Designing an HVAC System
What Size Air Conditioner Do You Need?
When designing an HVAC system, the first question that needs to be answered is how many tons of air conditioning does the house need? Of course other questions must be answered, but first, you need to determine the optimal size of the A/C unit.
In this article we'll discuss HVAC system design in the context of building a new home. However, the principles of the information here are applicable to older homes, too. The major difference is that you will not likely change all of your ductwork when replacing an old system. In many cases, you will adapt to the existing ductwork for the new unit, and though this can limit the performance of your new system in some cases, it's just not sensible to rip apart your entire home to update your ductwork to fit a modern system, particularly in a two story home.
Yes, the unfortunate truth is that in older homes, the ductwork is often basic and meant only to direct air to different rooms. Older homes that used gravity heat didn't have to concern themselves with air conditioning or static pressures because, well, they hadn't been invented or implemented yet.
How to Choose the Right-Sized Air Conditioner
There are quite a few questions that need to be answered to figure this out:
- How many square feet is the home?
- How many windows and doors are there?
- What types of materials were used to build the home? (Insulation, windows, etc.) NOTE: What your home is made of has a huge impact on how well your HVAC system works. It's a "Don't blame the car if the tires are bad" sort of thing.
It used to be that a "manual J" (mathematical load calculation) would be done to find out how many tons of air you'll need, but all you need is a rough calculation. Let's say I did a manual J and decided you need 3.286 tons of air, but they don't make a 3.286 ton unit, so you get 3 tons. In other words, a rough estimate would have been fine.
Square footage has become the biggest question and with the years of home building experience under my belt, I know what size you need. A newly built home with decent insulating materials will need about 1 ton of air conditioning per 600 square feet. This will go up and down a bit based on how well insulated or sealed your home is.
We start here with the A/C because cooling is the hardest part of controlling our home's environment and is much more finicky in how it works compared to other parts of the system. Cold air is also harder to move around and will also become an important factor in how we size our duct.
One last note on our air conditioner before we move on to our furnace is that the closer we locate our air conditioning condenser (outdoor unit) in proximity to our furnace, the better it will work. This is also true of its proximity to our electrical panel, if possible. We'd like to have the A/C unit close to both the furnace and the electric panel, but if you can't have both, priority to the furnace is most important.
Rule of Thumb When Sizing an Air Conditioner
When the call is borderline on which size unit to use, always go with the smaller. A larger unit will cool the home faster but will not dehumidify the home properly. Dehumidification is very important when talking about air conditioning performance.
Choosing a Furnace Size
The furnace size (or BTU) will be chosen based on the same information you used to choose your A/C requirements, but considering the fan potential of the furnace is also very important. The fan must be able to push enough air to properly assist the air conditioner. In this case, the rule of thumb is that 400 CFM (cubic feet per minute) is needed per ton of air conditioning.
We discussed locating your A/C unit as close as possible to your furnace. Also try to locate your furnace centrally in your home. This will not only help provide you with more options for A/C location but more evenly disperse the air supplied by the furnace. This, in turn, will help keep the home at a more even temperature throughout.
One last item to consider with furnace location is where you will be venting the chimney. With lower efficiency units, you will want to be near the "chase" or framed space where the chimney will run up through the home. Higher efficiency units will vent out of the side of the home and therefore will give you less limitations on furnace location.
Where to Place Your Thermostat
Thermostat placement doesn't require nearly as much thought as the rest of your system. Here are a few rules to follow when choosing this location:
- Locate the stat centrally in the home if possible.
- Do not place it on a wall where it will be consistently exposed to sunlight.
- Do not place it in a closed room.
- Place the thermostat approximately 5 feet off the floor.
- Don't place it directly above a supply air vent.
When installing thermostat wire for a heating and cooling system, you will need at least a low voltage 4 wire. You need at least 4 wires to operate the system. Personally, I suggest running 8 wire since this is a rather permanent installation and with the advancement of HVAC technology, the extra wires can potential be used to control "add-ons" you may choose to use later.
Ductwork Is Very Important to HVAC System Performance
Proper duct sizing has much to do with the performance of your HVAC system. Often, when people complain of having colder rooms or areas of their home, it is a result of improper duct sizing or register placement.
A duct-u-lator is often used in sizing duct these days though there is what is called a manual "D" formula for doing this as well. In order to size duct properly, we must start with the plenum, the duct work installed directly on top of the furnace and coil. This must be sized to handle the total amount of air that is produced by the furnace. From there, we decide how much is going in one direction and how much is going in the other. The first 2 pieces of horizontal supply duct will now be sized to handle that amount of airflow. As supply runs are taken off of this main trunk line, we will deduct that amount of air that is leaving the trunk and down size the trunk line to carry the remainder. This shrinking of the duct as it goes down the line helps to maintain the proper amount of air pressure and velocity until we finally reach the end of the duct where it will be sized to hold whatever air is left.
The return duct works the same however the last piece of duct is the one located near the furnace since this is where the air finally returns to the unit. This last piece is often referred to as the collector and must be able to handle the entire amount of air returning to the unit. The duct again will shrink as it goes away from the unit but in this case it is because it's collecting more and more air as it returns to the unit as opposed to the supply which is giving up air as it leaves the unit.
NOTE: Most supply runs will be ran out to the register in 6" piping. Some larger rooms may use 7" piping. As for return air runs, it's common that the entire joist space coming from the return register to the duct is sealed off to carry as much air as possible. "You can never have too much return air" some may say and it does help the unit to run quieter and with more ease.
If your ductwork makes a "BANG!" noise when the furnace comes on, it is often because the return duct is too small and "oil-cans". Meaning, it's sucked in and makes a bang noise as if someone has whacked the duct work. Adding support to the duct where it's oil-canning is a simple fix however adding more return air registers is better for the furnace fan.
Example of Duct Sizing Mathematics
Let's say you have a 2,000 (5 tons) CFM fan motor and the system has 10 heat runs going each way off the unit. You will need a 1,000 CFM to go each way for even flow and that 1,000 CFM will mean each run will get 100 CFM to it. Again, this is an example but let's go with 20" x 8" duct each way. In the first 8ft. of duct you take off 4 - 6" supply runs each supplying 100 CFM. Now, instead of needing your duct to carry 1,000 CFM, you only need it to carry 700 so we shrink it down to the proper size to maintain airflow. You would keep doing this as you go down the line until all the supply runs have been fed. The same train of thought would apply to return air ducting however you'd be looking to draw around 200 CFM per return. These are nice easy round numbers but the truth is if you only had 20 supply runs, your 2,000 CFM fan is likely a bit too big and wouldn't be working as well as it could.
Where to Locate Supply and Return Registers
Choosing the right location for your registers is important to how well your home evenly heats and cools. It is also important to your energy bills.
Putting supply registers in the middle of a room isn't a good idea though you'll find them there often in older homes. Now in the defense of our predecessors, they were working with gravity heat as opposed to forced air but this design allows the cold or heat to penetrate too far into our homes before being confronted. Registers are to be strategically placed around the perimeter of the rooms in our homes. Under windows, beside doors, etc...this is to stop the elements from getting much past our exterior walls and to attack at the coldest points of those exterior walls. Another added benefit of this placement is that it is less likely to be covered up by furniture and such.
As for our return air registers, they are better placed on the interior walls as far as possible from our supplies to help draw the supplied air through the room. It is again important to try to place them where they will not be covered up. As I mentioned above, you can never really have too much return air and when we choke these off, we starve our furnace fan for air. Think of your furnace somewhat like a lung only that it can inhale and exhale at the same time. We cannot do that and your fan cannot do them separately. Try to exhale for an extended amount of time without bringing any air back in...doesn't work out to well does it? This is what the fan is feeling if it doesn't draw air in and in time, it will overheat, work too hard, and ultimately it will break down.
NOTE: This is also where our furnace filter comes in. You can have all the return air in the world but if your filter is clogged, it means nothing because the fan cannot draw air from them through a clogged filter.
Are You Ready to Design Your Home's HVAC System?
Okay, I highly doubt that anyone is going to sit down with the wife and kids to work out a manual "J" or run the numbers for their exact BTU needs, but I do hope that if you're considering building a home or having a new system put into your existing home, this information will come in handy. Knowledge is power and if you know even the slightest bit about the principles of HVAC, you will feel more secure in the decisions you make and knowing that your eyes are wool free when dealing with contractors.