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Always remember this basic rule and design an air distribution system accordingly, Hot Air Rises and Cold Air Falls. Sounds simple enough. So don't think that distributing air from the floor is going to provide proper air conditioning or hot air distribution from a ceiling is going to result in a satisfactory comfort level.

The reason most HVAC systems fail prematurely is because the duct system or air distribution system is improperly designed and/or installed incorrectly. The reason is due to contractor ignorance or builder pressure to install as cheap a system as possible. Then also enters the customer into the equation who doesn't want too many of those ugly diffusers in the walls. They may interfere with physical décor or locations are sacrificed due to other items taking precedence such as different wall finishes or lighting. It seems as though every other trade takes precedence over the HVAC duct system. An example is the electrician who ran the wiring where the duct system is supposed to go or the plumber who ran pipes in the way of the duct system. All these sacrifices end up in sacrifices to the comfort level of the house.

You have to ask yourself if that great home is going to be so great if you're not comfortable??? A lot of times people just accept the way the system performs and come to accept the faults. After all when the dry wall is up and the house is finished, it's very unlikely any homeowner wants to have the walls ripped open and house torn apart to make duct changes. Worse yet is that when a house in uncomfortable the majority of the time the equipment is being abused as well. Improper air flow over a furnace or conditioner and especially heat pumps causes excessive wear and abuse to the system.

Like people a duct system has to be able to breathe properly to function properly. A duct system that is undersized is similar to a person trying to run with only one lung. Not only can you not run as well but running is also causing more abuse to the body. You breathe heavier and faster and more than twice as much.  

In a duct system as in a tree there are trunks, and branches. There are take offs and diffusers. Further there are plenums and the main unit which blows the air which is the furnace or air handler.

Every heating and/or air conditioning system has a requirement of so much air it needs to distribute based on the requirements of the house. The heating and/or air conditioning requirements are based on the heating and/or air conditioning load. The load is determined by the type of construction of the house, insulation, windows and size of the house as well as what outside temperatures are selected and the inside conditions selected. For example we will design a house in York, Pennsylvania at 95 degree outside temperature with a 74 degree wet bulb temperature and a winter design temperature of 5 degrees with an indoor temperature of 74 degrees at 50% relative humidity.

By now you may be thinking trees, trunks, design temperatures and branches; I just want to put in a heating and air conditioning system, not a tree. Actually you are building and designing a tree of air distribution. But the branches and trunks have to go where they are needed, not just where they'll fit.

So in order of the procession of designing a duct system the size of the equipment needs to be determined. Then the materials to be used need to be selected and then we need to design the system according to the materials used. Not knowing what materials can be used will not provide us with an idea of the many different ways a duct system can be installed as well as what materials work in the best applications and locations.

In this article we will focus more on the fiberglass duct system which is a premium duct system in comparison to sheet metal. Although you may not be familiar with the fiberglass duct systems, you will find that it is the best material and why it is so. Not being familiar with this material as a layman comes as no surprise and don't think that fiberglass is a new material since you're not familiar with it.

Fiberglass duct systems are superior to sheet metal because it provides a far superior tightness in comparison to sheet metal. A fiberglass duct system made form fiberduct is quiet, energy efficient, tight sealing, more consistent air temperature and no condensation. For these reasons and more a fiberglass duct system is the premium of all duct systems.

All standard air conditioners move 400 cubic feet of air per minute per ton or 12,000 btus of air conditioning. One ton equals 12,000 btus. One btu is the measurement of heat. It takes one btu to raise the temperature of water in degree fahrenheit. Knowing this we then first need to determine how the duct system is going to be sized.

Almost 95% of the time you will size the duct system to the requirements of the air conditioning. Exceptions would be where the heating requirements are significantly greater than the air conditioning requirements. This is what we call an unbalanced system.

In the far northern regions of the United States and Canada they need a disproportionate amount of heat in comparison to the air conditioning. So they will size their duct requirements based on the heat required rather than the air conditioning. Here go those exceptions.

But for now our focus is going to be on the system that is most common. That's the system where we design the duct system for the air conditioning and not the heating. Then I hear a question pop up.

Why would I size my duct system for air conditioning if I'm only putting in a furnace????
Because some day down the road you or the next person who might buy your house will want air conditioning so why put in a smaller duct system only to accommodate the heating. The heating system requires less size because hot air is lighter and it doesn't require as much duct to supply hot air as it does heavier colder air.

The next step we need to do is make a drawing of the house. A calculator, tape measure and lots of paper and PENCIL and lots of ERASERS will work just fine. Make a layout of the floor plan of your house. Lay out each room with the dimensions of each room on the paper. If you want to do a more professional looking drawing, I recommend Punch Home Professional Suite or Architect or Home Plan Pro which can be downloaded free from Home Plan Pro. Their link is here: www.homeplanpro.com. It's a very nice program for a trial and it costs you nothing. As long as you can get to a good final design, that's all that matters. However if you want us to review your design I would prefer the layout was on Punch Pro or Home Plan Pro or Cad or a nice detailed drawing that is clear and understandable.

For our example we need a design house to work off of. I've chosen a 2,000 square foot house located in Pennsylvania as an example. The house has double insulated average window exposure and has R13 insulation in the walls and R19 in the roof area. R is the insulation value. So what does it matter whether the house is in Pennsylvania or what the R values are and how many windows and what type they are? When we do a load calculation to determine the amount of Air Conditioning required, the Manual J program used in our software takes a look at the climate data for the area which is different for all parts of the United States. For example in Texas we may design a house for 100 degrees and in Utah we may design a house for 89 degrees and for Pennsylvania we use 94 degrees for the design temperature for air conditioning. We use 8 degrees for the design temperature for the heating.  Well who determined what those design temperatures are and what does it mean?

The temperatures were determined by ASHRAE, the organization that establishes the standards and design and engineering criteria for all HVAC systems.

 It means that the average high temperature based on a certain location based on the weather averages of the last hundred years determines the design temperature.

Also on our climate data it tells us how many hours on an average the weather will be between what temperatures on a typical summer and winter. It also tells us degree days and other important data such as humidity factors to determine what temperature an air conditioning system should be designed for. The same is true for a heating system. So we have a design point for the air conditioning and the heating based on the weather data. Then a well insulated house with double insulated windows is going to require far less heating and air conditioning than a house that is with little or no insulation and single pane windows. When you fill out the HVAC sizing form and return the information, our computer software takes all this in to consideration when selecting your HVAC system. So if you omit or provide incomplete information your results will be equally inaccurate.

In our layout we have a single story house with two bedrooms and 2 bathrooms, a living room and kitchen. Nothing fancy in our model home and you can learn off a more simplified lay out.  First we need to determine square foot for the entire house as well as square foot per room.

In our example house our computer tells us we need 2.95 tons of air conditioning. So we select a 3 ton air conditioner or a 36,000 btu unit.

Wait a minute! If you tell me I need a 2.97 ton system I'm going to be safe and install the next bigger system so when the temperature is over 94 degrees I'm going to be cool.
Typical American Attitude. If a little is good then a whole lot more should be even better!?! Wrong! So let's explain how this works. Our example house air conditioning system is properly sized for this house only when it is 94 degrees. That's right. Below 94 degrees the air conditioning system is oversized and above 94 degrees the air conditioner is undersized.

But we have to use a design point and so the system is selected for that temperature with a 10% safety factor already calculated in for other possible errors or design or installation problems. If you use a larger system than what was computer designed then the air conditioner will cycle less frequently on the 80-93 degree days which is where the majority of time the temperature is in the summer for Pennsylvania. We also have a high humidity level to contend with so the primary objective of an air conditioner in our area is to remove moisture from the air and lower temperature. If you select the next size larger size system which would be the 3-1/2 ton system, the air conditioner will operate substantially less time and provide a cool clammy house for most of the summer. Perfect breeding ground for molds, etc. and very uncomfortable to say the least.

So in air conditioning bigger is not better!!! But what happens when we have those 95 or even 100 degree days? The temperature in the house will naturally climb but the humidity levels will remain around 50%. The temperature may climb to as high as 78 or 82 degrees but the humidity levels will be bearable and that's what you're looking for. We're not designing an air conditioning system for those once in every 50 year summers where the temperature soars above 100 degrees in Pennsylvania and the other 49 years the air conditioner won't remove the humidity and you're uncomfortable. One factor to consider though is that if you live in Texas, Arizona, Nevada, New Mexico, etc. we are dealing with a very different design criteria. You don't have the humidity to contend with as much as the extreme in heat. When an air conditioner operates over 95 degrees fahrenheit, the capacity drops off rapidly and so when the outside temperature is about 110 degrees and we do have some areas that our computer designs for those temperatures, we will select a significantly larger system due to the inefficiency of an air conditioning system in those climates.  

An air conditioning system's rated capacity is for 95 degrees outside ambient temperature. Below that temperature it has more capacity and above it has less. We design HVAC systems for all of the United States and so it becomes very complex to select the right size air conditioning system that's right for your house. The idea that any contractor or an internet seller says this air conditioning system or furnace will heat or cool so many square feet is so absolutely stupid!! When you understand the complexity of sizing a system and the factors that need to be considered such as window types, sizes and location, wall construction, insulation, thermal barriers and climate for the location, what a mistake to state square feet per ton.

I know there will be some contractors reading this and saying I've been in this business forever and that's the method I've used and I've never been wrong yet. Unfortunately your customers were uneducated so they didn't really know what to expect in comfort level. But today's consumers are becoming more educated and they don't want that monster air conditioner and furnace where one size fits all. Also the most important problem for homeowners to be aware of is that over 95% of premature equipment failures are caused by improper or no design, improper selection of the equipment such as oversizing and poor installations.

Yes over 95%. So as a consumer keep in mind that it isn't the brand of equipment that's as important as the installation. A competent contractor can take any major brand of HVAC equipment and market that brand and be successful.  It's not the brand that makes a good contractor. What makes a successful contractor is the ability to select the proper size equipment and the quality of the installation.

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