What size of Air Conditioner & Heat system do I need ?
Load calculations ( Winter Heat Loss & Summer Heat Gain ). Every home should have a calculation done and in accordance with Manual "J".
To be eligible for utility / goverment rebates and incentives will require this to be done .

Unit size, (BTU requirement),  is based on the type of construction of your home, apartment,  or mobile home. The insulation it has or will have in the walls or attic, whether the attic is ventilated, The windows single-double-triple pane plus there type, The infiltration of air through your door, windows, receptacles, ductwork, in general is the home getting above average of infiltration,  and how hot or cold it gets in your area. All these things add up to the size of heating & air conditioning unit you must have to maintain your comfort. (See Residential Conservation).
 Every area has an outside temperature and inside temperature, ( see below example for design temperature ), taken by the hour & season from the weather 10 year average. This is called the Design Temperature.  Say a weather 10 year average is 95 degrees outside and a 75 degree inside your dwelling, that would be a 20 degree difference. The air conditioning requirement, (BTU's), would be calculated on it's "Design Temperature" and the grains of moisture. Sometimes the outside temperature may exceed say the 95 but that is not the normal and your unit is not sized with that in mind.  If you sized for the maximum occurrences of less than 1% of the year then you would be oversized at lesser temperatures and could cause a moisture problem.
That's one of the reason's that a larger unit will not remove as much moisture as an undersized unit. A certain amount of running time is very important. When air is discharged from you grille it is very close to 100% humidity level. Think about it, when it rains, (the air is cooled then the moisture (rain) is removed you get 100% moisture level.
Heating does the same thing but in reverse. To dry of air can cause a dryness condition not only for your tongue & groove walls but for you as well may be chapped lips. That's where a humidifier comes into play. Heating also has a design temperature for it's calculation.
Manual "J" is the reference for those type of multipliers. Even that came from the bible of the industry A S H R A E. But ASHRAE, (American Society of Heating Refrigeration Air Conditioning Engineers),  took to long to calculate and I mean several days for an average home. In the 1960's National Warm Air decided an accurate but faster way to calculate the home requirement was needed and they developed Manual "J". As time has gone on construction has changed and therefore the manual has been added to - to incorporate those changes. Other items have been added to Manual J to enhance the contractor abilities and provide more information such as operating cost in your area.
The present organization for the Manual "J" and other manuals that were originally developed by National Warm Air for good air conditioning & heating is ACCA, (Air Conditioning Contractors Association),  http://www.acACCA Manual "J" site

Click on item or read by paging down

(Existing or new residential)
Reference for calculating residential heat & cooling requirements.
Even computerized programs should be in accordance with ACCA'S ( Air Conditioning Contractor's Association ) Manual "J". Usually Code Building Departments, rebate programs, and others require this. When a survey of an existing home or one to be built is done there should be a load calculation to show the correct amount of BTU's for heating, cooling, & the volume of air that is needed to be comfortable, (Compiled from the weather report for the selected area, an Outside temperature and an inside temperature & grains of moisture = The Design Temperature),  there will be a "Design Temperature" for your area. More about Design Temperatures below.
New construction will also require an energy calculation to be completed.
When a calculation is not done and those "methods" (guess's) are done they may be OK or they may not be OK. Why would you as the home owner have your home comfort guessed?
Manual "J" is usually done by an air conditioning dealer but by those that are schooled & trained to calculate a Manual "J"
 below shows Manual "J" 7th edition
Your comfort:
Calculations of your home using Manual "J" (R's - HTM's - U values - BTU'S) is an accepted way to determine the BTU capacity of air conditioning & heating for the home and the selected equipment to maintain your comfort at Design Temperatures & the air volume (CFM) needed for your home. Basically your home envelope is measured and multiplied by the heating & cooling BTU'S that are penetrating that material (Don't forget the amount of BTU penetration is from the "Design Temperature for your area"). Be it glass, ceiling (roof), walls, & doors. These calculations should be done by someone that is familiar with the aforementioned values & tables that are in a computerized program as well as conservation. BTU sizing for the home & chosen equipment for Heating, Cooling, Dryness, & Humidity ( your comfort ) will be determined.
A room by room calculation has a lot of advantages and doesn't take much more time in doing. It not only is the room by room Btu requirement for cooling & heating but It can help in air duct sizing design or survey of existing duct.
Each conditioned room shall have: (to be shown as normally required by building departments), the amount of BTU's for heating and cooling, & the amount of air, ( cfm = cubic feet per minute ), needed to obtain & assure comfort at "Design Temperatures".
BTUHG = Heat Gain  -  BTUHL = HEAT LOSS  -  CFM = Cubic Feet per Minute

For those of you that want to know, BTU stands for British Thermal Unit.  (1 match = 1 BTU )
Heat gain means during the summer the heat comes into your home thus it's called a gain. During the winter the heat inside your home transfers to the outside thus it's referred to as heat loss. Think about it. Heat always transfers to a colder area, Heat gain for the summer & heat loss for the winter. That's basically how a unit is sized for you comfort. A given temperature outside with the desired temperature inside your home.

When the required cooling & heating BTU's and air volume are not distributed into the area then unsatisfactory results ( your comfort) may occur.

Always have a load calculated ( heating & cooling BTU's & Air volume ).  ( approximately 30 BTU's per cfm of air )
Originally load calculations were very complicated and for a residence it may take a day or two. The primary source & bible for the HVAC Industry is ASHRAE, (American Society of Heating,  Refrigeration & Air Conditioning Engineers).
Manual "J" was developed so that air conditioning & heating contractors could calculate a load quickly & accurately. National Warm Air developed "Manual J" in the 60's then later the organization became known as NESCA, another one but they were short lived then ACCA, (Air Conditioning Contractor's Association) and that is what it is known as today.
My first school to learn Manual "J " was in the 60"s. It was taught at a State University & was a Monday through  Friday course. Manual "J " has been around for quite some time. It seems that the current manuals are getting more complex than the old ones. I wonder how long it will take to get back why the manuals were created in the first place to make load calculations simple so the dealer or his rep could do a professional calculation and be accurate in doing so.
The recommend source for "Load Btu Sizing Calculations" and unit capacity sizing is ACCA's Manual "J". For Manual J's site click here.
Most Building Departments (codes) & rebate programs require the load calculations to be in accordance with Manual J.

Top of the page
First a little about the transfer of heat. Heat always moves to a colder object. When it is hotter outside your home and colder inside then the heat will come in from the outside. Thus "Heat Gain" (BtuHG) for air conditioning. When it is colder outside & warm inside then the heat will transfer from inside to outside. Thus "Heat Loss" (BtuHL) during the furnace season. Heat tries to stabilize at the same temperature.
That transfer of heat is slowed by precautions taken by the home owner. Insulation, air leakage, and other measures.
Heat transfer is measured by K factors, C factors, R Values, HTM Multipliers, & BTU's.  
We normally do this by BTU's of cooling or heating that is needed in your home. Btu's are by per square foot of the type of material and by outside to inside temperature  
Insulation is rated by the "R" value. The larger the "R" value the better is it's insulating value.
The objective is to slow the rate of heat transfer as much as possible but also be cost effective.
 A calculated load takes these and other things into account & then the size of air conditioner &/or heater is determined.  That is the big reason a "Load Calculation" is suggested.
Top of the page
Load calculations are NOT done for various reasons............ Laziness, I will replace with the same size unit, Oh I use 500 square feet per ton, don't know how to do that (they will never say that), I would install a ? ton if it were me,   and other reasons I call excuses.
 GET a calculation done.
A copy or print out can be given to you for your records. The dealer should provide you, (the customer),  with a copy at signing of the contract, (see a copy of a contract on AC survey).
Each room is calculated because that will tell you the Btu's & air volume needed in that room for heating / cooling. Each room will have "Heat Gain (air conditioning), Heat Loss (heating), & CFM (air volume)"

Take each wall of each room select the direction it faces, the type of wall (frame, concrete block, insulation R value. Then the windows in that wall & their type, single pane, double pane, etc. Ceiling in that area if it has an attic above it. Again select the insulation R value & it's type of ceiling. The floor in that area, whether it's a crawl space, no basement or space below it,  the insulation value.

For existing homes one can use graph paper for each floor. Don't forget for air conditioning there is approxmately 30 cfm (air) for each Btu.

I would always recommend a Manual "J" calculation or in accordance with Manual "J" in lieu of those rule of thumbs, (500 sq. ft per ton, so many cfm per outlet, replace with the same size unit, etc).  A helpful and important procedure in giving a good estimate is the correct size of equipment.   
1 Btu is equivalent to 1 match

A load calculation determines the amount of cooling and heating, (in BTU's), needed for your home at the design temperatures, (see examples below), for your location. Design temperature is obtain from Manual "J " and is the outside temperature with the inside temperature and some other factors. Lets say a Design Temperature cooling load was 95° outside & 75° inside, a 20° difference. So if  the outside was 100° then all you could expect inside is 80°.
If you wanted an inside temperature to 70 degrees when the outside is 100 then a larger unit is needed but that's where codes, design temperatures according to Manual J come into play. Too large of a unit could create an uncomfortable situation for you such as a moisture or a humidity problem.
Heating basically works the same way. If the BTU calculation was for 30° outside & the inside is 72° then you would have a capacity of 42°. So if it got to 10° outside then the most you can expect inside is 52°.
When heat is too oversized & no humidifier is used then the air in the home can become too dry. Your skin or lips will have that cracking. Even tongue & groove walls have pulled apart due to excessive dryness. The dealer thru the load calculation can advise you.
Cooling equipment size should be based on the calculated sensible and the calculated latent, (moisture), cooling loads. Procedure "D" of Manual "J " says that when the cooling equipment is operating at the outdoor design temperature it's sensible load and it's latent capacity must be equal to or greater than the calculated latent load.
Load Btu sizing calculations are NOT done by the maximum temperature outside.  That's why you should see & select the design temperature for your area.
Your outside temperature doesn't reach it's peak and hold there. Usually your local news paper will reflect the hour to hour temperature. Summer temperatures climb to their highest until 1:00pm or 2:00pm in the afternoon then they start to drop.
Also look at the outside temperatures in the "Bin Method". This "Bin Method" provides the hours per year at said temperatures (see Questions-Answers-Tips & select operating cost). This is one of the methods used to figure annual heating & cooling cost.
The highest or hottest temperature is only 1% or less of a given year. So you are going to be oversized at other times. We certainly don't want to add to this dilemma.
Air conditioning, (cooling only), equipment should be selected to keep over sizing to a minimum.
Top of the page
By using the "Design Temperature" the humidity in your home is better controlled & unit sizing will be more correct. Select the "Design Temperature the very 1st thing for your area & always do a load calculation.
When conservation or additions are added to the home after your installation and Load Calculation, they change the homes required BTU's capacity for cooling & heating. The unit could now be sized improperly.
The Design Temperatures are not changed but the capacity required is.
This is the design temperature for Patterson, New Jersey.
This particular example was from a computerized program. Any "Design criteria" is taken ultimately from Manual J.

This design selected will affect all other calculations. Wall, ceiling, windows, etc.

That's one of the reasons that someone familiar with doing this calculation should be selected.
Notice the summer outside temperature of 91 degrees,
 the inside temperature of 75 degrees, & the moisture of 95 grains. That is what your home is designed for in the Patterson, N.J. area.

These Design Conditions are for Phoenix, Arizona.
Note there is a difference in design locations.

Every location's design conditions will be different and should be selected for your location.

Notice the summer outside temperature of 107 degrees, the inside temperature of 75 degrees, & the moisture of 56 grains. That is what your home is designed for in the Phoenix, Arizona area.

Top of the page
The below Btu's will be based on the Design Conditions selected.
Direction of the glass is very important.

Notice the top left hand corner shows this direction is North.

Top of the page
Insulation amount for the ceiling can be a lot or little heat.

A BTU is the same as 1 match.

Top of the page
The type of wall & the amount of insulation comes into play.

Also the direction. In the top left hand corner this one  direction is showing North
Top of the page
The fireplace &
it's air leakage are taken into
Top of the page
The number of people are determined at usually 2 people per bedroom.
For the people selection generally two people in each bedroom is used. Each room is calculated until all air conditioned areas are done. Sounds like a lot but it really isn't.
Top of the page
The total calculation is on a form such as this example.

If done by a computer or long hand there is a form that can be kept by the contractor & a copy can be given to the customer.

REMEMBER this is only an example and is not a complete load calculation.
Top of the page

What is shown is a computerized example of some but not all of the factors and they are derived from Manual "J". This intent is to give you an idea of what it takes to do a load Btu sizing calculation and is NOT intended to be a tutorial.
Top of the page

 The number 1 item for heat or loss of heat is the glass & the direction it faces, type, (single - double pane), shaded, draperies, blinds, and more come into play.

The 2nd is attic, then walls, the amount of insulation, people, etc.
An accurate calculation should always be done by a competent person or air conditioning / heating contractor.
The load doesn't take long to do, (20-40 minutes),  and there was never a charge for it but one needs to understand how to do it.
There are a lot of quick methods, guess's, aka "Rule of Thumbs", (500 sq. ft per ton, so many cfm per outlet, replace with the same size unit, etc.), and some are pretty good but there is NOTHING that can take the place of a Manual "J"  load calculation.

The customer should be aware of the "Load Calculation",  the why & how it is done.
DON'T try it if you are not experienced in these calculations.  Let the competent dealer or person do it.
Top of the page
A central air conditioner is for a 24 hour operation. Not like the wall / window unit that sometimes is oversized. The main reason of not over sizing the central air conditioning is to get a certain amount of running time which helps in moisture control.
Building Departments may require a plan of your installation including the "Load calculation" with the amount of air to be delivered room by room. In other words for each conditioned residential room the "Heat Gain-(air conditioning BTU's), Heat Loss-(heating BTU's), and the Volume of air-(cfm)" will need to be determined.
( A quick method of determing the amount of air is 30 BTU's per CFM. That should help the ac contractor in sizing the duct & grilles).

Load Calculation Programs:
Over the years many various load calculation programs have been developed be it manual or computerized and there are a lot of them from free to costly and from easy to difficult to use. The following program is easy to use but most importantly it is widely accepted by State & local building requirements. HVAC-CALC is in accordance with Manual "J" and that is a prerequisite with a lot of building departments rebate programs.
A lot of state or local departments also require one to be fluent in this calculation process.

Support via toll free phone and email is very important and at no cost.  I find HVAC-CALC is superior.  Don Sleeth who is the developer was also in the air conditioning / Heating profession.
Many programs especially one that is rated as being good is lacking in their support, (let the dealer just ask a question about how to use the program then find their response may take several weeks if the dealer is lucky),  plus any of their programs are very complex & difficult to use.
Others can say their support is good but actions speak louder than words.
HVAC-CALC talks the talk and walks the walk.
HVAC-CALC may be the easiest, most accurate to use and the least expensive.
The trial version not only shows you the program and how easy it is to use but it has a video, at the Menu selection, that guides you thru.
I highly recommend taking a look at HVAC-CALC and it's help video.
HVAC-CALC also does commercial loads.
 click here for Website                                                            
          Ask the dealer.........
After discussion with the home owner & what the system should do is the measurements for a load calculation. The home owner should receive a copy of this calculation and please have it written on your contract that a load calculation in compliance with Manual "J" was done.
Ask the dealer was this done?   If they won't or don't do it then find another dealer.

The other load calculation program that seems to also be accepted by cities, municipalities, states, federal, etc is Right J, Wright J, Wright soft, and Elite. All these programs are & were available thru ACCA, (Air Conditioning Contractors of America), which is another story. No they didn't invent the Manual "J" as that was done by "National Warm Air" in the 60's, (this is discussed on my other web pages).
I started and learned on Manual "J", National Warm Air, and we did it by hand at first. Later the computer version came along and ACCA was the third maybe 4th  organization to purchase National Warm Air then the program started to become more technical and harder to use. (That was why National Warm Air invented Manual J to get away from causing dealers a 2-3 day laborious calculation just to see what a residence required in units heating &/or air conditioning.
All calculation information was taken from the ASHRAE manual which what was used in colleges while degrees were achieved. This was very complicated for the contractor and that's why "National Warm Air" made it a simpler process but as accurate as possible. In fact I attended a 4 1/2 days course at University of Florida where they also certified you were capable and eligible to do load calculations for residence's.
Because Wright J or what ever their name is now has become too complicated to use and the rules that have been set in place certainly don't favor the contractor   I recommend HVAC-CALC
Qualify your ac dealer !

Have questions just send me an Email  clik here