How do you determine what size central a/c unit a house needs?

[blue83ranger]

Our central a/c unit went out many years ago and we were just using a single window unit for the entire house and we are now ready to just replace our central unit. Does anyone know of any sites where we could look up what size we should be getting? and does anyone know of any good deals online? thanks for the help.

 

[98 ranger xlt]

Most places go by the square footage of your house.

 

[rboyer]

One question that comes to mind is what size unit did you originally have? And was it enough to keep the house cool? Sizing for central air or heating takes in alot of factors and square footage is the biggest one of course. The amount and type of windows in your house, how well it's insulated, type of ductwork (fiberglass vs. sheet metal), etc. This is one of those things where you really have to look into everything because you could end up either getting a larger unit to suffice or spending the same amount of money overall by getting a slightly smaller unit and making updates such as updating ductwork or sealing air leaks which will keep your cooling costs down in the long run.

 

[daniel3507]

there are a couple ways of figuring out how big of one you will need. by square footage, or like rboyer said. you calculate windows, positon of windows, amount of sunlight, a bunch of things. the best way to figure that out is to just call places out for a free estimate and they will tell you what size would suit your house best. then you may be able to find a good price online for what you need.

 

[LearjetMinako]

According to the guys that fixed our heater in the winter and looked over our central air. For a 2,000 sq/ft, we have a 3ton unit. Our consists of high cellings in the living area and master bedroom. 13x double pane windows around the house. As it is, its okay. It will keep the house cool at 72*F in the summer until outside temps start hitting above 88*F, then the house stays around 75*F. When ours goes, we're planning on getting a slightly bigger unit or just a better one.

FYI: A/C units are changing their free-on. They are going from R-22 to something called Pure-on. From what I've been told, companies will stop servicing R-22 units since they are stopping production of the stuff.

 

[blue83ranger]

thanks for the replys. I did find one site that had a list of questions about the size of your house, where the windows were, shade, where the ducts were, etc. but i got to the end of the questions thinking it would tell me what we need then it just said to print out the questions and take it to a contractor.
Our old unit did a pretty good job of keeping everything cool but i think my dad bought it just because it was cheap (I think my uncle use to assemble them) so it's not really sized for our house, but he could of guessed right I guess.

 

[shadetree]

Rule of thumb for a good insulated house is 1 ton for every 600 square feet. However, all the factors mentioned above will alter this somewhat.shady

 

[Onebadezgo]

Rule of thumb for a good insulated house is 1 ton for every 600 square feet. However, all the factors mentioned above will alter this somewhat.shady

Yeah thats how it was done in the ol days now we perform whats called a load calculation it involves all of whats above but we have a nifty new computer program that can do it all for us. Had to spend some money and a couple of days in school to get it but its worth it.

 

[LearjetMinako]

Rule of thumb for a good insulated house is 1 ton for every 600 square feet. However, all the factors mentioned above will alter this somewhat.shady

That would mean my A/C unit is undersize. I find that true since the unit is from 1994 when the house was built. And from the sound of it, it may not last too much longer. Not only that, when they built the houses in my area. The houses weren't really built to their own. More like buy in bulk and just throw it together.

I'm glad someone posted something like this. Its got me thinking.

 

[Legend]

There are a lot of additional factors including average wind speed, wind breaks, amount of shade, location of course, SEER rating, etc., etc.
Here is a link to a worksheet that is supposed to help calculate the size you need:
http://www.consumerreports.org/cro/appliances/heating-cooling-and-air/air-conditioners/sizing-worksheet/
The SEER rating is very important, nowadays they range from 13 to 22. I would sure suggest getting one that is at least 18. As far as I know, the government tax incentives for getting high efficiency have expired, but you should check to see if any have been reestablished. And your local utility company may give you a rate break or a one time payment if you go for a high SEER.

 

[skippy]

I can not stress how important it is to have the system properly sized by a reputable licensed contractor.

Sizing is done according to ACCA manual J and takes into account heat loss/heat gain of the structure based on location,building materials,elevation,exposure,etc..

If not properly sized,health issues such as mold can develop.

Most reputable contractors can give a free estimate,which includes a calculation on the trusty laptop.Then get at least 3 estimates.

We will service hcfc-22 systems forever,the refrigerant is being phased out,not outlawed.Puron is the refrigerant of choice for most manufacturers going forward and performs well,but runs almost double the pressure of 22.

 

[skippy]

Yeah thats how it was done in the ol days now we perform whats called a load calculation it involves all of whats above but we have a nifty new computer program that can do it all for us. Had to spend some money and a couple of days in school to get it but its worth it.

 

[OilPatch197]

Rule of thumb for a good insulated house is 1 ton for every 600 square feet. However, all the factors mentioned above will alter this somewhat.shady

I got a quesiton, at work we got ten 60 ton units.

Does that mean that each unit weight "60 tons"? What is the "tons" about in specs of a AC unit?

Is it the total weight of the unit, or the weight of the coils/compressor? I know in die presses they are rated at "100 ton" but weigh a fraction of that weight, is this the same thing applied to AC units?

 

[skippy]

I got a quesiton, at work we got ten 60 ton units.

Does that mean that each unit weight "60 tons"? What is the "tons" about in specs of a AC unit?

Is it the total weight of the unit, or the weight of the coils/compressor? I know in die presses they are rated at "100 ton" but weigh a fraction of that weight, is this the same thing applied to AC units?

1 ton is 12,000 btu/hr.60 tons =12,000 btu/hr x 60.

 

[blue83ranger]

thanks for the link. I'll fill it out sometime soon and see what it says.