Pure water window cleaning has largely replaced the traditional squeegee-and-chamois method among professionals, and once you understand why, it makes sense: water with zero mineral content dries completely streak-free without needing to be wiped off at all. It sounds almost too simple, but the science behind it is straightforward.
This guide explains how TDS meters and deionisation vessels work together to produce pure water, and the technique that gets consistent, streak-free results.
Why ordinary tap water leaves streaks
Tap water contains dissolved minerals, calcium and magnesium primarily, that stay behind as the water evaporates, leaving the faint streaky, spotty residue anyone who's ever let a car air-dry will recognise. Traditional squeegee cleaning gets around this by physically wiping the water off before it can evaporate and leave marks behind.
Pure water removes the problem at the source: with no dissolved minerals in the water itself, there's nothing left behind to dry into a streak, so the glass can simply air-dry after rinsing.
How TDS meters and DI vessels work
A TDS meter measures total dissolved solids in water, in parts per million, and is used to confirm the water coming out of the system is genuinely pure, ideally reading close to zero, before cleaning begins. Water this pure is produced by passing mains water through a deionisation vessel, which contains resin beads that strip out dissolved minerals through an ion exchange process, sometimes combined with reverse osmosis filtration for larger or commercial systems. Mains water hardness varies across the region too, so the exact filtration setup needed can differ slightly from one street to the next.
Checking TDS regularly matters because DI resin has a finite capacity and needs replacing or regenerating once it's exhausted; using water that's no longer properly pure defeats the purpose and can leave the same streaking pure water is meant to avoid.
The technique that gets a streak-free finish
Pure water is applied through a soft brush head on a pole, scrubbing the glass and frame to loosen dirt while the water flows continuously, then rinsed thoroughly with more pure water until every trace of soap-free dirt is gone. The key technique point is thoroughness of rinse: any residual grime not fully rinsed away, rather than the water itself, is what causes an imperfect finish, since pure water alone has nothing left behind to cause streaking.
Because the whole process can be done with a water-fed pole from the ground, it also reaches upper-storey windows safely without ladder access, a genuine practical advantage alongside the streak-free result.
Costs and how it compares to traditional cleaning
Professional pure water window cleaning typically costs a similar amount to traditional squeegee cleaning, often priced per visit for a standard semi at around £15-£25, though frequency and property size affect this. The main practical advantages are safer access to upper floors without ladders and a genuinely streak-free result that some customers find more consistent than traditional methods, especially on frames and sills that get cleaned along with the glass rather than left dry-wiped.
One thing worth knowing: pure water can look slightly dull on the glass for a short while as it dries, compared with the immediate shine of a fresh squeegee finish, before drying completely clear. This is entirely normal and not a sign of a poor clean, just a different drying process to what people are used to seeing.
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