Heating 24 August 2026

Unvented Cylinder Running Out of Hot Water? What Dartford's Newer Homes Should Check Before Winter

If the hot water in your house runs out faster than it did a year ago, the boiler is rarely the thing at fault — the cylinder is. On an unvented system the usual causes are limescale on the coil or immersion, a cylinder thermostat that has drifted, or an expansion vessel that has lost its air charge, and all three are checkable long before winter puts the system under real load.

That matters in Dartford more than in most of the towns we cover, because so much of the newer housing here runs on exactly this kind of system.

Why Dartford homes see this more than most

Dartford splits fairly cleanly into two sorts of property, and they fail in two different ways.

The older centre and the streets off East Hill are largely interwar and post-war semis, most of them on their second or third boiler by now. Out towards Stone, Greenhithe and Ebbsfleet it is a different picture: new build, with system boilers, unvented cylinders and pressurised systems throughout. Plenty of those are still inside their manufacturer warranty and yet well past the point where anyone has looked at them.

On top of that, Dartford is a hard water area. On an unvented cylinder that matters more than people expect. Scale builds on the immersion element and on the coil that carries heat from the boiler into the stored water, and the first sign is almost never a dramatic failure — it is simply hot water that does not last as long as it used to. If you want the full picture of how the two cylinder types differ, we have covered that in vented or unvented hot water cylinder.

What an unvented cylinder actually is

An unvented cylinder holds stored hot water at mains pressure. There is no cold tank in the loft feeding it, which is why these systems give you a strong shower upstairs without a pump, and why every outlet in the house can run at proper pressure at once.

The trade-off is that everything is pressurised, so the system needs safety and expansion controls that a traditional vented cylinder simply does not have: a pressure-reducing valve on the incoming supply, an expansion vessel or internal air gap to take up the expansion as water heats, a temperature and pressure relief valve, and a discharge pipe run to a safe place outside.

Those controls are the reason unvented work is notifiable under Building Regulations and must be carried out by an engineer holding the relevant G3 qualification. It is not a job to have a go at, and it is not a job for a general handyman.

Why does my hot water not last as long?

Work through it in this order, because the cheapest checks rule out the most.

The cylinder thermostat. The stat strapped to the side of the cylinder is normally set to around 60°C — hot enough to keep stored water safe from bacterial growth, and no hotter than it needs to be. If someone has turned it down to save money, or the stat has aged and is switching early, you get less usable hot water out of the same tank. It is a two-minute check.

The timer or programmer. On a system boiler the cylinder only reheats when the programmer tells it to. A heating schedule that was set on handover and never revisited is a common find — the cylinder gets one short reheat in the morning and nothing else all day, which is fine in July and hopeless in December when everyone is showering after dark.

Limescale. This is the Dartford-specific one. Scale on the coil insulates it, so the boiler works harder to put less heat into the water. Scale on the immersion element does the same and eventually kills it outright. You cannot see any of this from outside the cylinder, but the pattern gives it away: reheat times getting longer, and the hot water fading before the tank is empty.

The expansion vessel. If the vessel has lost its air charge, water has nowhere to expand into as it heats. That usually shows up as the pressure relief valve discharging outside — a trickle or a run of water from a small pipe on an outside wall, often noticed as a damp patch or an unexplained drip rather than as a fault.

What pressure should a hot water cylinder be at?

There is no single number to quote, because it is set by the pressure-reducing valve to suit the cylinder that is fitted. What matters is that the incoming pressure is within the range the manufacturer specifies for that unit, and that the expansion vessel is charged to match it. If the incoming main is unrestricted and the vessel is flat, the relief valve becomes the thing absorbing the expansion — and a relief valve is a safety device, not a working part of the system.

So if you have water running from the discharge pipe outside, do not ignore it and do not cap it. It is telling you something upstream needs adjusting.

What you can safely check yourself

• Look at the cylinder thermostat setting and the programmer schedule.
• Look for water at the outside end of the discharge pipe, and note whether it drips constantly or only after a reheat.
• Look at the base of the cylinder and the pipework above it for staining or corrosion.
• Note how long a full reheat takes, and whether that has changed.

Everything past that point — recharging an expansion vessel, adjusting a pressure-reducing valve, replacing an immersion element, or anything at all involving the gas side of the boiler — needs a qualified engineer, and gas work must be done by someone on the Gas Safe Register. We are Gas Safe registered under number 951167.

Check the warranty before you call anyone out

This is worth two minutes if you are in one of the newer Stone, Greenhithe or Ebbsfleet properties. Several manufacturers make their warranty conditional on the appliance being serviced annually, and a missed service can void cover that would otherwise have paid for the repair. If you are still inside that period, we would rather tell you to go to the manufacturer than take your money for a job they should be covering.

Where the warranty has lapsed or the fault is outside it, our servicing and repair rates are straightforward: a boiler service is a flat £95+VAT, and repair work is £95+VAT for the first hour, always billable, then £47.50+VAT for each hour after. Any parts needed are confirmed and agreed with you before we fit or order them.

Get it looked at before the heating goes on

Hot water problems are far easier to sort in August than in the first cold week of the year, when every engineer in Kent is on breakdowns. If your hot water is fading, your reheats are getting longer, or there is water coming out of a pipe on an outside wall you have never really thought about, that is the moment to book — not January.

Call us on 01732 792 454 and we will tell you honestly what is wrong before we do anything about it. You can book a boiler service online, read more about boiler breakdowns and repairs, or see how we cover the area on our page for plumbers in Dartford.