AC help please from the AC experts

[AUswimKC]

Located in Atlanta GA. Home built 2004

Need some AC help from the AC experts out there. A little bit long but trying to put out the full story in the OP.

Upstairs unit has a tiny leak (1lb to half a pound needed per year). Given the leak, I talked to a couple AC contractors a couple years back about a new upstairs system, if I recall both seemed to think both units were sized properly. It just doesn’t make sense to me given the below.

I have a two story home with a full size walk out basement (2 walls open, 2 in dirt). The basement stays 78 in the summer. 80 tops in the peak peak of summer. Basement is not finished and not serviced by the AC.

Have two AC systems that are both with integrated natural gas furnaces for heat. One up and one down. Both are 2.5 tons. Trane XB10 manuf 10/2003.

First floor has about 1175sf liveable measured interior wall dimensions. Basement ceiling and walls are insulated. AC furnace and blower in basement so AC ducts in basement ceiling so registers in the floor. 9 ft ceilings with one 15x15 area with 10ft.

Second floor has about 1625sf plan measured interior. The front door entry is about 150sf of double height space so about 1475sf liveable. Master bed and bath located over the two car garage. AC blower, furnace, and ducts in the attic with registers in ceiling. 8ft ceilings with high treys in the master and master bath.

We keep the house at about 77-78. I’d prefer to keep it a bit lower. Maybe at 75. Upstairs can barely keep up with 77, even immediately after charging.

Ok, so given all of that it doesn’t seem to make sense that with the upstairs being bigger, under a hot attic, over a hotter garage, with the ducts in the attic and a hot open entry foyer that the upstairs unit is the same size as the downstairs unit.

Looking at getting some estimates for a new system instead of keeping this one going. I understand that the units need to be sized correctly or they run too much or not enough (not pulling out the humidity).

I also understand that sizing a unit can be done by doing a Manual J calc. Nobody around here seems to want to do one until you’re already under contract for a system. And I understand that if you know what you’re doing to can manipulate a Manual J to whatever you want the answer to be? Is that true about the Manual J?

Can someone help give me a ballpark of what size I might need to get to replace the upstairs unit?

Thank you in advance for any help and advice you can give

[RKMBrown©]

If I want my upstairs to be 75 I first set my downstairs to 73. Why? Cause if I don't, the upstairs unit has to cool not just the upstairs space but also all the heat rising up from downstairs, including heat from cooking, garage, etc. Additionally downstairs does not have to fight the heat from the roof.. The two floors can be the same size, but the top unit has to work harder in most scenarios. You can also add thick curtains with a white backing and / or awnings to reduce the amount of heat from the sun during the hottest parts of the day. Trust me just set downstairs to 2degrees cooler than upstairs.. and all will be well.

[Phil Thien]

I’m no expert, but 2.5 tons seems undersized for your 2nd floor, for your climate.

[RKMBrown©]

I've got two 3 ton units one for upstairs one down about the same size house.. located in central Texas. Don't have a problem keeping it as cold as I want, 68 no problem. Middle of the summer 118 outside yeah then I keep it around 74.

He might have dirty coils.. clogged ducts.. Who knows.

[Phil Thien]

I've got two 3 ton units one for upstairs one down about the same size house.. located in central Texas. Don't have a problem keeping it as cold as I want, 68 no problem. Middle of the summer 118 outside yeah then I keep it around 74.

He might have dirty coils.. clogged ducts.. Who knows.

You're right, the rule of thumb for Atlanta is one ton per 600-SF.  So he is right in there at 5 tons total.

[Snipe Hunter]

I always have the same answer in HVAC threads... PM Blackhat

[Duane N]

"General" rule of thumb for residential is 600 sq/ft for 1 ton of cooling. 

I do commercial HVAC so I'm certainly no expert in residential but if you're square footage is accurate my guess is you're short on the upstairs and a little over downstairs (I would leave the downstairs unit the size it is).  Also take into consideration if you do go with a larger unit upstairs make sure the existing duct work can handle the increased air (supply and return).  Vapor barriers on duct work break down over time especially in attics due to the heat.  Once the vapor barrier breaks down the insulation gets wet due to the cold air in the duct being exposed to the unconditioned, humid air in the attic and once insulation starts holding moisture the "R" value decreases drastically decreasing the efficiency of the unit.  If duct wrap was used to insulate the duct work most likely it's R-6...code now is R-8.

Have you checked the attic insulation?  If it's blown in insulation it does compress over time decreasing the "R" value.  Do you have attic fans and are they working?

In my home during the summer I used to run the temperature up 4-5 degrees while I was at work and the turn it down when I got home.  I now only run the temperature up 1-2 degrees because the unit has to work less removing the heat load when I turn it down.  Running dehumidifiers will help the unit as well because a lot of the work the unit does is remove humidity.  My unit is a 14 SEER 3 1/2 ton Trane dual fuel heat pump system that's 15 years old.  I'm in the process of re-insulating my duct work in my attic due to the vapor barrier breaking down and replacing the R-6 flex duct with R-8 flex.

Good luck.

[AUswimKC]

Appreciate the comments so far. Informative. Black hat was one of the experts I was thinking of. Will need to PM him if he doesn’t reply in the thread. Thanks for reminding me of his name.

Attic insulation should be pretty good. It’s significantly deep up there. Attic fan running as it was replaced last year when I noticed the motor was seized. Will have to check the duct barrier and insulation. I’ve been thinking already where I could put additional supply and return vents if I go bigger. Will check the ductwork r-value. I think it says on the outside.

Indoor and outdoor coils should be good. Serviced every year. I know the outside coils are cleaned. Will check the ducts although filters are usually pretty clean.

[AUswimKC]

How many more square inches of registers are required (both supply and return) between 2.5ton and 3ton? Assuming the current 2.5ton have the right supply and return.

[blackhat]

I have to say that any kind of HVAC equipment in an attic is a damfool idea.  I know why its done but I don't like it.  Duane offered a couple of good points to consider.  You called the attic hot, as in 120 degrees hot or 150 degrees hot?  If it gets much above 120, you need to improve the ventilation up there.  Over time its not just the vapour barrier on duct that breaks down, joints that were poorly done and not sealed well can leak a lot of conditioned air on supply side or suck superheated/cold air on the return side.  Pushing an extra couple hundred CFM of air is usually not a problem, more often its the return duct and filter sizing that starves things.  Duct leakage in an attic is money thrown away daily.

If you are going for quotes, ask for pricing on a replacement furnace and 2 stage AC as well.  Ask if they can do a leakage test on the duct after the changes are made.  Repairs to existing would be an extra to any quoted price.  Personally, I would avoid a modulating system, the technology is too immature and parts pricing is outrageous at this point in time.  Ask if there are any energy efficiency grants available but the extreme efficiency systems aren't worth it at this time IMO. You could also ask about a system setup to run in a dehumidify mode. Usually requires nothing more than a control added and can be worth it in some climates.  If the return duct, filters and blower will support it, I wouldn't flinch at putting a 3 ton system in for the upstairs.  Ask what each system carries as a standard warranty and what an expanded warranty covers and costs.  Also ask for referrals from nearby customers, if they can't or won't provide referrals, thank them for their time and show them out.

Any other questions, fire away.