Despite looking similar, not all dryers and subsequently not all dryer vents are the same. However, there are a number of specifications, including those pertaining to size, that dryer vents need to meet when installed.
Read on to find out what these specifications are and what impact it could have when installing your new dryer.
In the US, where the IRC is adopted as the main residential building code, dryer vents are required to be 4". However, not all conform to this. There are risks linked to using ducts that are too large or too small for your dryer’s blower motor, so it’s best to stick to the manufacturer-recommended size.
US Conforms to IRC
The International Residential Code (IRC), is used to ensure that appliances like your dryer are installed as safely as possible. The IRC specifies the dryer vent diameter and regulates its length and is, therefore, an essential document, along with your dryer manual, to consult when installing your dryer.
Although the IRC is applicable throughout the US, local guidelines may differ. It is, therefore, important that you also consult your local building codes to ensure that your dryer is installed according to the regulations in your state.
If your dryer does not conform to your local IRC, then you may be looking at a fine or even having to uninstall and reinstall your dryer to fit in with safety standards.
Dryer Vents Should Have a 4″ Diameter
Section M1502.4.1 of the IRC says that exhaust ducts must be 4″ (102 mm) in diameter.
The diameter of a duct influences the effectiveness of the dryer’s blower, which would have been designed for use with a code-compliant duct, and the ability to properly vent the exhaust outside. In addition, the hose attachment for most dryers should also be designed to match the 4″-diameter hose.
However, just because it should be 4″ in diameter doesn’t mean that it is. So, I cannot say that all dryer vents are 4″; they just should be.
What Happens if Smaller Ducts Are Used?
If smaller pipes are attached to the dryer by way of an adapter, it means higher pressure. In a duct, you have velocity pressure (air movement) and static pressure (resistance to air movement).
Reducing the duct diameter increases the static pressure, and the blower motor has to work harder to overcome this and generate enough velocity pressure to move the air. This reduces the effectiveness of venting, increases drying time, creates more heat in the appliance, and reduces the dryer’s lifespan.
In addition, a pipe with a smaller diameter can more easily become clogged with dryer lint. Lint is flammable, and when it accumulates in the duct, it reduces the dryer’s ability to expel heat. This can cause clothes to burn or can even lead to a fire in the dryer itself.
What Happens if Larger Ducts Are Used?
Now that you know a smaller duct inhibits proper airflow, you can’t swing the other way, thinking that a larger duct will make your dryer super-efficient. The blower motor is still only designed to push air through a 4″ hose.
If the hose was wider, then yes, the duct’s static pressure would decrease, but so would the velocity pressure, reducing the effectiveness of ventilation.
Think about it like this, if you blow air through a drinking straw, you can feel your breath if you hold your hand at the other end of the straw. If you were to have the same length of PVC plumbing pipe and you were to blow with the same force, you would not feel the air nearly so strongly.
Dryer Hose Won’t Fit Over Wall Vent: What to Do
Once again, just because the ducts and connectors should be 4″ doesn’t mean that they are and you may be dealing with a dryer hose that won’t fit over the dryer vent. This often happens when trying to fit an older hose in a new dryer or a new hose in an older dryer.
The only real solution to this, despite what you may read out there, is to replace the hose.
If your hose is slightly too big for the vent, you may be able to try one of the following:
- Flexible hose: clamp it around the metal vent using a dryer vent clamp (amazon link).
- Non-flexible hose: use silicone caulk (amazon link) to fill in the gap between the vent and the dryer.
If the flexible hose is too short, you can lengthen it by stretching out the pleats. But, you are not allowed to join two pipes together to make the transition duct longer as this contravenes the “single length” requirement set out in the IRC.
Please note that when flexible hose is mentioned, this is only referring to the transition ducts. The rest of the dryer ductwork has to be rigid, smooth metal.
If your dryer hose is too small for your vent, replacing the hose is the only option you have.
Dryer hoses are not built for expansion, so it may be often more difficult to manoeuver, and trying to open the hose with your fingers may cause more damage than anything else. It is often safer and easier to buy a hose that is compatible with your new dryer.
Allowable Length of Dryer Ducts
Section M1502.4.6 says that:
“The maximum developed length of a clothes dryer exhaust duct shall not exceed 35 feet from the dryer location to the wall or roof termination.”
This excludes the transition duct, but the transition duct can only be up to 8 feet long (Section 1502.4.3).
For every bend or elbow added, the maximum length of the duct is reduced by the dimensions found in Table M1502.4.6.1. This is because these turns can restrict airflow and, ultimately, be the cause of dryer fires.
A couple of exceptions exist:
1. If the manufacturer’s recommendation allows for a longer exhaust vent, it will most likely be accepted despite the restriction of the IRC.
2. The IRC will allow for larger bends to be installed as long as it complies with performance engineering calculations under the ASHRAE Fundamentals Handbook, however, this is not recommended.