Radon mitigation system not working, or readings still high

First: was the test itself valid?

More re-measurements fail on conditions than on hardware. A short-term test requires closed-building conditions — exterior windows and doors closed except for normal entry and exit, for twelve hours before the device starts and for the whole measurement — with whole-house fans and window air conditioners off and permanent heating and cooling running normally.

It also requires the right placement: lowest lived-in level, at least twenty inches above the floor, several feet in from exterior walls, away from vents, fireplaces, drafts, and the sump. A device moved to a different room between the before and after tests produces two numbers that cannot be compared.

If any of that was not true, re-run the test properly before concluding anything about the system. That is a ten-dollar experiment against a several-hundred-dollar one.

• Was the house closed for twelve hours before the test started?
• Was the device on the lowest lived-in level, in the same place as the original test?
• Were whole-house fans, attic fans, or window units running?
• Was a window left open by someone else in the household?
• Did the test run its full stated duration?

Second: is the fan actually moving air?

Radon fans fail quietly. There is no alarm, no error code, and no obvious symptom in the house — which is exactly why the system has a manometer on the pipe. That is the U-shaped or inclined gauge with coloured fluid, usually mounted at eye level near the riser.

When the system is pulling, the two fluid columns sit at different heights. When they are level, the system is not developing suction: the fan has failed, a pipe has been disconnected, or something is blocking the run. Note the reading and compare it to the mark the installer made at commissioning, if there is one.

You can also stand near the fan. A working unit is a continuous low hum. Silence from a fan that used to be audible is the simplest diagnosis in this entire subject.

What not to do yourself

Do not open the fan housing, rewire the circuit, work on the roof discharge, or climb into an attic to service a unit. Radon fans run on a dedicated circuit and are usually mounted where working on them safely requires the right access and the right tools.

Checking a gauge, listening for a hum, and confirming the breaker has not tripped are homeowner tasks. Everything past that belongs to the person who installed it or to another certified mitigation contractor.

Third: the design may not reach the whole slab

If the test was valid and the fan is running, the remaining explanation is that the suction field is not covering everything it needs to. This is a real and reasonably common outcome, particularly where sub-slab material is inconsistent, where a house has an addition on its own slab, or where a crawl space was left out of the original scope.

The professional answer is diagnostics — measuring pressure at test holes across the slab to see how far the field actually extends — followed by an additional suction point, a fan better matched to the conditions, or sealing a pathway that is bleeding the suction away. Common culprits are an open or poorly sealed sump lid, an untrapped floor drain, an unsealed floor-wall joint, and an exposed soil section behind a finished wall.

Note that this is a redesign conversation, not a repair conversation. It is why the written scope matters so much: a quote that named a remedy up front turns this into a procedure, and a quote that did not turns it into a negotiation with a contractor who has already been paid.

What a reasonable contractor response looks like

A contractor worth keeping treats an elevated post-mitigation result as unfinished scope. They come back, run diagnostics rather than guessing, explain what the measurements showed, and propose a specific change with a specific price — or absorb it if their original scope promised a result.

A contractor to stop working with tells you the number is fine, blames the weather without measuring anything, suggests you simply retest until you get a lower reading, or stops answering. In that situation, a second certified contractor performing an independent diagnostic is money well spent, and their report is what you need if the first bill becomes a dispute.

When the answer is "the house changed"

Systems that worked for years can drift. A finished basement alters the air balance and often puts drywall over the very joints that were sealed. New drain tile or a new sump opens a pathway. Replacement windows and air sealing reduce unintentional ventilation and can strengthen the stack effect. A furnace swap changes the pressure relationship between the lowest level and the soil beneath it.

If a previously good system now reads high and nothing is wrong with the fan, ask what has changed about the house since the last verified test. That question resolves this more often than any inspection of the equipment does.