A valve is only as reliable as its smallest part. After 25+ years of manufacturing brass valves and faucets in Yuhuan, Zhejiang, we can tell you exactly where most field failures begin: not in the casting, the machining, or the plating — but in a 3-gram O-ring that was specified for the wrong service. This guide explains how EPDM, NBR, silicone and PTFE behave in hot, cold and reclaimed water, and how to specify seals that will still be sealing in a decade.
Elastomers and plastics are not inert. Immersed in water they absorb moisture, swell, harden, extract plasticiser, and react with disinfectant residuals. Select the wrong compound and you get leakage at the spindle, seized ball valves, crumbling gaskets, or — worst case in a potable system — taste and odour taint and premature failure of a fitting that should have lasted decades. In Europe and the UK, seal materials in contact with drinking water must also satisfy materials approvals such as WRAS or the relevant national requirements; that is a compliance issue, not just a performance one.
EPDM is the default elastomer for water. It shrugs off hot and cold water, steam, ozone, UV and weathering, and remains flexible across roughly -40°C to +150°C, which comfortably covers domestic hot-water storage up to 90°C and the brief thermal spikes of pasteurisation-style legionella control. Food- and water-grade EPDM compounds are available with WRAS, NSF/ANSI 61 and similar approvals.
Its one famous weakness: poor oil and grease resistance. Keep EPDM away from petroleum lubricants, mineral oils and solvents. It is also only moderately resistant to strong oxidisers at high concentration.
Typical roles in brass fittings: O-rings on valve spindles and bonnets, body gaskets, and seal rings in check valves and hose connections.
Nitrile is the oil-country cousin: excellent resistance to mineral oils, greases and fuels across roughly -30°C to +100/+120°C, with good abrasion resistance and low cost. It is the right choice wherever a valve or fitting is exposed to lubricants, hydraulic fluids or oil-contaminated media.
It is, however, a weaker choice for long-term potable hot-water service. Nitrile has only moderate heat-ageing performance and is attacked by ozone, weathering and strong oxidising disinfectants such as chlorine at elevated concentration — all of which are normal realities in a water system.
Typical roles: seals in oil-handling and heating circuits, hydraulic and pneumatic fittings, and certain radiator valves where system additives are present.
Silicone is the extreme-temperature specialist, remaining elastic from roughly -50°C to +200°C and above, with excellent resistance to ozone and UV. Food-grade and potable grades are common, which is why silicone appears in so many domestic appliances and thermostatic cartridges.
Its trade-offs are mechanical: lower tear and abrasion strength than EPDM or NBR, higher gas permeability, and a tendency to soften or swell in some oils and fuels. It is also usually more expensive.
Typical roles: diaphragm seals and thermostatic-element seals, membrane seats in mixer cartridges, and applications needing a very wide temperature envelope with light mechanical duty.
PTFE is not an elastomer — it is a fluoropolymer — and that changes how you use it. It is chemically inert to almost everything, handles -200°C to +260°C, has the lowest friction of any common seal material, and never rots, hardens or absorbs water. That makes it the classic seat material for ball valves and the standard thread-seal tape for jointing.
The catch: PTFE does not "spring back." An O-ring must recover its shape to seal under pressure fluctuation; PTFE cannot, so it is rarely used as a compression O-ring without an energising element (a metal or elastomer spring behind it). Use it for seats, back-up rings, bearings and static seals where it is held in compression by geometry.
Typical roles: ball seats in full-bore ball valves, back-up rings behind elastomer O-rings, gland packings, and sealing tape for threaded joints.
| Service | Water quality realities | Recommended seals | Avoid |
|---|---|---|---|
| Cold potable water | Chlorine or chloramine residual, occasional UV exposure, up to ~25°C | EPDM (approved grade), PTFE seats | NBR for long service; compounds without potable approval |
| Hot potable water / heating | 60–90°C, thermal cycling, scale, periodic heat-up for legionella control | EPDM; PTFE seats in ball valves; silicone in thermostatic elements | Standard NBR at sustained high temperature |
| Reclaimed / grey / rainwater | Variable chemistry, higher disinfectant dosing, ozone or chlorine dioxide, sometimes oil traces from runoff | EPDM for water-side seals; PTFE seats; NBR only where hydrocarbons are genuinely present | One "universal" compound assumed safe for all three |
| Oil / fuel / lubricated systems | Hydrocarbons present | NBR or FKM/Viton-type where temperature demands | EPDM (it will swell and fail) |
Two notes from our production floor. First, in our brass ball valves you will usually find the combination PTFE seats + an elastomer stem O-ring — PTFE gives a smooth, low-torque, drip-tight shut-off, while the elastomer handles dynamic sealing. Second, if a buyer tells us a valve will sit on a roof in the Middle East handling hot water with heavy chlorination, we steer them to EPDM or perfluoroelastomer grades rather than a cheap general-purpose ring, even though it raises the unit price slightly. A seal failure inside a finished wall costs a hundred times the price of the right O-ring.
When you write the seal requirement into an RFQ or datasheet, include:
Service medium and temperature range (continuous and peak), and pressure rating.
Water-contact approval needed — e.g. WRAS / NSF 61 / ACS / KTW for potable contact in your destination market.
Hardness — 70 Shore A is the practical default for most water O-rings; softer (60) for low-pressure, harder (80–90) for high pressure or extrusion resistance.
Compound traceability — ask for material data sheets and batch certificates, not just a colour.
System additives — heating-system inhibitors, biocides or glycols change the answer; disclose them up front.
Spares strategy — for a large project, order spare seal kits in the same batch so compounds match for years of maintenance.
A good OEM partner treats seal selection as an engineering decision, not a cost line. At Xindong, our in-house laboratory tests assembled valves for torque, leakage and seal compatibility, and our engineers will challenge a specification that will fail in service — before we quote, not after your first container arrives with weeping glands. That is the difference between buying a brass casting and buying a sealing system.
Need a compound or approval we have not mentioned? Send your service conditions and drawings to our engineering team via [[email protected]](mailto:[email protected]) or use the [contact page](https://www.cn-xindong.com/) — we will recommend the seal stack, confirm approval feasibility, and quote with full material traceability.
*Xindong Engineering Team — Zhejiang Xindong Sanitary Ware Co., Ltd. (Est. 1999, Yuhuan, Zhejiang, China).*
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The company, founded in 1999 and located in Yuhuan,specializes in taps, thermostatic shower valves and shower kits. With over 30,000 sqm of space and 380+ employees, we hold numerous certifications and 65+ patents.
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