking about doing... What's better?



This is a good one - but just tell your guy that you want one that will last. The 40 and 50 dollar models are only good for about a year, in my experience.
I'm not usually into the whole 3-way thing, but you got me a little excited with that. - Skymama
BTR #1 / OTB^5 Official #2 / Hellfish #408 / VSCR #108/Tortuga/Orfun

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billeisele

***I have a high efficiency 5 ton 14+SEER unit running my whole 1900 sq ft house. Not sure why you would buy a second. When our old system packed it in, the new system cost $6K, used the existing ducting and my electric bill dropped by $50/month.

Unless you have some poorly designed ducting system, I think a single unit should do it all. And all the newer systems are far more efficient each couple years.



Quote

And if you had put in a 3-3.5 ton the bill would have dropped a lot more. The trick is in the duct system, it must be properly sized for the unit.



Your the pro do you mean the ductwork need to be properly sized and "installed". Every improperly sealed joint in the duct work system is a money pit.

Do you have any advice for John Mitchel since he lives in the PNW about I
stalling

I have 2800 sq feet, 2 story. Monthly bill is $129. The units are properly sized. The neighbors house is almost identical. Their bill is $300+. The units were "upsized" because more is better BS. Normalize these 2 houses to account for variables, thermostat setting, lifestyle, etc., and his bill is about 70% higher.
One Jump Wonder

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Hi bill,

Quote

HVAC guy in the house



I like learning from you type of guys.

On 'Ask This Old House' this evening on PBS they had some guy who a previous contractor had put a 4-ton unit in his house, which was either 1300 ft or 1500 ft.

The 'Old House' guy said that his unit was way too large for the size of house and it kept freezing up. So they tore it out & put a new 3-ton unit in.

From the small amount of studying that I have done on heat-pumps and/or a/c units; in an a/c unit you usually do not have to upgrade the ductwork if your heating system is something like nat. gas or oil. However, if you're using a heat-pump for heating, then you should upsize the ductwork. Heat-pumps move the heated air at a slower velocity than the heated air from systems such as nat. gas or oil. They need to move a larger volume of air at a slower velocity vs other systems.

JerryBaumchen

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Sorry folks, been at the DZ getting ready for Carolina Fest. Finished building a 23' x 39' shelter in honor of one of our jumpers. A great addition to the DZ that will be well used.
Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

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JohnMitchell

Thanks for the advice. We have a 2000 sq ft home in Olympia, WA we may install AC in. We'll be using the existing F/A Gas ducts, of course. What kind of tonnage will we need with our milder, although humid, summers and how do I know the AC is optimized to my ductwork?



John -I've left out a lot of details and been talking in generalities, you have a specific question that can't be answered without information, sq footage is one piece of info but HVAC deals with volume so ceiling height is critical, your best bet is to find an HVAC dealer that can do a heat loss/heat gain calculation following ACCA Manual J, from that data the unit can be sized using the local weather conditions, if you have a tech school that teaches this stuff they could do a class exercise, it takes about 30 minutes for someone that knows what they are doing

to answer your 2 other issues, this info applies to everything else being discussed

the design of the duct work is critical, it should be done following ACCA Manual D, it is designed for the specific floorplan, volume, windows, doors, orientation and unit size, this is why you can't simply add an addition and upsize the unit

it's all based on proper air flow - velocity and volume, the return should be properly sized to have a certain face velocity (of the air), too low and it won't collect the dust, too high and it is noisy

the duct work is designed to deliver the air (BTUs) where it needs to be based on the amount of "conditioning" needed in the space, this is done by using larger or more ducts/registers in larger rooms and small ones in small spaces, for instance the guest 1/2 bathroom will have a 4" duct and small register, a small bedroom might have one 6" duct and a larger register, just like the return - the registers need to have a certain air velocity to properly throw the air into the room, too low and it just falls out and doesn't properly mix, too fast and it is noisy and drafty

this all comes back to the available amount of air, this is dictated by the unit size, the larger the unit the more the air volume

most dealers will simply count the rooms, add up the duct sizes and runs, they know the air needed per size and run, they total the air flow and then pick the next larger size unit, this is the easy way and is backwards of how it should be done, but they sell it based on tons and don't want to get the "my house is hot" call, so they add a ton

on my house I had to sign a paper relieving them of liability for the sizing, they still don't believe how well it works, been here 22 years

it's all about heat gain and heat loss, talking extremes - a house that is 50% glass is much different than the identical floorplan with 25% windows, but the 50% window house with the majority of the windows facing north (in SC) could be identical to the 25% house that has the windows to the west

I've seen a 1-ton difference in identical homes just based on the direction they face

bored yet :)
on to humidity control, if the unit isn't running it is not controlling humidity, an oversized unit will turn-on, run 15 minutes and turn-off, about 12 of those minutes would be removing moisture, a properly sized unit will turn-on, run 30 minutes and turn-off, 27 of those minutes it was removing moisture

the speed of the air across the cooling coil also effects moisture removal, the blower will usually have 3 fan speeds, factory setting is normally middle speed, if the return or ducts are noisy and the unit is oversized, it will help some to lower the blower speed

to be comfortable in the home with the large unit (for instance) - Tstat setting will be 75 and humidity 65%, AC bill will be ~25% higher than properly sized unit,
house with properly sized for the same comfort will be 78 degrees and 50% humidity

if what you need is humidity control, the unit needs to run

as to your existing duct work, I have no clue if it is properly sized for the cooling system you need, a dealer that is willing to do it the right way will have to look at it

the ducts also need to be properly installed, that means NO LEAKS, all joints screwed together and sealed with mastic, insulation properly installed and air tight

back to my house: good duct system, 1400 sq ft upstairs that needs more air flow than a 1.5 ton can deliver, simple, install a 2-ton blower to get the air volume I need, set the fan speed on low to get the humidity control, proper speed at the return and registers, add a return in the FROG, on a 95 degree day the unit never shuts off, that's perfect, the temp is 78 degrees and humidity between 40-50%, cool and crisp, the summer peak bill is around $225 but quickly drops in shoulder months, smaller unit pulls less energy so running longer isn't expensive

for the skeptics, my lake house (other house) is 1668 sq feet, big windows that face ESE, tree shade, 2 ton variable speed Trane package heat pump, on a super hot day with 10+ guests I'll set the temp at 75 to collect the extra water from the bikinis and towels, everyone is comfortable or cold

John - in SC, not knowing anything about your windows, orientation or quality of construction, if it's a typical house, i'd be using a 2.5-3 ton unit
Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

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