Need some advice here:

I've got a 2 story 3100 sq. foot house located in North Alabama (Madison), with about 1500 sq. ft. on the second story.  I currently have, I believe, a 2.5 ton unit for the second story.  During the summer, the AC will run upstairs until around 10 PM before it finally gets the 2nd story cool.  No one is home during the work day.  The original builder told me that he put the largest size he could get away with with the city inspection unit, since the city was fining home builders for putting larger than necessary AC units in houses (something to do with energy conservation or such).  I don't know if I believe him except that I know that the unit isn't as large as it should be.

In addition, I've added onto the original 2nd story that will add another 240 sq ft. of heated and cooled space.  I've spend a bunch of time, energy and money adding radiant barrier insulation to my attic and powered fans in an attempt to reduce the cooling load on the house.  The AC still runs until 10 PM and the upstairs never really gets cool during summer nights.

I want to replace the upper unit with a 3 ton unit (or 1/2 ton more if its another size).  Can I do this?  Will I have to change out any of the basis AC structure such as the wiring/electrical, the ouput/room ducts or the filter/intake duct sizes?

BTW, I've currently got a Janitrol (Junkatrol, my brother-in-law calls it; I'll replace it with a Rheem when its due to be replaced).

Any help and/or advice would be appreciated.

Thanks,

Merlin

edited because I can't spell advice.:)

Lot's of questions:
How well are you insulated? Shade trees help to keep the sun off the roof.

Do you have an attic ventilator? If not, the wind turbine style or powered ventilators will help immensely.

Is your unit in good working order? fully charged? no blockage of the condenser or A-coil? Keep them clean and free of blockage - (you wouldn't drive your car with a blocked radiator would you? - same thing applies to AC)

What's the SEER rating of the AC? Here in Commiefornia the enviro-nazis are more interested in a high SEER than the tonnage. They actually give .gov-sponsored rebates for SEER 14 or greater.

Dn,

There are 2 Pear-something trees in the front (the front faces South) and I have one other small maple growing slowly.  I worked on the house while it was being built and added insulation, mostly in the interior walls and floor between the 1st and 2nd floor for noise control.  I did, however have extra insulation added to the attic.  As I originally posted, I have powered fans in the attic, 2 auto, 1 manual and I've added several hundred square feet of Refletix insulation.  The house still stays hot until around 8 or 9 or so and its not even summer yet!

The filter is clean and the outside heat exchanger is clear.  

Where do you find the capacity, Seer and other ratings?

Thanks for the help!

Merlin

Seasonal energy efficiency rating (SEER) is calculated by dividing the BTU output by the watt input plus a factor for inefficiency during startup and shutdown. Call Janitrol, they will be able to tell you the SEER. A low SEER sucks a lot of electricity for little cooling output.

From your description, it sounds like you have a 2 zone system (upstairs and downstairs). Can you physically isolate the upstairs from downstairs (usually a door in the stairway)? I have the same problem because of an open stairway (wife refuses to install a door because it's "ugly"). All the hot air rises to the 2nd floor while cool air sinks to the 1st floor. In my house, I've measured 68 downstairs and 80 upstairs with the upstairs zone going full blast. A door in the stairway will trap the cool air upstairs. Another solution is to keep the doors to upstairs rooms closed to trap the cool air within the rooms.

Dn,

There's no way for me to close off the 2 floors: I have a foyer in the front, and the stairs are open at the top.  I do have a split unit set up.  The downstairs unit was replaced last year when lightening knocked it out.  I really like it (its a Rheem unit) and it gets the downstairs cool pretty fast.

Can you or anyone tell me what size I should have for the area I live in and the size of the house (2nd floor).

Thanks,

Merlin

Stay AWAY from "energy-efficient" compressors.  All they are is a marketing campaign to get you to buy a compressor too small for the application.  It costs less to run BUT IT RUNS ALL THE TIME and usually won't keep your house cool at the peak of summer.  Clever way for the electric utility to make more money in the name of conservation.

Find out the largest compressor your air handler recommends use and then get a compressor one size larger.  When my AC kicks onb I want ICICLES shooting out the registers and then SHUT OFF.

I'm an engineer not an HVAC guy, but the HVAC charts for a 1,500 sq ft living space requires minimum 24,000 BTU/hr. To that add 4,000 BTU/hr for every room that has a heat generator (like a kitchen or workshop) and 600 BTU/hr for every person (warm body) in that space.

If your upstairs is bedroom area with 4 persons (26,400 BTU/hr), I would expect a 2.5 ton unit to be plenty for you (that's 30,000 BTU/hr where 1 ton = 12,000 BTU/hour).

You don't want to go grossly oversize because of the inefficiencies of electric motor startup (compressor and blower). In other words, it's better to have a unit that comes on and runs a while than one that is constantly starting and stopping. Pay a little more for a SEER 14 to cut down on the electric bills (and buy more ammo). Cheers.

A unit that is too big will cause problems. AC that comes on for only a few min. and cools down every thing will not run long enough to remove any moisture from the air. A significant amount of cooling effect is drying the air out. You could end up with a cool, damp "bat cave" that is not comfortable or healthy.

Interestingly enough, high SEER units often cause high humidity problems!  They run higher evaporator temps resulting in less moisture removal. (Moisture is latent load).  There are two ways to improve SEER.  Higher pumping eff by the compressor as a result of higher suction pressure - ie warmer evaporator.  Other is lower condensing pressure from oversize condenser.  The second is a win-win situation so long as system is well designed.  Snake oil is two speed compressors, variable speed blowers, low energy motors, high eff compressors (scroll being exception).

2.5 ton is none too large for the area/climate/second story.  3T may be OK -  I F!! you can get enough indoor air to make it work.  350 CFM per ton required, 400 better.  Nearly ALL home units are short on air.  That may even be source of your current problem.

Your unit is clearly down from where it needs to be, but why?  Hard to give you accurate answer without some info.  What are system pressures?  Suction line temperature and liquid line temperature?  Pay somebody to find out if need be and post here or e-mail me and I will help you interpret them.

For the most part, avoid "high eff. or high SEER" units.  They do not perform as well and never save enough electricity dollars to pay off.  The Rheem/Rhudd unit is a good choice.  Uses a Copeland Compliant Scroll compressor that is a good compromise between cost/eff./performance.  As an added benefit it will tolerate SOME floodback such as happens with too little indoor air and/or freeze-ups.  (VERY common disease!)  Janitrol is the Jennings of the HVAC world.

Yea, I do this for a living.  Have for over 25 years so I have seen the snake oil guys come and go!

Last year I had a Train high efficiency air conditioner and furnace installed, I had them install a 1/2 ton larger unit because I have no trees to shade my home.  I also had them install high and low returns in every room and a duct under every window and place that needed one. Volume is important too, if you can not get the cooled air out of the unit or get it to circulate properly the system can not do it's job. My home is comfortable with no hot or cold spots and it has been trouble free.

I assume that you have a high ceiling over the downstairs living room area that extends up to the foyer. My parents have a similar floorplan, and the temperature differential here in the south Texas heat can be amazing — at least 25 degrees warmer upstairs! The most effective solution for them was to install a ceiling fan over the living room area, located to pull air from the high ceiling downwards during the summer, reversed for winter. 10 minutes after it's turned on, the differential drops to no more than a few degrees.

 Are you turning the AC off during the day?  AS mentioned before, dehumidification must take place prior to coolong the air.  The capacity sounds about right for the size unless you have a greenhouse or something up there.
 Here's an old servie mechanic's trick: Get the unit running on a fairly warm day and after about 5 minutes, grab hold of the large line at the outside unit.  It should feel like a glass of ice water as far as temp goes.  If it is warm you have a short charge, if it is freezing you have airflow problems.  If the line is too cold, you may have a dirty coil in the air handling unit indoors.  Physically inspect it and if matted over, sweep it with a stiff brush in the direction the fins run.
 Brand name has NOTHING to do with how well the unit cools.  Rheem uses the same compononts and controls as Janitrol, and most other manufacturers in the business.  I ran HVAC service for twenty years, and believe me, no make has a corner on either reliability or efficiency.  Believe it or not, i have Janitrols in my house, b/c I dont like paying double just to have a brand name on the same components.
 I also second the comment on air flow.  Check in the attic, you may have that damn junk ductboard (fiberglass with foil on it) which is prone to splittting apart after a few years and spewing cooled air into the attic.  Sheet metal is all we used for commercial work, and I have tin in my house too.
 If I ever meet the idiot who invented ductboard, I intend to shove it up his @$$..crosswise!   Email me if necessary, it's on the profile.

Merlin, I'm a sixteen year A/C tech. #1 rule of thumb is 1 ton of cooling for each 500 sq. ft.. Depending on type of construction. And yes 90 percent of the homes built in the USA are not properly duck sized. Or ducks have leaks. Example a 5 ton unit only producing 4 tons of cooling due to lack of properly sized ducking. For each ton of cooling your house is load caculated at should have 400 CFM per ton. It sounds like you need a real company that can come out and not only do load calculations but as well as air balancing. Everyone thanks like Tim the tool man Taylor (bigger is better) WRONG! And as far as your craptrol get ride of it. And don't screw up and buy a rusting Rheem/Ruud. Buy a Trane XL1200 and get the full ten year parts and labor warranty. And as for anything higher than a 12 SEER Beware of the repair cost of the two speed compressors and variable speed blowers. Trane and Lennox are the two companies pushing them. Variable speed blower replacement about 900.00 vs. direct drive 360.00. Trane uses two compressors for there two speed. An acid burn out and you have to replace both compressors. Lennox uses a single compressor with hi/low speeds. Cost for Lennox compressor 2,500.00. Air flow is most important issue. And as for units run time. They should run only 2 or 3 cylcles per hour. And yes you want them to run for 12 to 20 minutes per cycle. An A/C system takes about 5 to 10 minutes of run time for pressures to get to unit SEER rating. And anyone having a A/C unit built prior to 1986 should have it replaced. Reason prior to 1986 all units had a SEER rating of 6 or lower. And SEER stands for Seasonal Energy Efficient Rating. Also homes should have about 10 percent within of .5 inches of mercury static pressure. Example of your 2.5 ton unit. With a flowhood return air should have 1,000 cfm and all of your supply ducks added together should equal 1,000 cfm. Also have them check air flow with room door open then closed. We find some bedrooms which should have say 300 cfm. Will with door open. Close door and only have 75 cfm. And yes never shut an A/C off when your not home. Adjust it up only about 6 to 8 degrees. And don't listen to engineers. I have to deal with their theory crap every day.

OK, thanks guys for all the good info.

A couple of points:
- I have pretty good insulation in the attic, I had them add it while the house was under construction, but:
- Front of the house faces south, with no shade for most of the day.  House is 3/4 brick.
- There are 6 windows approx. 24x72 in 3 rooms plus the approx. 5x5 window over the 1st to 2nd story foyer.  There is a large chandalier (sp?) over the foyer, so no place to install a ceiling fan.
- I don't turn off the A/C during the day, but I do turn it up to 75-77 degrees then turn it down to 70 when I get home, 65 when I'm getting ready for bed, then back to 71 or so before I go to sleep.
- Last night I got my first batch of window tinting for at least 5 windows.  It supposed to reduce incoming radiant heat by 55%.  
- I've also installed about 1000-1200 square feet of radiant barrier insulation in the attic and bonus room.  This, I feel, has helped tremendously, although I've got more to go.

Questions:

- How do I determine the current rating in tons of an A/C split unit?  Will it be on the lower unit or the one in the attic?
- How do I determine the cfm of the return duct or outlet ducts without calling in an expert (My b-i-l is the family A/C guy, but I hate to call him in unless its an emergency).

Thanks again for all the help!

Merlin

Some good info here, some not so good.  Sort!

Rating of unit is best determined from outdoor unit.  Will be buried in model number.  A ton = 12000 BTUH.  2 T unit will have 24 in model #, 30 =2.5T etc.  Some manufacturers do it differently but that is good start.  Similar data on compressor.  Post numbers here if you can't figure it out.

All the "rules of thumb" will do is give you a hint.  Proper heat gain calculation will give concrete answer and is well advised in problem situations.  The temp and pressure data I mentioned earlier will give a wealth of answers to what is really going on.

Comments on duct board echo my own opinions.  Damn stuff has been outlawed in some areas (thank God!!).  Flex duct for runouts is a little better but not much.  If draped around like Spanish Moss it tends to colapse.  Will never flow as much air as metal of same size.  Needs to be at least one inch larger than metal  but never is.  These things are usally installed by idiots and engineered to be CHEAP.  By the time homeowner finds out the thing doesn't work, intaller is long gone.

Quit adding to your own problems!!!!!  STOP "JACKING OFF THE TEMPERATURE" set it where you are comfortable and LEAVE IT ALONE!  SHOOT anyone who suggests a "setback thermostat".  Temperature "fiddling" saves very little if anything and increases capacity requirements a lot.  NEVER open a window.  Not even a little.  Keep shades drawn if no tint on windows.  Can make a huge difference.

MM,

You mean its easier and cheaper to let it run the same temp all day when we're not there??  I never knew that, although I probably exagerated the upper temp that I set it when I leave in the morning, its probably closer to 74-75 or so (its dark when I leave, give me a break!!).

I'll check the units label this afternoon and report what I see.

My ducts (or ducks) are the sheet metal through the main runs, the flex duct from the sheet metal to the air outlet.  The main air inlet (return) is a large 24" flex duct.

Any idea how to measure air flow?

Thanks,

Merlin

Changes to temperature result in minute, if any, energy savings.  In some cases they actually RAISE your bill!  When system capacity is over stressed, small things mean a lot.  If you cool the house to the desire temperature it has considerably less heat to remove during the occupied period.  All AC units work better when it is cool outside so that is ideal time to get space temp. down - when it is most eff.;  To profit you need to KEEP it down all day.

I assume ducts (not ducks as in quack quack) are insulated?  (If not, it is easy to tell as they will sweat in unconditioned areas.)  Carefully examine the flex ducts.  If poorly supported, such as by a wire or plastic strap, they can sag and somewhat collapse, restricting airflow.

Count number and size of runouts to registers.  About 80+ CFM each.  (VERY rough estimate!)  Measuring airflow requires some instruments.  Several ways such as flow hood or temp rise using two thermometers and known input to furnace as well as it's eff.  Bottom line is it requires someone that knows how and has the stuff.  Quick, dirty way is check amps on indoor blower with door in place.  If it is near nameplate you are great shape.  Not very accurate but is an indicator.  Make sure filters are clean before any tests.  Forward curved blowers draw MORE amps as airflow increases.  More duct restriction makes amps go DOWN.

Be sure to WASH outdoor condenser coil!!  Turn off power, remove top if you can, use small strong stream of water directly into coil from one side then the other.  Be careful to not bend fins.  Wash until water runs clear.  Repeat at least once per summer.  More often is better.  Coil can look spotless but still be full of crap.  This is my most common trouble call in the summer on both AC and refrigeration!!!!!!!!!  Avoid coil cleaners.  Most remove some metal, aging the coil and provide little if any real improvement to the grunge situation.  The foam they generate is their attack on YOUR aluminum fins!!!

At some point it is imperative a set of pressure gauges need to be connected to this thing.  Line temps plus the pressures really tell the story as to what is going on.  Most of these things are installed by morons.  You would be amazed the number I find improperly charged!!  Those readings tell me coil cleanliness, indoor airflow, refrigerant charge, compressor contition, unit size vs load and sometimes other more obscure things.  Get the readings!!