Fluorescent UV dye testing is the leak-tracing technique we reach for when thermal imaging, acoustic listening, and tracer gas have each ruled out the obvious but the leak still isn't pinned down to a single source. A small measured dose of non-toxic, water-soluble fluorescein tracer dye is introduced at the suspect entry point, a WC cistern, a header tank, a central heating fill loop, a shower waste, or a soil stack, and allowed to circulate or migrate with the water flow. Concealed surfaces and emergence points below are then surveyed with a 365 nm UV-A blacklight, under which the tracer fluoresces an unmistakable yellow-green glow at parts-per-million concentrations. Where the dye appears tells you exactly which fixture, joint, or seal is the source.
We carry sodium fluorescein for warm and circulating systems, Rhodamine WT for cold mains and storage work where a contrasting orange-red signal is needed, and WRAS-approved tracers for any potable application where dye is added on the clean-water side. Surveys are run with Spectroline OPX-365 high-output UV-A torches and a Mini Maglite UV for tight cavities, with all dosing, dwell time, and observation logged to the BSRIA TM50 "Tracer dye testing in HVAC and plumbing systems" methodology. Dye is dosed at the minimum effective concentration so there is no staining of fittings, finishes, or downstream drainage, and the system is flushed clean once the source is confirmed.
Where this service applies
- Shower-tray and shower-pan leaks where failed silicone, grout, or a perished waste seal is staining the ceiling below
- Soil-stack pinhole leaks and hairline cracks behind boxing, with dye traced down each branch to isolate the offending floor
- Central-heating header-tank, feed-and-expansion, or fill-loop leaks where pressure loss can't be located by thermal alone
- Hidden hot-water cylinder weeping seams, immersion-boss leaks, and slow drips from poorly soldered connections at the cylinder crown
- WC cistern and pan-connector leaks where the seep is intermittent and only shows after flushing
- Multi-source leaks above a single damaged ceiling, dye is run one fixture at a time to attribute each contribution
- Swimming pool, hot tub, and ornamental pond leaks where water loss exceeds evaporation but no surface defect is visible
- Wet-room and tanked floor leaks where the failure is in the membrane rather than the pipework
- Confirmation surveys after another contractor's repair, dye proves the seal is now sound before finishes are reinstated
How we deliver
No surprises, no upselling. The exact path every job follows.
- 1
Customer interview and system review
We confirm where the symptom is showing, when it appears, and what has already been tried. Dye testing is most powerful as a final-step confirmation, so we map the suspect fixtures and circuits before any dye is introduced. - 2
Dye selection and dose calculation
Sodium fluorescein for warm and heating systems, Rhodamine WT for cold-water work, and WRAS-approved tracer where dye contacts potable water. Dose calculated to BSRIA TM50 at the minimum effective concentration for the volume under test. - 3
Controlled introduction at the suspect source
Dye added to the cistern, header tank, shower waste, soil-stack rodding eye, or fill loop, one source at a time so emergence can be unambiguously attributed. - 4
Dwell, flush, or circulation period
Typically 15 to 60 minutes depending on the system. Heating circuits are run to circulate, drainage runs are flushed to migrate, shower trays are flooded and held to load the failed seal. - 5
UV-A blacklight survey of emergence surfaces
Spectroline OPX-365 high-output torch at 365 nm across ceilings, voids, cylinder seams, soil-stack joints, and the underside of trays. A Mini Maglite UV is used for tight cavities and inspection hatches. - 6
Pinpoint and photograph under UV
The leak point is marked, photographed with a UV-pass filter, and cross-checked with a second observation cycle to rule out cross-contamination or staining from earlier escapes. - 7
System flush and insurance-grade written report
The dosed system is flushed clean through its normal outlet, dye concentration drops below the visible threshold within a normal cistern or tank turnover. Findings are written up with photographs, marked plans, and recommended access cut, in the format your insurer expects.