HVAC Static Pressure

[Chefs Chef]

Hey all, found some interesting info for anyone setting up multiple ducts in their room.

 

Chef

 

 

Read On:

 

Flow of Air

 

Now, the challenge is to get the proper amount of air into each room at the same time. This is where things begin to get complicated. Here is the problem. The furnace blower fan only creates a certain amount of energy at any given instant. This energy is constant. If one room gets too much air (energy), there won't be enough for the remaining rooms. Remember, different amounts of air (energy) are going to each room.

 

To achieve the goal of getting the proper amount of air (energy) into each room at the same time is to have equalized static pressure throughout the entire duct system. Static pressure in a HVAC system is the same thing as blood pressure in your body. It's a time tested principle that works. It is not that difficult to achieve in your HVAC system.

 

The principle is very easy to apply. Remember the initial calculation that indicates the total number of cubic feet per minute of air that your house requires? It is vitally important that you or your HVAC person correctly calculates this number. If you decide to do this calculation, you must obtain heating and air conditioning books which tell you how to do it. It is not that difficult, it just takes time. Your local library probably has the books that you need. Once you have this number, and you have the separate number for each room, you are ready to design your Duct work

[Chefs Chef]

Duct Work Installation

 

Duct work is like any other piping device. You can only get so much air through a duct at a given pressure. It makes sense. You must not forget this concept. We also know that the furnace blower fan can only produce so much energy. This energy translates to pressure, because it is pushing the air out of the furnace. The pressure at the blower will not be equal to the pressure at the air supply register in each room. This is because the total pressure of the blower becomes divided by the number of air supply registers. However, the pressure at each register must be the same regardless of the size of the room or register.

 

Here is how it works. Once the air (energy) leaves the furnace blower it begins to travel down the main supply duct. This duct has a certain cross sectional area. An example would be a duct 8" high by 22" wide. Before the blower turns on, this duct as well as all of the other ducts are full of air. The blower is going to have to push this air out of the way in order to get the hot or cold air into the rooms. It takes energy to push things. Remember, the furnace blower only creates a constant amount of energy at any given moment. When the furnace blower turns on, it begins to push all of the air down the ducts. Every time a branch duct turns off the main duct there is less energy to push the remainder of the air. If the main duct does not reduce in size after several branches are taken off, the remaining air (energy) begins pushing extra air that it shouldn't have to push. If this happens, this wasted energy results in lower energy (air) at the remaining supply air registers. The pressure of the air coming out of these registers will be lower than the pressure at the registers closer to the furnace. As you can see, the main air duct must continue to get smaller and smaller as more branches go off to each room. This is exactly how your blood supply system is designed. The arteries close to your heart are much larger than the arteries in your hands or feet.

 

The downsizing of the main duct depends upon the amount of energy which is left after each branch duct leaves the main duct. Many factors come into play as to when this duct begins its downsizing. However, if for some reason your main duct does not get smaller as branch runs are taken off, ask your HVAC person why. I hope they have a good explanation.

 

Other Air Balancing Tips

 

There are some other important points you should consider when installing a new furnace or replacing an existing one. Try to locate the furnace in the center of the structure. Did you ever stop to wonder why your heart isn't in your head or your feet? By placing the furnace in the middle of the structure, you minimize the distance to the farthest room. This means less air has to be pushed. Make sure that the HVAC person installs a damper control in each branch duct going to each room. This damper control is very similar to a water faucet. You can limit the amount of air going to each room by adjusting this damper. This extra control will also help you to balance the system. Ask to see the calculations that your HVAC person used to enable him or her to properly balance your system. These calculations will show each room and the amount of air each room needs. The air volume will be expressed in cubic feet per minute (CFM). Ask questions so that you fully understand the calculations. Spend a few minutes at the library or local bookstore. Believe me, you can't know enough about your heating and cooling system.

[HashMakerOilProducts]

Sort of but not quite right....your total flow is based on your velocity pressure, this is how a VAV system works and calculates flows. not all ducts downsize on there path, it depends on a lot more factors than just how many rooms there are. we found through various studies that if we ran duct static pressure at 2 inWc we seemed to be able to saisfy most flows. we usually used varible fan speeds to control static pressure and Automatic dampeners connected to velocity pressure sensors to control flow. at the ends of runs on really big buldings we would add fan powered boxes or makup air units to help boost pressure down the duct.