4” or 6” Duct for Bathroom Fan Duct size depends on the bathroom fan capacity. Installing a duct as big or bigger than the fan port is OK. Avoid using adapters. If the fan housing has a 6” connection, always use a 6” duct even if the fan comes with a 4” adapter.

What size duct do I need for bathroom exhaust fan?

When sizing a vent fan, a factor to consider is duct size and length. Most 50 CFM fans will run well with 4-inch round duct. But as you get up into the higher CFM fans, duct size will have to be increased to 5- or 6-inch round duct.

Can I use 3 duct for bathroom fan?





Taking a bathroom fan that is 80 to 120 cfm from a 4 inch outlet to a 3 inch duct is not an issue. It will work just fine and suck the bad air out very well. This will not be any difficulties and you will not notice any problems (as there will be no problems).

What kind of pipe do you use for a bathroom fan?

PVC Schedule 40 Foam core pipe is the ideal pipe for bathroom fan venting. This type of piping is non-toxic, easy to install, and is code compliant. The PVC pipe fittings can be safely cemented and sealed, preventing any water that might collect in the pipe from leaking and causing water damage.

How many CFM can a 4 duct handle?

Reviewing the chart, a 4” pipe will convey 395 CFM at 4500 FPM. This means that a 4” pick-up on a machine will use 395 CFM from the filtering system; or, if you know that a machine will require approximately 400 CFM to remove the waste, you should use a 4” pipe for that purpose.

What is code for venting a bathroom fan?

Section 1203.4. 2.1 of California’s building code requires all bathrooms with a bathtub, shower, spa or similar fixtures to be ventilated by an exhaust fan. The fan must be Energy Star-compliant and vented to the outside.

How long can the vent pipe be for a bathroom exhaust fan?





Generally a 4 inch flex duct can carry a fan exhaust for up to 25 ft. Most codes require that.

Can flex duct be used for bathroom fan?

Flexible duct is perfectly acceptable for a bathroom fan.

Can you connect 2 bathroom vents together?

If you have two bathrooms that are close together and one has an exhaust fan and the other doesn’t, you might be wondering if you can tie a new exhaust duct into the existing one. Well, you can’t! You’d often blow air from one bathroom into the other, and local building inspectors wouldn’t approve it.

Can a bathroom fan be too big?

Duct dimension



A higher capacity bathroom exhaust fan will need a larger duct. You can’t just slap a 300 CFM bath fan on a 4″ duct. Well…you could, but it will be loud, inefficient and it won’t meet the specified CFM. There is no point in connecting a high capacity bathroom fan to a small duct.

What size ductwork do I need?



To calculate the required equipment size, divide the HVAC load for the entire building by 12,000. One ton equals 12,000 BTUs, so if a house or office needs 24,000 BTUs, it will take a 2-ton HVAC unit. If you get an uneven number, such as 2.33 for a 28,000 BTU load capacity, round up to a 2.5-ton unit.

Does duct length affect CFM?

Texas A&M did a study on the effect of flex duct not pulled tight and the results are astounding. In my article on this research, I showed from their results that a 6″ duct moving 110 cfm when pulled tight will move only about 70 cfm with 4% linear (longitudinal) compression and about 40 cfm or less at 15% compression.

How many CFM do I need for a 12×12 room?

One cfm per square foot of floor area is the average air quantity required for a room or entire building.

How do you vent multiple bathroom fans?



Install a Damper For Each Bathroom Fan



A back-draft damper is a simple flap that is installed inline in the system. Normally it prevents cold air from the outside from blowing back to the bathroom. But in this case, it will also prevent one bathroom fan from blowing air into the other bathroom.

Can I use dryer duct for bathroom fan?

A bathroom fan and a dryer should never share the same vent. International building code states that dryer vent exhaust systems must be separate from all other systems and must expel moisture out of the house through a dedicated vent. This is because of the potential hazards associated with improper dryer venting.

Should I use insulated duct for bathroom fan?

Regardless of where or how you’re installing your bathroom fan, the accompanying exhaust fan should always be insulated. If you don’t want to deal with condensation, mold, and water damage, insulation is best to avoid it.

Can you use PVC pipe to vent a bathroom fan?

I’d seriously consider using foam core PVC pipe as the vent pipe for your fan. The fittings can be glued, and any water that might form in the pipe will never leak onto your ceiling or down a wall.

Is it OK to vent a bathroom fan through the soffit?



A bathroom fan can be vented through soffit so that the warm air from the bathroom comes out through it and goes outside. However, It is important to keep in mind it can not be vented into the soffit, as this will lead moisture back into the attic.

Is it better to vent bathroom fan through roof or soffit?

Whether it’s new construction or a remodel, a bathroom vent should always vent through the roof instead of an eave, overhang, or soffit. The point of a bathroom exhaust vent, or any vent really, is to remove hot, moist air from the house.

How do you vent a bathroom with no outside access?

Installing a ceiling vent is probably the most efficient way to ventilate a bathroom with no outside access. A ceiling vent is a unique machine that allows air to escape from the bathroom. In other words, it’s a machine that, like an open window, allows moisture to escape from your bathroom.