Post-mitigation retest

A radon system is a claim until somebody measures it. The fan hums, the gauge shows a pressure difference, and neither of those tells you what is in the air you breathe. The retest is the part of the job that converts an installation into a result, and it is the step most often skipped or quietly dropped when a quote gets sharpened.

Step one: a short confirmation test, 24 hours to a few days after commissioning. This is a screening measurement placed in the same location as the original test. Its job is to catch a problem early, while the crew is still nearby and the invoice is still open. If a system was going to underperform badly, this usually shows it. A good short result here is encouraging. It is not the number you file away.

Step two: the long-term test in the next heating season. Health Canada's measurement guide is clear that the real confirmation is a long-term measurement of at least 91 days, run during the heating season after the system goes in, with the detector in the same spot as the original where possible (Guide for radon measurements in residential dwellings). That is the number that goes in the file, goes to a future buyer, and tells you whether you are under 200 Bq/m3 as an annual average rather than on one mild week in April.

Step three: repeat testing. The same guide asks for a retest roughly every five years on a home with an active system, to confirm it is still doing its job. Test sooner after a fan replacement, an addition, a basement renovation, new windows, or a mechanical change like adding an HRV or replacing a furnace, because all of those alter how air moves through the house.

Who should place the detector. There is a reasonable argument for independence here. The person who installed the system has an interest in the retest passing. Health Canada's guidance favours a professional certified under the Canadian National Radon Proficiency Program, and it is entirely acceptable to have the installer do the work and then buy your own mail-in kit for the long-term confirmation, or to have a different certified measurement professional place it. Check any certification claim on the C-NRPP certification check page.

How long it takes. The short confirmation is 2 to 7 days plus lab turnaround. The long-term confirmation is 91 days minimum, and in practice October to February is the natural window in Ottawa. Lab results generally come back within one to three weeks. So a system installed in November is properly proven around March, and a system installed in June is properly proven the following spring.

What it costs in Ottawa in 2026. Indicative ranges only. A post-installation short test is often included in the installation price. Where it is charged separately it is usually about $100 to $200. A long-term 91-day mail-in kit you place yourself, lab fee included, is roughly $40 to $80. A professionally placed long-term retest runs about $150 to $300. The main cost driver is whether someone has to make a site visit, and whether you want a signed report rather than a lab printout.

Reading the result honestly. Under 200 Bq/m3 means the system is doing what it was bought to do. Well under 100 is common with a properly designed system, since Health Canada notes that depressurization reduces levels by more than 80% in most homes (Reducing radon levels in your home). Still over 200 means something is wrong and it is the installer's problem to solve, usually by adding a suction point, upsizing the fan or finding an entry path that was missed. A result that is better but not good enough is a common and fixable outcome, not a reason to start again from scratch.

What makes an Ottawa house different. Timing is the practical issue. The heating season is the measurement window and it runs roughly October to April here, so a system installed in May sits unproven for five months. Never delay a repair over that, but do agree in writing at quote stage who supplies the long-term kit and who follows up, because five months is long enough for everyone to forget. Ottawa's deep frost adds a second thing to check on the retest visit: an exhaust that ices up or gets buried in a snowbank degrades a system in exactly the months you are trying to measure.

What to ask before you sign off the install. Is a post-installation test included. Who supplies the long-term retest kit and when. If the 91-day retest comes back above 200 Bq/m3, what happens and at whose cost. Will you give me a written report with the dates, the location and the lab name on it. How long is the warranty, and does it cover a retest failure rather than only a broken fan.

Keeping the paperwork. Three documents are worth keeping together: the original test that justified the work, the installation details including fan model and pipe route, and the long-term retest that proves the outcome. Home inspectors and buyers ask for exactly this set, and it turns an unexplained pipe in the closet into a documented improvement.

A last word on limits. A retest measures one location over one window. It does not certify the whole house forever. If your basement result is fine but you have turned a crawlspace into a bedroom, that space deserves its own detector. And if the gauge ever levels out, stop trusting the last retest and call the installer, because the number on file describes a house where the fan was running.