Why more A/C tonnage downstairs instead of upstairs?
 

At my house I have something like 4 or 5 tons of AC for the first floor and basement and about 3 tons for the upstairs. If I turn all A/C off and walk out of the house - when I return the upstairs will be much warmer than the downstairs. We all know hot air rises. So why don't I have 4 or 5 tons for just the upstairs and 3 for the bottom two floors? The basement is frigid regardless of A/C.

Same situation here. And I've wondered the same thing.

Load generation far more likely to occur on first floor. Poor insulation on second floor. Two possible factors.

usually the units are sized based on the square footage of the area, and the amount of glazing.

rule of thumb = 500 to 750 s.f. per ton, depending on the ceiling heights (vaulted ceilings = 500, flat 8' ceilings = 750)

If the ground floor is 1.5X the size of the second floor, and has large expanses of windows, then the heat load is larger, requiring more tonnage

I think I have a problem in this respect. My units are 2.5T upstairs and 4T down. My Upstairs is 40% the size of upstairs, and the downstairs is high ceilings. But there is a large open area connection from downstairs and my upstairs unit does not keep up.

ive never understood this either, especially in the south. I see the formula above, but that rule of thumb simply isnt enough. My units run non stop to keep up. That formula should be doubled! I have a three story house with @ 3800 sq ft with 2 systems equaling 4 1/2 tons. Not near enough but pretty much standard all around here.

I've read about people putting air circulation systems in their homes. Basically a duct and fan, moving air from top floor to basement, runs intermittently on a timer.

You guys have separate a/c systems for upstairs and downstairs? That must be a TX/South thing, I've never had that. We have terribly ineffective a/c on the upper floors, but it is because of the difficulty of fitting central heat/air ducts into an old house.

If I'm not mistaken, my HVAC guys always placed the higher tonage AC up stairs. Heat rises and cold falls. Higher rated furnaces were placed down stairs for the same reason.

Maybe it's a design of the house, with a smaller footprint upstairs, it gets less tonage?

I suggest getting smaller houses

Loonybin is right, cubic ft & glazing etc. size units.

Red - same thing in my house, with an open foyer & family room. We keep the bedroom doors shut in the summer, and I have the upstairs on 2 zones, master BR & kids bedrooms.

Typical Subdivision BS..always the cheapest units they can buy...

I am running two units for 9 tons...on a 3700 sq. 1 story trailer. But then I live in an oven in the summer. The 5 ton unit carries the most of the load, as the trailer faces N/S. The bedrooms get the morning sun and have the garage in front which insulates the bedroom area..so even during the heat of the day the 4 ton unit hardly even runs. I have planted 5 trees to the west to block the late afternoon sun and have one in front. My thinking is this would help shade the house in late afternoon. One thing about trees is that even if they don't shade the house they do keep the ground temp down around the house Further I have LOW E windows and Solar Screens all in an effort to keep it Kool without running up the E Bill. Also to help with cost I have the Pwr Co have a cut off during heavy usage times of the day, this usually saves about $40 a month in the summer plus I get a check for about the same at the end of the year. I am probably going to put in an attic fan to blow out the heat. It gets up to about 160 in the attic in the summer. Costco has a Solar powered attic fan for $270 which is a consideration.

Just had to replace a Compressor on the 5 ton unit..OHH JOY! that was 14 I coulda done without...

Also one might consider that people spend most of their time downstairs.

I'm in a ranch now but my previous 2 story had a larger unit upstairs. It was a WW2 brick building with a flat roof so the upper floor was considerably warmer.

We have the same setup - downstairs unit is larger although the SF is also larger and there are lots of windows and doors.

Dumb question - how do you tell the tonnage rating? I don't see anything obvious on the labels.

We have them here in so cal. I have it in my own house when we put in HVAC a few years back. It is a much more efficient system. We completely shut off the upstairs AC system when we go to bed. Our bed rooms are downstairs. Our AC is use 3-4 weeks out of the year if we are lucky. Heat is set and forget for a couple of months.

Usually is a cost issue. The AC man knows by giving the client a bid for a split system, the installation cost sky rockets. They will never get the work. It may require ripping up walls for ducts. Most of time, they just have the cost of a giant 5 ton unit and call it the day. Not acceptable to me BTW. My client and I usually have a long talk over the situation so they can make better decisions.

Mine is a 4 ton Payne, it sits on the roof, the place is 2k sq ft, we NEVER turn the air on but in the winters use the heater. The problem here is the bedrooms are downstaris and in the winter they never get warm. The builders didn't put very big ducting in the place so it doesn't pump down that far. Always cool downstairs even in the summer. We have to use an electric heater during the winter to keep the chill off downstairs. The unit is on the roof behind the chimney. Yes, they had to have a crane to put the new one on last year.

http://forums.pelicanparts.com/uploa...1306764785.jpg

Dampers should fix some of your issues. readjust them for the winter months. You can have electronic ones install so you never have to craw into the attic for adjustment. I am not sure if that's worth the money since you live in Dana Point and almost never need AC.

Jeff

I believe the split is to accommodate energy savings during night time hours.
While the bedrooms are unoccupied, they can be left with a higher setback temperature to save energy during the day. Downstairs comfort cooling is being maintained (while occupied).
At night the unoccupied 1st floor setpoint is set back to save energy. The smaller 2nd floor unit takes over. No solar load at night so it should be able to keep up.
The thing about setting temperatures back to save energy is that if the structure (mass) of the building heats up too much, it may be a long run time before it becomes comfortable.

With the roof top a/c problem, you might need to see if there are any balancing dampers to get the distribution right. You may need to speed up the supply air fan to provide more static pressure to the system to get air out to the furthest zone. You may be able to get a transfer fan installed to pull the warm air down to the lower floors in the winter.

UPDATE:

As of writing this post I started leaving the upstairs on all day and the downstairs off or much higher. The whole house is cool and I am running the bigger downstairs compressor much less if at all during the day. I'll ask it again: WHY AREN'T UPSTAIRS COMPRESSORS HIGHER TONNAGE!!! Looks like this is going to be my method from here on out. The upstairs may work more but it takes less energy to maintain the same thing. The upstairs used to be steamy while the downstairs was cooled by its own system.

Because, like Billy said, in most 2-story homes, the upstairs contains (mostly) bedrooms and is rarely in use during the day.

And at night, one doesn't need as much tonnage to do the job with the night time cool down.

The way you are adjusting your temps and run times, you are taking advantage of the different capabilities of the two systems, but in the future, if one of the units needs replacing, you could change the tonnage if you wanted to.

I always wonder why the AC guys have the "formula" that just is OK for moderate days but sucks for the hot part of summer. At my house when the heat really cranks up the unit just runs non stop and the inside temp will creep up, especially if I have several guests.

A friend had a house custom built for him. You could play volley ball in his living-dining room. He told the HVAC folks that he wanted to keep his house 70 degrees on a 110 degree day while having a house full of people coming and going. He ended up with two of the largest home units on the market. I don't know what his electric bill is but when my wife & I go over there in the summer my wife dresses for winter.