Sizing Air conditioners

the only benefit is to the salesman......they tend to feed slightly erroneous information to their customers.

oversized units actually cycle more that a proper unit, wasting a LOT of power.

a properly sized unit will run longer on each cycle and temp fluctuations will not be as great as with an oversized unit.

Here here Charlie R cannot argue with you as you are 100% correct

This question popped up at a very valid time for me. I'm in the market for a new electric heating & cooling system for my home.

OK how do I figure out what size A/C I actually need if I can't depend on the salesmans calcs.

The 2.5 ton unit that is currently in the house seems to cycle quite a bit. Its 30 years old and thirty years ago you didn't have all the heat generators (i.e. computers & TVs) that you have now. So I would assume you would need to move up a step to cover the heat put out by the various items in a house these days. Or do they not generate enough heat to factor in?

Hello fellow Scotch drinker.. a couple of years ago I moved into a new home in a new subdivision, the a/c unit was too small. It was 3 tons. In the summer it never shut off. It ran and ran, as did with most of my neighbors.

I went on-line and searched "Load Calculator" and paid $50 for a limited time (I think it was 3 months use) of a truly good program. It took me about an hour or two to do it right, measuring rooms and window sizes, but it also let me add in a fudge factor for just about anything.

I then took the 8 page printout to the builder to show that I needed an 3.5 ton unit. Now that sounds like a small difference but the 3.5 ton is about 75% larger then the 3 ton unit. When they changed out my unit, they also had to change out all the duct work as that was too small for the house as well.

I shared this information with my neighbors, and in all about 9 or 10 houses needed to be changed all at the builders expense. (the builder didn't like me much, but I get free beer from all my neighbors now.)

Now a good unit should run about 20 min. out of every hour, and if your looking for year over year savings on your electric bill, pay the extra now for a better (higher) SEER rating. be sure to get a 15 or higher.

>>the builder didn't like me much,

Show me a builder who likes engineers and you've shown me a guy who did the job right the first time...

Seriously, scotchdrinker and labguy have this pretty well nailed. There is no reason to pay for overcapacity.

If you would like to save even more on your AC/operating cost. Go to

www.heatpipe.com and read about their unit. I've had one in my home for 18yrs and

my ac bills were about half that of my neighbors. an I'm also a scotch drinker.

Or do they not generate enough heat to factor in?

You bet they do! 1 watt = 3.4129 BTU/hr so a standard 100 watt light bulb puts out about 340 BTU in an hour. Another reason to replace standard bulbs with flourescent. A compact fluorescent that generates an equivalent lumen output only uses about 23-27 watts of energy and heat load so you are actually saving on two fronts of energy use.

.....thirty years ago you didn't have all the heat generators .....

30 years ago was 1977. The TVs were all CRT tube type and generated a lot of heat and in fact semiconductors TVs were still ramping up to replace the vacuum tube TVs which generated a heck of a lot of heat. But, you are absolutely correct that the number of electronic devices (heat generators) has grown dramatically just the heat each produces has gone down significantly. Shoot, even wireless phones generate heat when they are just sitting in the docking/recharge cradle! Also, computers put out a good bit of heat even when not being used.

One of the most important things to consider is the insulation in the walls and ceiling plus the weather stripping around the doors and windows. The recommended R values have gone up since 1977. Also, the type of windows play a major factor in heat load with which way they face and if they are double, or even triple, pane.

I took this from the HGTV website for reference.

The power of an air conditioning unit is measured in "tons," with one ton equal to 12,000 British thermal units (BTUs) per hour. The size of the air conditioner needed for any given house is based on the heat load, which is determined through a complicated formula involving the square footage, location, number of windows and doors, ceiling height and several other factors. To get a rough estimate of how many tons are required for your home, divide the square footage of living space by 500.

Here is a link from Consumer Reports for calculating the correct size.

http://www.consumerreports.org/cro/appliances/air-conditioners/reports/sizing-worksheet/index.htm

Guess I wasn't clear on my TV comment.

What I meant was you had one TV for the whole family not a TV in every room like you do now. And then add in Computers, Game Consoles, etc.

And you bring up a good point though. Modern TVs LCDs & Plasmas don't generate the heat the old CRTs and tubed electronics did.

Yep, I thought about that as I was writing and figured that was what you were pointing out which is what I rambled on to conclude, sort of, I think.....

did anyone notice that the original poster almost never comes back to let us know if we helped?

Anyway I'm glad we helped scotch drinker.

Dear All,

Thank you very much for your genuine effort to help me on this regard. But still I'm confused with all kind of different answers. Please let me know, whether over sizing of A/C plant is a measure of energy saving? (which will avoid continuous operation of the compressor)

If I understand correctly you do NOT want to over size or under size the AC/Furnace due to the fact that that it will be inefficient either way.

Couple more questions how much of a fudge factor can you get away with without hurting the efficiency thus hurting your energy cost.? 1/2 ton 1 ton?

If you upgrade to a larger unit. What about the duct sizes?

I don't want to have to replace all of the duct work because I added 1/2 ton to my unit.

How much does the duct work effect the unit size and efficiency?

No, do not oversize. Oversizing will cause cool down the area rapidly and of course shut off. The problem is the unit will then need to come back on rather soon. This is the rapid cycling mentioned previously. The starting phase is when a large amount of energy is consumed. You can compare it to City driving vs. highway driving.

Also, the ducting and fan are sized to move the amount of air necessary for the chilling coil. You put in a larger coil from a higher tonnage unit and the airflow will not be adequate which will reduce efficiency and could potentially cause icing on the coil.

One other factor to consider...an oversized unit may not run a large enough % of time to dehumidify the air... a concern for the more humid climates

Dear All,

Thank you very mush for your valuable thoughts.