Sizing your HVAC System to go green and save energy

It's hot, the beads of sweat are rolling off your forehead in the kitchen. Time to replace that old air conditioning system with one of those green high efficiency units, right? Yes, but only after you make sure that your existing unit is sized properly using the what HVAC contractors call "Manual J." By using Manual J, you'll take going green a giant step further by basing your unit's size on good science and sound engineering, not on short cuts or what seems cool. We offer tips on how to ensure that your furnace or air conditioners are properly sized for your situation using Manual J and working with a HVAC contractor.

Why not just use the same size AC or furnace?
Independent studies show that well over half the existing AC and heating units are not properly sized. In fact they are geneally oversized because HVAC contractors used semi-qualitative methods to size your air conditioning and heating units. That could mean that you're shelling out extra money on electricity and natural gas every month, year after year. Besides having an older AC or furnace, maybe you have made some improvements to conserve energy and taken advantage of those federal energy tax credits. Finally, AC and heating manufacturers have designed more efficient units that you can take advantage of. It's time that you got serious and pocketed some savings.

When resizing your AC or Heating System really matters
HVAC fanOn any remodeling project that involves a new addition, home owners are faced with determining whether or not to update their HVAC system, aka Heating Ventilation and Air Conditioning. Some people ignore their HVAC system or let the professionals take care of it. There's nothing wrong with the latter, but you're the one who knows the most about your home and how you live, not your HVAC contractor. 

Think hard and demand proof if your general contractor tells you that your existing AC and heating units can handle the new addition you are planning.  

Your decision could translate into a HVAC system that doesn't  heat and cool your home properly and ends up costing you thousands of dollars in natural gas and electricity bills over a 15-20 year period. A better alternative is to work with your HVAC contractor to make sure they really understand your home, and who and how you will live in it.

It really pays to rethink the sizes of the AC and furnace if you're doing one or several of the following:

  1. Doing an extensive remodeling project that includes a new addition. The temptation may be to rely on your existing units to carry the extra load and save some money. However that may result in poor performance in the new addition and reduce the performance in the rest of the house.
  2. Installed new energy efficient windows- you lose a great deal of heat and cold through windows so these could reduce your cooling and heating bills.
  3. Adding new skylights.
  4. Adding new insulation to the roof or walls.
  5. Adding new appliances.
  6. Adding people to your household- this includes the elderly or new arrivals.

How a HVAC systems are sized
Air Conditioning Contractors of AmericaThere is no longer an excuse for improper sizing of the HVAC system in your home. HVAC contractors and manufacturers have a greater understanding of how to size a unit. Any HVAC contractor that is affiliated with the Air Conditoning Contractors of America should be able to properly size your air conditioning and heating system using Manual J.

In the past, HVAC contractors used the square footage method of your home to size an AC and furnace. Using square footage is easy, but ignores all the unique characteristics about your home. For example, your home's orientation to the sun, the size of and quality of your windows, insulation, how the home is constructed, and of course the average temperature during the summer and winter in your locality. All of the above factors and more determine how fast your home loses heat during winter and gains during the summer. This information is then used to determine the proper size of an air conditioner or furnace.

Enter Manual J- applying science and engineering
Manual J for Residential Load Calculation HVAC contractors and manufacturers have taken the scientific and engineering knowledge about cooling and heating and documented it in what is called the Manual J. Simply put, the Manual J systematically asks numerous questions about all aspects of your house that you know based on the area where you live. It asks the above questions and many more, room by room and then uses formulas to determine the proper sizing of an AC and heating system.

Manual J is pretty rigorous and using it takes a great deal of time. Thanks to computer software, an HVAC contractor can properly size an AC or heating system a lot easier. So there's really is no excuse for not using the Manual J to size an AC or furnace. If your prospective HVAC contractor says that square footage is just as good, we recommend that you not use them. You may not be convinced of that, but the example below will get the point across. 

Besides the the Manual J, HVAC contractors can also design duct systems using what is called the Manual D - Air Flow and Duct Design. Again, science and engineering a brought to bear on the problem.

Myths and Sizing Air Conditioners
One of the biggest myths about sizing AC units is that bigger is better. AC units have to do two things: reduce the room temperature and also reduce humidity. The larger the AC unit the less likely it will remove humidity. The large unit will run shorter periods of time and not have an opportunity to remove the humidity.

The second myth is that sizing a HVAC system using a "Rule of Thumb" or the square footage of a home will provide an acceptable solution. The following explanation is provided courtesy of HVAC-Computer.

"Some HVAC contractors still feel they can accurately size an air conditioner or furnace with a "Rule of Thumb" such as 800 sq.ft. per ton. Don’t put too much value on the number 800, in my 33 years of load calcs, I have heard it all the way from 250 to 1200!. Thumbs can do a lot of things, but they can’t size HVAC equipment. Here is an example that blows the "rule of thumb" method right out of the water.

Picture a scenic lake and a lovely custom home on the south shore. The house is 3,200 sq ft with a view of the lake in almost every room. Of course those windows face North. Mr. North who owns the house calls Mr. Sizebythumb, the A/C contractor, to install an air conditioner. Mr. Sizebythumb, who’s been doin’ this for a long time, divides the square footage of 3200 by 800 and installs a 4 Ton air conditioner.

Mr. South admires the house from across the lake where he has a building lot and has one built on his lot, with a lovely view of the lake, to the south of course. He too calls Mr. Sizebythumb who, trusting his thumb, does the math and installs a 4 ton unit.

Mr. South absolutely cooks in August with the sun streaming in the glass, and must close the blinds to achieve any relief, thus losing his $100,000 view of the lake. Mr. North finds he has to turn his thermostat colder and colder to try to achieve a feeling of comfort. He can never find the right setting for his AC because an oversized AC does not remove humidity. Rules of thumb can’t work."

Manual J for the Do It Yourselfer
Using computer software that mimics Manual J will not make you an instant HVAC contractor or specialist. What it will do is put you on a more equal playing field with your HVAC contractor to ask the right questions and make sure they took into consideration every ascpect of your house.  We say that because you know your house best and how you live your life much better than the HVAC contractor.
Here are two examples:

  • You know that you spend most of your time at home and like a lot of natural light. Therefore, you seldom close the binds or curtains. Your HVAC contractor may think otherwise and assume you close the blinds 50 percent of the time. That fact alone can significantly affect the size of the of the heating or AC unit. 
  • Your elderly parents are moving in and are always cold when the thermostat in winter is set to 75 degrees F. You'll want to design the heating system to accomodate them.

Our Experience
We're working with some clients on an extensive remodel that includes plans to install a new three zone HVAC system. The architect provided the specifications for the furnaces and air conditioning units. According to a reputable Maryland HVAC contractor, the units were greatly oversized. The discussion were so "heated" that the HVAC contractor told my client that he would not take the job it they insisted on installing these "oversized units.

I recommended to my client that he use a piece of software called HVAC-Calc (see the Video Overview). Two versions of the program are available from software publisher HVAC Computer Systems Ltd. at My client used the residential version, which is downloadable only and costs $49.

The intent of this program is to take what you know about your house — the size of the rooms, the climate you live in, how many windows and doors your house has, etc. — and then calculate all the stuff you don’t know. Namely the heat gain and loss of your house. Once you understand how your house loses and gains heat energy, you can use a formula included in the software tutorial to determine the proper size furnace or central air unit for your house.

What happened next
My client, an engineer, eagerly downloaded the $49 version of HVAC-Calc and started to enter data. After a few hours he learned a great deal about what went into the calculations and compared his results with the HVAC Contractors' printout. H found significant differences in the data used to calculate the size of the AC and furnaces.

The conversation and communication between my client and the HVAC contractors changed dramatically when he pointed out some errors in the data used by the HVAC contractor. He went on to explain that he was not trying to design the HVAC system, but wanted to make sure they understood all aspects of the house and how many people would be living in it along with their lifestyle.

The HVAC contractor looked over my clients data, accepted some poinits and explained why others were not applicable. After several discussions the AC and heating units are being modified. The Architect's earlier calculations were entirely off. While the HVAC contractor's calculation had to be adjusted. The results were that two of the three unit sizes were reduced and the one on the main floor was increased.

What you want your HVAC contractor to do
Here are some steps that a good conractor should take to size your HVAC system:

  1. On replacing your HVAC system, make sure they use Manual J to size the units (either manual or software).
  2. On a brand new HVAC system for a remodeling project, have them use Manual D for the ducts.
  3. Obain building plans and measure walls, ceilings, floor space, and windows to determine the room volumes,
  4. Assess the R-value of the home's insulation, windows, and building materials.
  5. A close estimate of the building's air leakage using a blower door test is the best way to measure air leakage.
  6. The placement of supply and return registers should be appropriate for the system type and size.
  7. The orientation of the house also affects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows.
  8. Make sure the contractor uses the correct design for the outdoor temperature and humidity in your area. Using a higher summer design temperature results in oversizing air conditioners.
  9. When the contractors are finished, get a copy of their Manual J calculations, assumptions, and the computer printout or finished worksheet. his is your only proof that they did their work and compare it to your own calculations.

Also don't forget to get a written warranty on equipment and workmanship and see if you can hold the final payment until you are satisfied with the new system.

We are not associated with any of the organizations liste in this article and are not being compensated for mentioning them.