Deionized water cleans glass because there is nothing dissolved in it to leave behind. Strip out the calcium, magnesium, sodium and silica, and what dries on the pane is nothing at all. No soap, no squeegee, no film. Here is the chemistry, and the jobs where it fails.
Tap water is a solution. Everything you see as a spot on dry glass was dissolved in that water and got left behind when the water evaporated. Calcium and magnesium carbonates mostly, plus sodium, sulfates and silica. A TDS meter reads that load in parts per million, and most Treasure Valley taps land somewhere between 150 and 350 ppm. Deionization pulls that to zero. Water at 0 to 1 ppm can dry on glass in full sun and leave nothing, because there is nothing in it to leave. That is the entire mechanism. Pure water is not a stronger cleaner the way a degreaser is stronger. It is a rinse that disappears completely. Everything else in the process, the brush, the flow rate, the order of the final rinse, exists only to get soil into that water and then carry it off the pane.
Deionization is an ion swap, not a filter. Water passes through resin beads. Cation resin charged with hydrogen ions trades them for the positives, calcium, magnesium and sodium. Anion resin charged with hydroxide trades for the negatives, bicarbonate, chloride, sulfate and silica. The hydrogen and hydroxide released meet each other and become more water. Mixed-bed resin does both in one vessel, and mixed bed is what gets you to a true zero rather than close. Resin has finite capacity, roughly 20,000 grains per cubic foot, and the math is unforgiving: usable gallons is about 342,000 divided by your feed TDS. At 200 ppm that is roughly 1,700 gallons per cubic foot. At 100 ppm, about 3,400. Resin runs $130 to $200 a cubic foot, which is why serious rigs put a reverse osmosis membrane in front to strip 90 to 95 percent first and stretch resin life close to tenfold.
Soap is a residue by design. Surfactants work by staying on the surface and lowering surface tension so the water film relaxes and drains. That film is real, and if you do not physically drag it off with a squeegee blade, it dries into a haze that grabs dust and pollen within days. Pure water rinsing has no squeegee stage. The water sheets and evaporates on its own, so any surfactant added to it is guaranteed to stay behind. Pure water, at about 72 dynes per centimeter of surface tension, actually beads more than soapy water, which looks wrong on the glass while you are working. It does not matter. A bead of 0 ppm water dries to nothing. Soap earns its place on real grease, restaurant exhaust film, cooking residue, road tar, where you need chemistry to break oil apart, and there you should use it and then squeegee it off.
Pure water dissolves and rinses. It does not cut, etch, remove bonded scale or repair anything. Hard-water spotting that has sat on glass through a couple of Idaho summers is no longer a deposit sitting on top. The silica and calcium have reacted into the glass surface. Early-stage film comes off with an acidic gel or a cerium oxide polish, which is a separate restoration job. Late-stage etching is physical damage to the pane and is not fixable at any price. The honest answer there is replacement or living with it. A fogged insulated unit is a failed seal between two panes, and no cleaner of any kind reaches that space. Paint overspray, stucco splatter and construction silica need a blade and a trained hand. Interiors are not a pure-water job either, since you cannot flood water down a wall, so the inside of your glass is still soap, microfiber and squeegee.
Snake River Plain groundwater is calcium-bicarbonate hard and carries meaningful dissolved silica, which is the ingredient that turns neglected spotting from ugly into permanent. High feed TDS also burns resin fast, so purified water genuinely costs more to make here than on soft municipal supplies. The bigger local culprit is irrigation. Plenty of valley yards run untreated canal or well water through pop-up heads, and that water is both higher in dissolved solids and carrying silt. West- and south-facing glass that gets hit nightly will spot again in weeks no matter how clean it started. Rotating a sprinkler head ten degrees is free and does more for that window than any service you can buy. On timing, we do not run pure water below about 35 degrees F because it ices on glass and walkways, and past roughly 95 degrees we chase shade, because hot glass dries faster than the rinse can sheet.
On an exterior the sequence is simple: brush the frame and glass with a soft boar and nylon head to lift soil into the water, flood from the top down, then pull the pole back so the final rinse sheets off the pane in one pass. Frames, tracks and grids get cleaned on the same trip, which a squeegee cannot do. The finish dries with nothing on it, which is what we call the RainShield finish, and the 7-Day Rain Guarantee covers the weather turning on you after. Where this costs us money to say: most valley homes do not need the Monthly membership. Quarterly is the right interval for a house that is not next to a field or gravel road, and it saves $100 per cleaning. Cleaning starts at $199 by pane count, screens are $50 more. If your only problem is one etched shower door, do not book a window cleaning.
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Under 10 ppm is generally spot-free, and a properly running mixed-bed system reads 0 or 1. Between 10 and 30 ppm you will start seeing faint spotting on dark glass in direct sun. Above that you are just wetting windows with slightly improved tap water. Any crew claiming pure water should be able to hand you the meter and show a reading at the tank.
No. Distilled water is boiled and condensed, which is pure but impractical at sixty-plus gallons a house. Reverse osmosis pushes water through a membrane and removes roughly 90 to 98 percent of dissolved solids, which is not enough on its own to dry clear. Deionization ion-exchanges the remainder to zero. Most professional rigs run RO first and DI as the polish, because that pairing is far cheaper per gallon than DI alone.
No. Pure water is a rinse, not a biocide. It lifts loose algae, pollen and dust off glass and frames, but it kills nothing and does not slow regrowth. North-facing glass and shaded frames that grow green film need either a tighter cleaning schedule or, on siding and concrete, a soft-wash mix at $0.18 per square foot, which is a different service and does use chemistry.
Yes, and on a single-story house it can pencil out. A small DI vessel with a water-fed pole runs roughly $300 to $600, and refill resin is $130 to $200 per cubic foot. On 200 ppm feed water that resin makes about 1,700 gallons, which is a lot of cleanings if you are only doing your own house. The real limits are second-story reach and the ladder work we would rather you skip.
Four usual causes. Spent resin, which the meter tells you in seconds. Sprinkler overspray re-depositing within days. Etched glass, where the marks are in the surface and were there before anyone showed up. Or the inside of the pane is dirty and being blamed on the outside. Stand at an angle and touch the interior surface. That tells you which side the problem is on.
It is safer than soapy water around landscaping, since there is no surfactant running into beds. Deionized water absorbs carbon dioxide from the air and drifts slightly acidic, around pH 5.5 to 6, which is about the same as rainwater and harmless to paint, seals and screens. The genuine cautions are mechanical, not chemical: do not aim flow at a failing seal, and do not scrub an old exposed exterior coating.
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