A rubber flexible hose is typically a single-layer or reinforced rubber tube, sometimes with a fabric or wire reinforcement moulded inside rather than wrapped around it. It is generally the cheaper option, flexible enough to route around awkward bends, and perfectly serviceable for lower-pressure, lower-duty applications. Its weak point is the outer surface: rubber degrades under UV exposure, ages with heat cycling, and can perish or crack over time in a way that is not always obvious from the outside until it fails.
For a trade buyer, the practical upshot is that unvented spares are not interchangeable with vented ones. A pressure relief valve rated for an unvented system is a specific, tested part, not a generic valve that happens to fit the thread. The same goes for the expansion vessel, sized to the cylinder's stored volume, and the tundish and discharge pipework that safety fittings rely on to work as intended. Buying the wrong-rated part to save time on a job is a false economy at best.
Material choice, water softener supplier UK braided stainless outer versus a plain rubber hose, changes the durability picture too, and is worth its own consideration alongside bore, length and rating.
Pressure rating is where corners genuinely should not be cut. Every flexible hose has a working pressure it is designed and tested to, and mains-fed systems, unvented cylinders in particular, run at higher sustained pressure than an old gravity-fed tank system ever did. A hose rated for low-pressure use has no business being fitted on a mains-pressure job, regardless of whether the thread happens to match.
Cost is the other real variable. Braided hoses cost more than rubber equivalents, and for buyers working through volume on multiple jobs, that difference adds up over a year of trade supplies. The right call is matching hose type to the duty and visibility of the specific connection, rather than defaulting to one type across every job regardless of application.
Plumb2U stocks safety-rated spares for unvented cylinders, including expansion vessels, relief valves and the fittings that go with them, alongside parts for standard vented systems, all at trade prices with next-day despatch. Getting the right, correctly rated part first time matters more on an unvented system than almost anywhere else in a plumbing job, and it is worth ordering from trade supplies that treat that distinction seriously.