Why a running fan isn't proof
A fan that's on and a manometer that shows suction tell you the system is active. They don't tell you whether the number's actually below 4 pCi/L. Suction point placement, how well it's communicating under the slab, sealed penetrations, all of it affects the real result, and none of it shows up just from watching the fan run. The only way to know is to test the air again after the system's been working.
Why it's included, not an upsell
I build post-mitigation testing into every install because a job that isn't confirmed isn't really finished. You shouldn't have to pay extra to find out whether the thing you just paid for worked. It's part of doing the job right, not a line item I add on afterward.
When I run the retest
The system needs time to actually condition the house air before a test means anything, so I don't test the same day the fan goes in. I place the retest in the same spot as the original test whenever possible, for the same reason closed-house conditions matter on any test: consistency is what makes the before-and-after comparison mean something.
What the number tells you
If the retest comes back under 4 pCi/L, the system's doing its job and you've got a documented result. If it doesn't get there, that's not a failure on your end, it's information. Sometimes it means the suction point needs adjusting, sometimes it means there's a second zone, like an addition with its own footing, that needs its own point. I'll tell you what I'm seeing and what the next step is, not just leave you with a number and no explanation.
Retesting on a closing timeline
This is where post-mitigation testing gets time-sensitive. If mitigation went in ahead of a sale, the retest is usually what the buyer, seller, or agent is actually waiting on before closing. I build the timing into the job from the start rather than treating it as an afterthought once the system's installed, because closings don't wait and neither does everyone's patience.
Retesting an older system
Not every retest is tied to a brand-new install. If you've got a system that's been running for years and hasn't been tested since it went in, or since whoever owned the house before you had it put in, that's worth checking too. PA DEP's own guidance leans toward testing during the heating season, since that's typically when levels run highest. A system that worked at installation can drift for a lot of reasons, an aging fan, a sump lid that got opened for pump work and wasn't resealed right, a finished basement that changed air pathways. Retesting is the only way to know if any of that's happened.
Where I retest
I cover York County the same as I do for testing and mitigation, from the rowhomes and older housing stock downtown and in Northwest York to the newer builds out toward Springettsbury and Spring Garden Township, and the farmhouses and additions further out that sometimes need a second look at more than one foundation zone.