Why HVAC Ductwork Design Is Just as Important as a New Unit

Have you ever tried to drink a coke through a tiny coffee straw? Don’t. It’s a horribly inefficient and tiring way to quench your thirst.

But that’s what you ask your air conditioning unit to do when you hook it up to old, poorly designed ductwork.

Do you have that one room in the house that’s always hotter (or cooler) than the rest? Does condensation form on your AC vents? Did you install a new, “high efficiency” AC unit but are still paying high electric bills?

Having your AC system not live up to your expectations is rough. It’s like you don’t even use one room of the house because it’s hot all the time! The air feels sticky from the humidity that the condensation on the air vents causes. And you don’t understand when these “savings” from your fancy new AC unit should start piling up. As far as you’ve seen, the bill hasn’t decreased at all.

The cause of those symptoms is high static pressure. 

What Is a Normal Static Pressure in Ductwork?

The normal static pressure in residential ductwork should be no greater than .5 inch of wc (achrnews.com).

Static Pressure – simply put – indicates how well your AC ducts work. I use a professional gauge called a Magna-Helic to take readings in your ducts. These readings tell me if you have normal or high static pressure. 

Getting the static pressures right may include replacing AC ductwork or – if the problem is isolated – an AC ductwork repair. When your AC contractor properly designs your ductwork, you can have an evenly cooled house that lowers your electric bill.

Replacing your AC old ductwork with ductwork that we design specifically for your space ensures that your unit can pull in new air easily without adding stress to the outdoor unit. 

Think about when you try to drink something through a tiny coffee straw. You have to work harder and longer to drink the liquid in the cup. But when you use the right size straw, you can drink the same amount of liquid in less time without tiring out your throat.

Now think of your AC unit. Instead of sucking liquid, it’s sucking in air through your home’s ductwork. If you are not using the right size ductwork for your AC unit or if there’s not a clear path for the air to take, your unit has to work harder to provide the same amount of cooling.

And when your unit works harder, it breaks down quicker.

You want your unit to last as long as possible, so don’t stress it out with bad ductwork. 

What Causes High Static Pressure?

If you have any of the problems I’ve already listed, you probably have high static pressure. There are three common mistakes that cause high static pressure in your AC ductwork.

1. The filter is wrong. Maybe you’re using the wrong kind – like those really expensive ones you get from Home Depot. They say it blocks sorts of junk from entering your house. 
   
   Well, sure it blocks the junk. But it also blocks the air flow. So now your unit is working overtime and your house has a high static pressure situation. 
   
   Now Accuclean is specifically designed for this situation. It’s an electrostatic filter that cleans the air coming into your system without blocking the airflow. You can read more about it here.

2. The filter is dirty. Be honest. How often do you change your filter? If it’s been more than a month, you’ve waited too long – even if the filter says it’s a 3 month filter. Unless your contractor designs your system for a 3-6 month filter, you should be changing it every month.

3. The AC ducts are the wrong size. The size of your ductwork depends on the amount of air that your space needs to cool it. If the ductwork is too small, your AC unit works harder. 

4. The AC ductwork design is wrong. Just like tworly straws are harder to drink through, ductwork that doesn’t have a clear path without sharp turns makes it harder for air to move through.

5. The AC ductwork was installed poorly. Ever tried drinking through a straw with a hole? Yeah, it doesn’t work well. The same goes for leaky ductwork except with the added side effect of pulling the dirt from your attic into your AC system.

Yes, that’s gross. But I see it all the time! 

When your AC ducts have holes or gaps from poor installation, the fan that pulls the air through the ducts also pulls the dirt from your attic into the air flow that gets blown into your house. 

If you have dust sticking to the ceiling in front of your air vent, you probably have leaky ducts. 

Also if your installer didn’t hang the flex duct properly – without any sharp turns – then you have a high static situation.

How Do you Design HVAC Ducting to Fix High Static Pressure?

Proper AC ductwork design calls for imputing specific variables for various calculations. I don’t have time to do all the calculations myself. That’s what computer software is for. But one main calculation I make before I design HVAC ducting is a load calculation.

The load calc. gives me the amount of air the space needs to cool it. I take measurements of the rooms of the house, the number of windows, roof type, roof ventilation type and a few other things. From there I plug in the variables and the answer gives me the volume (or amount of) air I need to cool the space.

Then I determine what velocity (speed) of air I need to properly circulate air. The velocity of the air is important because it makes sure the AC system cools the rooms evenly and keeps condensation from forming on your vents.

Finally, I draw out the paths the ductwork should take. Air needs a clear path with no obstructions to flow properly. Otherwise you get turbulence – that’s when an obstruction, like a sharp turn or dead end, causes the air to turn back on itself.  

But you don’t always have to redesign your whole house to fix high static pressure. If there’s a specific area that’s causing the problem, I can either repair that ductwork or redesign that area without tearing everything out.

For example, houses built in the 90s tend to lack return vents. So I can go back through and add returns to the existing ductwork without redoing the whole house’s ductwork.

But how do you know when to repair and when to replace your AC ductwork?

Replacing vs Repairing AC Ductwork

Obviously the immediate cost to replace AC ductwork is more than to repair an isolated place. But the cases where it’s better to repair than replace are not common. 

Repair your ductwork if you have…

➜ Leaky ducts that I can easily seal. 

➜ An isolated area of the ductwork that I can work on to fix the high static pressure

Generally, if you’re getting hot and cold areas of the house, condensation, or a new AC unit that never shuts off, you need more than ductwork repair.

 Replace your ducts if…

➜ The ducts are literally falling apart

➜ Mold or water damaged the ducts beyond repair

➜ There’s a design flaw in the way you ductwork currently runs

AC Ducting Design Is Just as Important as a New AC Unit

When your AC unit can’t breathe freely, your home and comfort suffer. You get humidity issues from the condensation that forms on your vents. You have high electricity bills and an overworked AC unit that is more likely to break down.

Your AC unit can evenly cool your home without jacking up your electric bill. So let’s check your ductwork before the summer heat kicks into high gear.

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Written by Elon Jones

MMAC Services, Inc.