Duct Design 2 — Available Static Pressure

10 Comments Read/write comments

Static pressure measured with a magnehelic pressure gauge

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.

Air handler with blower attached to return and 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.

Blower capacity

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.

Blower speed, static pressure, and air flow table

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).

Static pressure drops and available static pressure

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.


Other articles in the Duct Design series:

The Basic Principles of Duct Design, Part 1

Duct Design 3 — Total Effective Length

Duct Design 4 — Calculating Friction Rate

Duct Design 5 — Sizing the Ducts


Related Articles

The 2 Primary Causes of Reduced Air Flow in Ducts

Don't Kill Your Air Flow with This Flex Duct Disease

The Science of Sag - Flex Duct and Air Flow

The Secret to Moving Air Efficiently through Your Duct System


NOTE: Comments are moderated. Your comment will not appear below until approved.


May 26 2017 - 1:09pm

you wrote: "For example, if you're using wooden grilles, the pressure drops will be significantly higher."

Not a good example. Given your long-standing focus on the dark side of residential HVAC, any mention of wood grilles within these hallowed walls should only be in the context of admonition!

May 26 2017 - 2:30pm

Yes, you're absolutely right, David. I got caught up in the main subject and neglected my duty there. I've just added this to the end of that paragraph: "But please, please...don't use wooden grilles! They will make it very difficult to get good air flow."

May 28 2017 - 8:43am

At some point it may be worth pointing out that like many other budgets, it is worthy not to expend every available asset if avoidable.

In the case of air handlers with smart ECM blowers (SEER 15 and up, typically) the system will either punish or reward for ductwork. If the ductwork (and air filter) are sized or otherwise selected and installed for lowest feasible static, then an ECM blower will reward that happy condition with very quiet and efficient operation.

Conversely, if the ductwork and air filter conspire to gobble up the entire static budget (or more) the ECM blower will punish that typical condition with noisy and inefficient operation, followed, perhaps, by an early demise.

May 28 2017 - 11:15am

One question for consideration: What happens when an existing duct system is employed with a downsized air handler installed as part of a highly effective energy retrofit. For example, when a 5 ton system is downgraded to a 3 ton because of a newly encapsulated attic with continuous rigid foam under the siding. Now a nominal 2000 cfm capable duct system might present much lower low static pressure when driven by a 1200 cfm air handler.

May 28 2017 - 4:23pm

Low static in airways is a non-issue, although dropping from 5 tons to 3 tons would likely require re-balancing (assuming system was ever balanced in the first place!) since fittings have non-linear friction losses.

In my experience, the duct system was likely undersized for the original system, so a smaller system will end up being significantly less oversized than the nominal tonnage would suggest, especially if the new system operates at greater than 400 CFM/ton, which is often beneficial. I've seen situations where the original duct system was still undersized after downsizing by a ton!

In general, when it comes to duct static, lower is better, especially on the return side. As for supply side static, there's one caveat to keep in mind when dropping by such a large percentage. When supply velocity is reduced, register throws will get shorter. Whether or not that becomes an issue depends on how well the house is insulated and sealed. In general, the more efficient the envelope, the less important throw patterns become. I would argue that for homes built to 2012 IECC or better, throw patterns are irrelevant. Ditto for supply register placement.

May 29 2017 - 5:09pm

Great reply, David.

One thing I might add regarding supply register throws with high performing building envelopes is, in context of cooling mode, that a high performing envelope will not provide as much reheating of chilled supply air as a marginal performing envelope. Therefore it's cause to revisit the importance of good throw principles, meaning don't direct air onto occupants, whether the envelope is high performing or not. As building envelope thermal performance is improved, the burden of reheating supply air shifts toward internal heat gains vs. gains through the enclosure. If the internal gains are modest, and register selection and/or placement is poor, comfort complaints will come from the occupants.

May 29 2017 - 7:20pm

Absolutely, Cameron. I should have clarified that my comment (re: register placement being irrelevant in high performance homes) specifically referred to the presumption / rule-of-thumb that supplies should be located at the perimeter, where the load is. In most cases, you only need to get the air into the room.

To your point... when laying out a duct system, one must always avoid feedback between supply and return, as well as avoid directing supply air at seating or standing areas.

May 29 2017 - 2:03pm

Along the lines of what Mr. Parker said, I read that many many AC installations are criticized for having excessive ESP (External Static Pressure). And many AC installations are over-sized by any rational measure. It might be nice to think about making the duct system more appropriate, by reducing AC tonnage and therefore airflow. Perhaps a couple of case studies may be presented.

Jul 2 2017 - 9:00am

Is it necessary to replace the lineset when replacing you A/C unit.

Jul 2 2017 - 1:26pm

Marion wrote: "Is it necessary to replace the lineset when replacing you A/C unit."

That depends on several factors. Generally speaking, if new system is equal or smaller tonnage and uses same refrigerant, and the original lineset is not damaged, then replacement isn't usually necessary. However, each brand/model/size has specific limits on equivalent length and minimum diameter that must be followed.

When changing from R22 to R410 refrigerant, the lineset must be purged of incompatible chemicals from the R-22 oil. Established cleaning procedures must be followed, including post purge testing.

Add new comment