Radon Mitigation Fans: What They Do and Who Sizes Them

A fan does not tell you anything. It moves air. If you have not tested yet, a fan is the wrong purchase: start with a test or a monitor and let the number decide whether a system is needed at all.

What the fan is actually doing

A sub-slab depressurisation system is a pipe from under the concrete slab to above the roofline, with a fan in it. The fan holds a slight vacuum under the slab, so soil gas takes the pipe instead of the cracks, the sump and the gaps around service entries. That is the whole mechanism. Everything else in a mitigation system exists to serve it: the suction point, the sealing, the manometer that shows the system is running.

This is why a fan cannot be chosen from a product page alone. It is matched to the resistance of the soil under your slab and the pipe run above it, and those are measured, not guessed.

Where a homeowner's decision actually sits

Three situations bring people to this page, and only two of them end in buying a fan.

A system already exists and the fan has failed. This is the common one. Fans run continuously for years and eventually stop, usually announced by a manometer that has gone level or a radon reading that has climbed back. Replacing like with like is the straightforward case: the model is printed on the housing.

A system exists and the fan is noisy or underpowered. Worth a conversation with whoever installed it before buying anything. A fan that has been working for a decade against a partly blocked pipe is a different problem from a fan that was undersized on day one.

There is no system yet. Then the fan is not the purchase. Sizing, the location of the suction point, sealing the slab and venting above the roofline are an installation job, and the fan is one line on its bill of materials.

The two models here

Both are RadonAway, which is the brand most commonly specified in North American systems, and they differ by duct size rather than by quality.

RP145C takes 4.5 inch inlet and outlet and is the general-purpose fan for a typical residential sub-slab system. If you are replacing a failed fan on an existing pipe, this is the one that most often matches what came out.

GP501 is the 3 inch fan, for smaller systems and tighter duct runs, including some crawl space membrane systems.

We publish no airflow or static pressure figures for either. RadonAway does publish them, and the reason we do not repeat them is that the numbers only mean something against a measured system curve. A figure quoted next to a buy button invites exactly the comparison it cannot support.

What to check before buying a replacement

• The model on the old housing. RadonAway prints it on a plate; matching it is the safest route for a like-for-like swap.
• Duct diameter, measured rather than assumed. 3, 4 and 4.5 inch systems all exist in houses of the same age.
• Where the fan sits. Manufacturers require the fan outside the living space (attic, garage, exterior wall) so that a cracked housing cannot push soil gas into the house. A fan relocated indoors for convenience defeats the design.
• Electrical supply. These run continuously and are usually on their own circuit or switch; an installer wired it that way for a reason.

After the fan is running

A working system is a system somebody watches. The manometer on the pipe shows pressure, not radon, so it tells you the fan is pulling and nothing more. The number that matters still comes from a measurement: a continuous monitor or a fresh test after the system has run for a few days, and what the reading means once you have it.