What size PVC should I use for my radon system?

Short answer: 4″ schedule 40 PVC in most, if not all, homes.

International Standards

In radon mitigation, there are several acceptable radon system materials.  You can dig into the details at standards.aarst.org.  These consensus standards are the result of thousands of volunteer hours from real-life professional tradespeople over the last four decades.  These standards were written using empirical data from thousands of real-life mitigations.  They are the most complete radon standards available.  I’m not going to dig into the details in this article, moreso provide an overview of common acceptable materials.

Acceptable system materials

Exhaust

3″ or 4″ schedule 40 PVC are the most commonly used and accepted radon venting pipes.  Metal, most commonly steel, downspout is also used and accepted.  The reason you need schedule 40 PVC is for UV resistance and cold-weather performance.  If you see a thinner pipe, dwv, or green pipe on your system, then you know it was installed by an untrained professional.  Schedule 20 PVC will crack easily in cold weather or after a few years of UV exposure.  These materials end up costing you much more in the long run as they will break almost immediately.  If the break is not severe you risk leaking radon back into your home.  If you don’t test and maintain your system, this could go unnoticed for years.

Intake

For sub-membrane, sub-slab, and sub-soil collection piping, it is acceptable to use: corrugated piping, perforated piping, geotextile radon mat, and other drain-tile loops.  These materials are used to collect soil gasses and should never be used on the exhaust end of a system as they will leak and render your system useless.

2″ PVC is unacceptable

2″ PVC or smaller is never an acceptable intake or exhaust material.  It’s just too small.  If your house has a radon problem, then 2″ PVC will never move enough air to solve it.  3″ is the smallest diameter you can go.

Unfortunately, we keep seeing builders, plumbers, and other untrained radon contractors use this small pipe and ruin a system.

Why size matters: double your airflow

PVC size, radon fan size, and soil conditions dictate your airflow.  You can read this article for more detail.  In radon, bigger pipe is better.  The larger diameter pipe that you use: the more air you will move and the less noisy it will be*.  So for most homes, 4″ PVC should be your choice.

The reason you sometimes see 3″ PVC is two-fold.  One is that when radon mitigation was in its infancy, contractors were trying everything and just kinda seeing what worked.  Read more on the history of our industry here.  Professionals in Pennsylvania found that 3″ PVC worked most of the time, so that became a standard in the Eastern US via word-of-mouth amongst pros.

Over time, contractors began experimenting with larger PVC sizes and found that 4″ PVC performed better in nearly all regards.  Now trade schools across the world teach 4″ PVC as a standard.

If you use some basic math (πr²) you’ll find that the area of 3″ PVC is 7.07 square inches while the area of 4″ PVC is 12.57 square inches.  That’s nearly twice the area to move air with.  In radon, we share that often.  4″ PVC is about twice the airflow as 3″ PVC.

*Most of the time.  Extenuating circumstances can sometimes throw a wrench in this rule of thumb, but more often than not, bigger PVC will be better and quieter.

Pros and Cons

3″ PVC is able to hide in wall chases more easily.  That is it’s primary advantage.  3″ PVC moves less air and creates more resistance.  In radon, resistance is analogous to noise, so 3″ is often the noisier PVC size.  3″ PVC is lighter than 4″ and when you’re up on a ladder, weight is a consideration.

4″ PVC is more likely to solve your radon problem as it moves more air.  While an extreme amount of airflow can be noisy, in most instances, higher airflow, lower resistance means a quieter system.  If your system is moving a lot of air and is very noisy, then you probably used a fan far larger than you needed.  Read about noise in detail here.

Metal gutter material has a cosmetic appeal, as it looks like it belongs on the side of your home.  Steel is recommended as it holds up to weathering better than aluminum.  Aluminum is more likely to dent and ding during install and during hail storms.  3″ PVC has the equivalent airflow of 3″ x 4″ gutter downspout, while 4″ PVC has the equivalent airflow of 5″ x 4″ downspout.  5″ x 4″ downspout is extremely expensive and I have never actually seen it used in nearly a decade in this industry.

Metal gutter is also extremely light, which makes it easier to install on a one, or two-person, team on a ladder.  4″ schedule 40 PVC is the heaviest, so take that into consideration when you’re running it up the side of your home.

Again, using anything other than schedule 40 PVC or metal can result in brittle, easily damaged materials outside of the home.  Never use schedule 20 or other DWV sewer pipe as it will need replacement almost immediately

Can I use multiple pipe sizes?

You can, but it’s not practical.  Imagine you’re drinking from a straw.  Now pinch that straw to half it’s size and continue to drink.  Despite the straw being the same diameter at the intake (in you soda) and exhaust (in your mouth), you’re limited to the pinched diameter flow in the middle.  For that reason, once you “choke down” to a smaller pipe diameter, your stuck with that size.

Other seldom-used, but acceptable materials

While rare, here is a list of other materials that have been used successfully.  Always check with a certified radon pro before using or specifying these materials.

• Brass or copper drain pipe
• Other metal piping
• 6″ or larger schedule 40 PVC (more common for large footprint RRNC jobs like schools and warehouses)