The water question gets asked less than it should. Every dental autoclave has a reservoir, somebody has to fill it, and the bulk of the practices we visit are still doing it with whatever comes out of the tap. That works, briefly, and then it stops working, usually about three years before the unit was supposed to be replaced.
This is what HTM 01-05 actually requires, what hard tap water does inside an autoclave, and the practical decision between distilled water bottles, an under-sink RO unit and a centralised RO system.
The relevant passage in the 2013 HTM 01-05 publication is at paragraph 4.20:
“Before carrying out the daily tests, the user should: clean the rubber door seal with a clean, damp, non-linting cloth; check the chamber and shelves for cleanliness and debris; fill the reservoir with distilled water or RO water; turn the power source on.”
That’s the rule. Distilled water or reverse-osmosis water. Mains tap water is not on the list.
The supporting technical reason sits in BS EN 285, Annex B (Table B.1), which sets suggested maximum contaminant values for the feed water supplied to steam sterilisers. Conductivity ≤ 15 microsiemens per centimetre, chloride ≤ 2 mg per litre, hardness ≤ 0.02 millimoles per litre. Mains water in most of England comes in at around 300 to 700 microsiemens per centimetre, twenty to forty times the suggested limit, before you even start on the hardness numbers.
The mechanism is straightforward and predictable. Calcium and magnesium ions dissolved in mains water precipitate out as limescale when the water boils. Inside an autoclave, that means scale deposits on:
The heating element. Limescale is an insulator. A scaled element runs hotter to deliver the same heat into the water, which shortens its life and changes the temperature profile of the cycle. Eventually the element fails or the unit can’t reach target temperature within the cycle window. That shows up as cycle fault codes or, worse, as a failed validation.
The chamber walls and shelves. Scale on the chamber surfaces affects heat distribution, which is exactly what the validation thermometric tests are looking for. A scaled chamber produces uneven heating, which can take a load out of spec at the colder spots.
Valves and steam pipework. Limescale collects on valve seats and inside narrow steam channels. Symptoms include sluggish vacuum performance, pressure fluctuations, and eventually mechanical failure.
The safety valve. Scaled safety valves don’t seat properly, which causes pressure-relief problems and shows up at the PVI.
The cumulative effect across all of that is an autoclave that fails validation early, breaks down more often, and reaches end-of-life years before a comparable unit fed proper water. The economics are stark: the cost of an RO system over a decade is a fraction of the cost of premature replacement and lost clinical days.
Three ways to make water suitable for autoclave use. They produce subtly different products but all meet the HTM 01-05 standard.
Reverse osmosis (RO). Mains water is forced under pressure through a semi-permeable membrane that removes around 95 to 99% of dissolved solids. The output conductivity is well inside the EN 285 spec. RO is the workhorse option for clinical-scale water purification, relatively low ongoing cost, no chemical regeneration, just a pre-filter that needs changing every six to twelve months and a membrane that lasts two to three years depending on feed water.
Distilled water. Mains water is boiled and the steam condensed in a separate chamber, leaving dissolved solids behind. Conductivity is near zero. Distillation works well at small scale: a 25-litre desktop distiller, or bottled distilled water bought in cases. The downside is energy cost (it takes real electricity to boil and condense the water) and the practical limit on volume.
Deionised water. Mains water passes through ion-exchange resin that swaps the dissolved minerals for harmless ions. Output conductivity is very low. The catch is the resin exhausts, once all the active sites are used up, the unit produces tap-quality water again, often without obvious warning. Maintenance is more demanding than RO.
For most dental practices the practical choice is between bottled distilled water (low capital cost, ongoing supply cost, manual handling) and an under-sink RO unit (higher capital cost, lower ongoing cost, plumbed-in convenience). For larger or multi-surgery practices, a centralised RO system supplying multiple autoclaves and washer-disinfectors is the right answer.
The decision tree:
Under-sink RO unit. Suits single-autoclave practices. Mounts in the cupboard under the decon-room sink, feeds the autoclave through a dedicated tap or a direct plumbed feed. Capital cost lower; install straightforward; maintenance is a filter swap once or twice a year.
Centralised RO system. Suits multi-surgery practices, busy clinics with multiple autoclaves, and any setup combining an autoclave with a washer-disinfector that also needs RO. One larger RO unit in the plant area feeds multiple machines. Capital cost higher; install more involved; maintenance is centralised which can simplify life for the practice manager.
If your practice is currently running on bottled distilled water and you’re getting through a 25-litre bottle a week or more, the RO maths starts to look good. The point at which RO pays for itself depends on the bottle delivery costs, the staff time spent handling them, and how big your autoclave reservoir is.
Yorkshire Water publishes water quality reports by postcode. Most of West Yorkshire sits in the moderately-hard to hard range, conductivity around 400 to 600 microsiemens per centimetre, hardness 200 to 280 mg per litre as calcium carbonate. South Yorkshire (Sheffield) is similar. London and the South-East are typically harder still. The North West and parts of Scotland are softer.
Whatever the headline number, it’s well above the EN 285 feedwater spec. Even “soft” UK mains water at 100 microsiemens per centimetre is six times the limit.
We supply, install and service RO systems as part of our broader decontamination offering. Annual service visits check water conductivity going into the autoclave reservoir, if it’s drifting toward the limit, that flags either a pre-filter change or a membrane change before the autoclave starts having validation issues.
For practices considering the move from bottled water to an RO supply, we’ll do a site survey covering pipework, electrical supply, and plant-room space, and quote a complete install. For multi-surgery practices, we can quote a centralised system feeding multiple autoclaves and a washer-disinfector from one RO unit.
The Tethys H10 Plus washer-disinfector that we install also runs on RO feed and benefits from the same water-quality regime, which is part of the case for a centralised RO solution rather than separate point-of-use units.
A few patterns that tell you the water is the issue:
If any of those sound familiar, water quality is the likely root cause, and replacing the autoclave without fixing the feedwater will just shorten the life of the new one as well.
Ring us on 01274 265566 and tell us:
We’ll quote either an under-sink unit, a centralised system, or an annual service of an existing RO unit you already have. RO supply and service comes under our wider service and maintenance offering, so it can be bundled into the same annual visit that covers your autoclave service, validation and PVI.
The wider compliance picture sits in our HTM 01-05 validation explainer.
