Sizing Air Conditioning

By Tim Carter
©1993-2007 Tim Carter

Summary: Calculating the proper size of your air conditioner is critical. If your HVAC system is not the proper size, your house will not be comfortable. Calculating heat gain can be done with the help of the Manual J. Get a copy for your air conditioning sizing.

My friends - the Gibson's - purchased a tract home about 15 years ago. It was actually the sub-division model home. The front of the house has no less than 10 windows if I am not mistaken. One month ago their AC unit gave up the ghost. My HVAC man installed a new AC unit for them. The Gibsons had always complained about being uncomfortable. They thought the AC unit was low on Freon. It turns out the AC unit was never sized right from the get go. It was 1.5 tons too small!

To add insult to injury, the return air ducts from the second floor rooms were non-existent. There was just one central return duct in the second floor hallway. My HVAC man did a heat gain calculation to make sure that the proper sized unit was installed. He has a neat computer program that allows him to do this. You can do the same thing by hand - I have done it numerous times - and arrive at the same results.

One Size Doesn't Fit All

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Tract houses often have similar sized AC units. The builder or HVAC person simply does one calculation and often applies it to all houses that are similar in size. Guess what? This doesn't work! In the case of my friends, the glass on the front of their house happens to face west south-west, the WORST possible direction! The blazing sun late in the afternoon cooks the inside of their house. I'll bet that a number of you reading this bulletin have similar situations.

Some HVAC contractors try to size AC units by doing a simple square foot analysis. They use a gross multiplier that ranges from 400 (older homes) to 1,000 (newer homes) sq. ft. of floor space per ton to arrive at total BTU heat gain or total tonnage. This method is unacceptable. If your HVAC person proposes this, find a qualified technician who understands heat gain and knows how to use Manual J.

The Bible - Manual J

Calculating heat gain is really complex. You must use a booklet called Manual J. This booklet takes you step by step through the process making you account for each and every source of heat. Many scientists have worked over the years to formulate the tables, formulas, and values that combine to form this bible of the air conditioning industry.

You can purchase a copy - and I URGE you to do so - if you want to really understand how air conditioning works. The ordering instructions can be found if you keep on reading.

The manual is actually a rather technical booklet, but if you follow instructions well, you can easily work out a heat gain computation of your own. You will at least get close. Don't get bogged down in all of the technical mumbo-jumbo within the manual. Simply take your measurements and crunch some numbers.

Finding a Professional

The same outfit that publishes Manual J also has a new program that will allow you to locate a professional who is dedicated to a high professional standard. They have a new certification program called the Air Conditioning Excellence (ACE) Technician Certification Program. Air conditioning contractors who are ACE certified meet a high standard because they have completed a comprehensive, industry education and examination process and have demonstrated technical expertise with numerous types of air conditioning (and heating) units in the field. They also have a thorough understanding of non-technical and safety issues that impact the equipment in your home. If you want a professional in your home, I suggest you look for someone who has this ACE certification.

Oversizing Equipment

Some people want to oversize their equipment so they are assured of being cool on those hot days. This can be a big mistake. You actually want to undersize your equipment ever so slightly. Researchers at Texas A & M found that air conditioners that were undersized by 10 to 20 percent are more efficient and more effective at removing water vapor (humidity) from your indoor air.

Now I will agree with them to a point, but I don't know if I would undersize my own equipment to the 20 percent benchmark! I would go 5 percent at the most....a totally unscientific approach since I have always sized the equipment on my jobs to match or slight exceed the calculated heat gain.

Oversizing a unit causes problems because the unit doesn't run long enough. You want your air conditioner to run for 15 - 20 minutes at a time. This allows the recirculating air to come into contact with the cold coil inside the air handler. This is how humidity is removed from the air. You will feel most comfortable when as much moisture is squeezed from the interior air as is possible. Short cycling air conditioners don't remove humidity well at all.

Calculating Heat Gain

The first thing you need to do to calculate the heat gain of your house is to purchase a copy of Manual J. This booklet is very likely at your local library and/or you may be able to borrow a copy from a local air conditioning distributor. The point I am trying to make is that it contains 116 pages of tables, examples, and other valuable information that is physically impossible for me to include in this tiny Bulletin.

My copy of Manual J has an awesome example computation. You can see the floor plan of the house and the computation sheet. This allows you to see how they arrived at all of the individual heat gain BTU's. The example computation will allow you to easily compute your own house's heat gain.

The following instructions are meant to HELP you make sense of the rather technical Manual J. Without the following notes, I'm afraid that you might give up in frustration.

You can purchase your own copy of Manual J by contacting the following Association. The cost ($35) is steep, but there is no other booklet like it:

Air Conditioning Contractors of America (ACCA)
2800 Shirlington Road
Suite 300
Arlington, VA 22206
www.acca.org
Call Toll Free - 888-290-2220

Heat Pump Costs

Heat Pump Operational Costs

By Tim Carter
©1993-2007 Tim Carter

Summary: Does it make sense to buy the most efficient heat pump? It depends on your climate, location and energy costs. This chart makes a comparison of the heat pump efficiency, electric cost, yearly savings and lifetime operating cost.

Heat Pump Operational Costs - Different Geographic Regions

Does it make sense to buy the most efficient heat pump available? Well, it depends! Look at the table below. I selected some cities where friends of mine live. It is fairly well distributed across the USA and absolutely in different climates. Pay attention to the cost of electricity in each city and compare it to your cost. Reference your electric bill. The calculations to arrive at the results below were done by a computer. Just about every heat pump distributor in every city in America has the capability to produce these numbers. They just need to know three things: the size of the system in tonnage, the local electric rate per kilowatt hour, and efficiency of the unit you are looking at. The best thing to do is have a calculation run for three different efficiencies just as the table below shows. I extend my thanks to Jeff Urbanek who works very hard each day at Quality Crafted Systems - the local York distributor. Jeff was kind enough to take time from his busy schedule to run the numbers you see below. Thanks Jeff!

The numbers below are based on some constants. The system Jeff and I selected is a 3 ton system. The calculations were done taking into consideration the design temperatures for heating and cooling for that city. The lifespan of the heat pump is 15 years. The total lifetime cost value in the last column is adjusted for inflation over the life of the unit.

City	SEER Efficiency	Electric Cost per Kwt hour	1st Year Cooling Cost	1st Year Heating Cost	1st Year Total Operating Cost	Total Lifetime Operating Cost
Charlotte NC	10	$.076	$359	$602	$961	$16,619
	12	$.076	$299	$564	$863	$14,924
	14	$.076	$257	$502	$759	$13,126
Cincinnati OH	10	$.070	$258	$668	$926	$16,014
	12	$.070	$215	$626	$841	$14,544
	14	$.070	$184	$557	$741	$12,814
Orlando FL	10	$.060	$694	$191	$885	$15,305
	12	$.060	$579	$179	$758	$13,108
	14	$.060	$496	$159	$655	$11,327
Seattle WA	10	$.040	$31	$578	$609	$10,532
	12	$.040	$26	$542	$568	$9,823
	14	$.040	$22	$482	$504	$8,716
Toledo OH	10	$.11	$319	$1,386	$1,705	$29,485
	12	$.11	$266	$1,299	$1,565	$27,064
	14	$.11	$228	$1,155	$1,383	$23,917

Central Air Conditioning

By Tim Carter
©1993-2007 Tim Carter

Summary: Air conditioning a home properly will allow your family to be comfortable in the entire house. Central air conditioning systems must be sized correctly. Call a professional heating and air conditioning expert to calculate the proper size air conditioning unit for your house. If you already have a central air conditioning system, he can do the air conditioning troubleshooting for you.

The central air-conditioning system in a home, as well as the heating system, is one of the most important parts of a home outside of a sound roof and indoor plumbing. The heating and air conditioning systems create an artificial climate inside a home. It is a very realistic expectation that this climate can be the same in each room. In my own home, each room, no matter the outdoor temperature or time of day (this is very important), can be within one or two degrees of any other room in my home. That keeps my family and me very comfortable.

It is unrealistic for you to expect your home to be a certain temperature no matter what the outdoor temperature is. In other words, if the outdoor temperature rises to 115 F, I would not expect you to be able to cool your home to a chilly 72 F temperature without considerable discomfort on days when the temperature was say 85 F.

Central air conditioning systems are designed to operate within a given range of temperatures. For example, the design temperatures in your part of the country may be only 20 degrees. This means that your air conditioner can only produce a 20-degree difference in temperature from the actual outdoor temperature to the lowest possible temperature the system can maintain indoors. It is possible to create a wider temperature spread, but oversizing an air conditioning system can result in short cycling when the air conditioner has little work to do.

If an oversized air-conditioning system short cycles or only has to drop the temperature a few degrees, it simply does not run long enough to remove humidity from the air. When this happens, the temperature inside your home gets to the desired level, but you feel cold and clammy. Properly designed central air-conditioning systems will run for ten or fifteen minutes at a time which allows them to extract humidity as the air flows across the cooling coils inside the air handler.

The key to getting an air conditioning system to work properly is to have a real professional size the equipment properly and install a ducting system that delivers the right quantity of air to each room of the house. Each room must also have a return-air duct inlet that vacuums hot air from the ceiling and returns this air to the central system to be cooled once more.

A professional air conditioning person will take your new-home plans and analyze them using sophisticated computer software. This process will determine both the BTU (British Thermal Units) heat gain and heat loss for EACH room of your new home. This same exercise can be done on existing homes. With this data, the professional can ensure the proper sized equipment is purchased, and the ductwork that provides air to each room is sized correctly. This is of the utmost importance.

Heat gain is the measurement of heat your house gains each hour during the summer months. This number can range from 20,000 in a smaller home to more than 90,000 in larger homes. There are many variables including but not limited to: amount of wall and ceiling insulation, number and size of windows facing west and south, amount of air infiltration, compass direction each wall of your home faces, number of people living in the home, etc. Heat loss is the amount of energy your home loses each hour when it is cold outside and you are trying to heat it.

My own home has a heat gain of nearly 77,500 BTUS. To offset this, I have two separate central air-conditioning systems. One is 2.5 tons and the other is a 5-ton system. These two central air-conditioning systems are completely separate from each other. The smaller one handles the second floor of my home which has much less square footage of living space than the first floor of my home.

The advantage of the two systems should be obvious. Each system is controlled by a separate automatic setback thermostat. At night, the thermostat for the first-floor system tells the first floor system to turn off all night since we are upstairs sleeping. In the daytime, the second-floor system is set to not work as hard, since we are downstairs. But one hour before bedtime, the automatic thermostat for the second floor resets itself and tells the outdoor compressor to get to work so the bedrooms are cool when we go upstairs.

Air Conditioning

By Tim Carter
©1993-2007 Tim Carter

Summary: Air conditioning units in your home have to be properly sized to fit your home. Your heating and AC contractor (HVAC) figures out how many supply ducts and return air ducts need to be in each room for the best air flow.

I find it interesting how some of the coldest parts of our nation get hot - and I mean steamy hot - in the summer months. Ask any elderly person who still has a fairly crisp mind and he will tell you what it used to be like without air conditioning (AC) in the summer months. Yes, AC is a relatively newer innovation for the residential home. Tens of thousands of older homes that still stand in the U.S. were built without it. But I am convinced that your next new home will come with AC as standard equipment, not an option.

But don't think for a moment that the AC in your new home is going to work perfectly and keep all of the rooms in your home pleasant and comfortable. To achieve that goal your builder and heating and AC contractor (HVAC) have to engineer a system that is tailored and fit to your home like a custom suit or dress. Unfortunately, poor builders and HVAC contractors often install AC systems like clothes bought from the rack at a discount store.

The first step in the process is studying your actual building plans. Most of the local distributors who actually sell the AC equipment have special computer software that can analyze your plans. The software studies the compass orientation of the house, the sizes and types of windows and doors, thickness of wall and ceiling insulation, number of occupants, special appliances or lighting that generates extra heat, etc. to determine what the heat gain of your new home will be. With this number, they can specify the correct equipment that can adequately cool your home.

Strangely enough, two identical homes on the same street can end up with slightly different equipment. The primary reason being compass orientation. The house that has more window and door glass surface area facing due west will almost always more cooling capacity. The equipment size is measured in tons with 12,000 Btus of cooling capacity equaling one ton. It was determined long ago that it takes 12,000 Btus of energy to melt a ton of ice. Equating this to your new home, if the calculations show your house has a total heat gain of 59,750 Btus, then you need to make sure you have a 5 ton AC unit. It is that simple.

But the proper sized unit doesn't mean you will be comfortable. The amazing software that computes the overall heat gain does so by looking at the individual heat gain of each room. This data is extremely important as it tell the HVAC contractor how many supply ducts need to be placed in each room and most importantly the size of the pipes feeding those outlets. Think of fighting a large house fire with a garden hose. It doesn't work because you aren't putting enough water on the fire. You need to pump in enough cold air into each room to offset the fire or heat gain that is occurring within the room.

Getting the cold air to each room is not as easy as it sounds. The large fan within the air handler in your basement or attic creates pressure much like wind blowing against a sail on a boat. This static pressure needs to be maintained within the entire supply duct system. To do this the HVAC contractor must reduce the size of the main supply duct after every two or three branch runs run towards the individual rooms. If the main duct remains the same size, there is simply not enough energy left in the remaining air to push the cold air to the rooms farthest from the air handler.

I have news for you, we still are not finished. What happens to the hot air that is in each room? It needs to be sent back to the air handler where the heat is extracted. To do this, your builder and HVAC contractor need to make sure a return air duct is located in each room, preferably seven feet high off the floor. Ideally this return air duct should be on the opposite side of the room from the supply ducts. As the air handler is operating, it is both pulling air into the return ducts and pushing out an equal amount of air through the supply ducts. By placing these different ducts far apart, anyone standing in between the ducts feels the cool air floating over their body.

Beware of builders who do not understand these concepts. The HVAC component of your home is very important. It is what creates and maintains the artificial climate within the four walls of your new home. The HVAC part of your home is perhaps that last place to trim costs if the budget is running a little high. Be sure to specify top drawer HVAC equipment. If at all possible, try to tour a home that is under construction. Look at the overall HVAC equipment and ductwork. Pay attention to see if the main supply duct decreases in size. That is a good indicator the HVAC contractor is likely on top of his game.