How Long Should an Unvented Cylinder Last? When to Service, Repair or Replace

Unvented hot water cylinders are workhorses. The Megaflo, Gledhill StainlessLite, Telford Tempest and Vaillant uniSTOR units I service every week across Orpington, Bromley and Kent are reliable, efficient pieces of kit — but they don't last forever.

One of the most common questions I get asked is: "How long should my unvented cylinder last, and when should I be thinking about replacement rather than repair?" Here's the honest answer based on real cylinders I see in real homes.

Typical Lifespan of an Unvented Cylinder

A well-maintained, good-quality unvented cylinder should give you between 15 and 25 years of reliable service. The variation is huge because lifespan depends almost entirely on three things:

• Water quality — hard water across South East London and Kent is the single biggest factor in cylinder lifespan
• Servicing history — cylinders serviced annually tend to last longer than those left to fend for themselves
• Build quality — premium brands (Megaflo Eco Plus, Gledhill StainlessLite Plus) typically outlast budget alternatives by 5–10 years

The Three Stages of an Unvented Cylinder's Life

Years 1–10: The Reliable Years

During the first decade, a properly installed cylinder needs little more than annual servicing. Common jobs at this stage:

• Recharging the expansion vessel
• Testing the temperature & pressure relief valve (TPRV)
• Replacing the immersion heater element if used regularly
• Cleaning strainers and inspecting the discharge pipework

If you're paying for an annual service, this is the era where it pays back the most — preventing the small problems that, neglected, become big problems later.

Years 10–18: The Repair Era

This is when components start needing replacement. Common repairs at this stage:

• Expansion vessel replacement — the rubber diaphragm fails and the vessel won't hold its charge
• TPRV replacement — these valves are typically rated for 10–15 years
• Pressure-reducing valve replacement — scale damage is the usual cause
• Thermostat or thermostatic mixing valve replacement
• Limescale-related issues on the heat exchanger

None of these on their own justify replacing the whole cylinder — they're all sensible repairs with affordable parts. The cylinder body itself is still good for many more years.

Years 18+: Decision Time

Once a cylinder approaches 20 years, the maths changes. The cylinder body itself starts to show its age — internal corrosion, weld fatigue, deteriorated insulation, and increasingly frequent component failures. At this point I'm honest with customers: keep paying for repairs, or invest in a new cylinder that'll last another 20 years.

Warning Signs Your Cylinder Is Approaching End of Life

Look out for these — any one of them is worth investigating, multiple together suggests replacement:

• Visible rust streaks on the cylinder body or insulation
• Persistent leaks that come back after repair
• Frequent TPRV discharges even after expansion vessel servicing
• Reducing hot water capacity — runs out faster than it used to
• Heating times getting longer despite proper servicing
• Multiple components failing within 12 months of each other
• Cylinder no longer meeting current Building Regulations

Repair vs Replace: How to Decide

My rule of thumb: if the cost of the proposed repair is more than 40% of the cost of replacement, and the cylinder is over 15 years old, replacement is usually the better long-term value. You're paying for old technology with limited life left. If you're weighing it up, my unvented cylinder replacement page explains what's included and what it costs.

However, for younger cylinders (under 15 years) almost all faults are economically repairable. A failed expansion vessel, blown TPRV, or even a complete control gear replacement is far cheaper than a new cylinder.

What's Involved in Replacing an Unvented Cylinder

A like-for-like replacement of a domestic unvented cylinder is typically a one-day job for a G3-qualified engineer. The work includes:

1. Isolating water and electrical supplies
2. Draining the existing cylinder
3. Disconnecting and removing the old unit
4. Installing the new cylinder including all required safety devices
5. Reconnecting hot/cold supplies, expansion vessel, immersion heater
6. Pressurising, testing and commissioning
7. Issuing your G3 Building Regulations certificate

The new cylinder will come with a manufacturer warranty (typically 2–10 years on the cylinder body, 1–2 years on parts). Provided it's serviced annually, you should expect another 20+ years of reliable hot water.

Choosing a Replacement Cylinder

If you're replacing, this is the time to think strategically:

• Stick with the same size? Family changed? You may need bigger or smaller.
• Heat pump compatible? If you're considering a future heat pump, a cylinder with a larger heat exchanger coil is essential.
• Premium or budget? Pay extra for the premium model — the price difference is small relative to the lifespan difference.

I usually recommend Megaflo Eco Plus or Gledhill StainlessLite Plus as well-proven options, but the right choice depends on your specific setup, water pressure and future plans.