In part 1 of this duct design series, I discussed the basic physics of moving air in ducts. Now we’re going to take that and use it to figure out how to make all the parts work together properly. First we choose a blower that will give us the total air flow we need. Then we design a duct system that will deliver the proper amount of air to each room. To do that, we need to take the concept of pressure drops and apply it to blowers and ducts.
More about pressure drops
We know from part 1 of this series that there will be pressure drops all through the duct system. Whenever air encounters a filter, coil, heat exchanger (if there’s a furnace), registers, grilles, balancing dampers, and the ducts themselves, it loses pressure. So let’s sort this out.
The diagram below shows the components of our system. The AHU is the air handler (or handling) unit. That’s where the blower is. Air inside the home gets pulled back to the AHU through the return ducts. The air gets conditioned inside the AHU and then sent back into the home through the supply ducts.
When talking about pressures here, we’re not talking about absolute pressure. We’re talking about relative pressure. Our reference when we talk about pressures is the pressure inside the conditioned space. That’s our zero.
On the return side of the blower, the pressure will be negative. As the air moves from the room, into the return grille, and down to the AHU, the pressure gets more and more negative relative to the room. On the supply side, the pressure is positive. As air moves from the AHU through the supply ducts and out into the rooms, the pressure gets more and more positive.
The maximum positive and negative pressures occur at the air handler. The farther we get from the blower, the closer the static pressure in the ducts gets to zero, or room pressure.
To get a certain amount of air flow, a blower needs to operate against a certain pressure and at a certain blower speed setting. Here’s a table from one unit.
The blower speed is set by moving wires to different taps. In this case, there are 5 of them. The row of numbers across the top is the total external static pressure (TESP) the AHU is rated for. That’s the pressure change across the AHU when pushing and pulling air through the ducts.
You generally want to design a system to operate on medium speed (tap 3 in the table above). That way you have some room for adjustment when you commission the system. Also, most systems are rated to operate at an total external static pressure of 0.50 inches of water column (iwc). For the system above, those parameters yield an air flow of 899 cfm. If that’s the number you need, you just have to make sure you design your system to operate at 0.5 iwc.
So, from the return (most negative) side of the AHU to the supply (most positive), we want a total pressure change of no more than 0.5 iwc. (That’s the typical number. Some air handlers are rated higher. Some are rated lower.) That’s the total pressure change across the AHU. The actual pressure in the system will depend on the ducts and other components. As long as we’re at or below 0.5 iwc in this case, we’ll get good air flow.
Notice I said pressure change here, not pressure drop. The blower causes a pressure rise. It’s the force behind the air flow so from the negative side (return ducts) to the positive side (supply ducts), the pressure rises.
Got all that?
Finding the available static pressure (ASP)
What happens next is splitting up the two kinds of pressure drops in the duct system. First, we want all the external pressure drops of the components that are not ducts or fittings. Those things have to go into the duct system and generally have known pressure drops. We subtract them from the total external static pressure number (typically 0.5 iwc). What’s left is the available static pressure (ASP) for the ducts and fittings.
Here’s a screenshot from the software we use (RightSuite Universal).
At the top is the total external static pressure. That gets entered automatically after you select equipment, but you can override the numbers here. In the table above, I’ve got different numbers for heating and cooling just to illustrate the effect on the bottom line, but usually those numbers are the same.
Next, you enter all the external pressure drops. The coil and heat exchanger are zero here because the coil is already included in the total external static pressure because it’s inside the AHU and there is no heat exchanger since it’s a heat pump. With a furnace, you’ll have a coil that’s outside the AHU and will need to add it. I don’t think we’ve ever had a project where the heat exchanger was external and needed to be added here.
The other numbers shown there are pretty standard numbers, but you want to enter the actual numbers if you have them. For example, if you’re using wooden grilles, the pressure drops will be significantly higher. But please, please…don’t use wooden grilles! They will make it very difficult to get good air flow.
Your duct budget
Once you’ve entered your external static pressure rating and all your external pressure drops, what’s left after subtracting the drops from the rated pressure is the available static pressure. That’s how much you have left to “spend” on your duct system.
To summarize where we’re at now:
- The blower creates a pressure rise to move air through the ducts.
- It’s rated for a certain amount of air flow at a specific total external static pressure.
- The ducts, fittings, and other components cause pressure drops.
- Subtracting the pressure drops for all the things that aren’t ducts or fittings from the total external static pressure yields the available static pressure.
- The available static pressure is the pressure drop budget you have to work with when designing the ducts.
We now go to the next step and design a duct system that will have a pressure drop of no more than the available static pressure. To do that, we size ducts and choose fittings using something called equivalent length. And that’s the subject of the next article in this series.
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