Heating & hot water
Hot water cylinder problems (vented and unvented)
If your hot water comes from a cylinder, the boiler is only half the machine. Coils scale up, immersion elements burn out, motorised valves stick, and one small drip near the tank has a meaning all of its own. Here is the map, for vented and unvented cylinders alike.
By the Tickover team · Published 4 May 2026 · 5 min read
Vented or unvented: a thirty-second ID
A vented cylinder is the traditional pairing: copper tank in the airing cupboard, cold water tank in the loft, gravity doing the pushing. Upstairs taps run gently and the header tank refills after every bath. An unvented cylinder is the modern one: a pressurised steel tank fed straight off the mains, no loft tank, powerful showers, and a tundish (a small funnel-shaped fitting) visible on its discharge pipe.
Cut across that split another way: indirect cylinders, which is most of them, are heated by a coil of boiler water running through the tank, with an immersion heater as electric backup. Direct cylinders heat by immersion elements alone. Which kind you own decides who you call when the water goes cold.
Symptoms and suspects
| Symptom | Likely suspect | Who fixes it |
|---|---|---|
| Cold water, boiler seems fine | Motorised valve or cylinder stat | Heating engineer |
| Hot on immersion only | Coil circuit: valve, stat, pump | Heating engineer |
| Runs out faster than before | Stat set low or a scaled coil | Plumber |
| Immersion cold or tripping | Element or its own thermostat | Electrician |
| Tundish dripping (unvented) | Expansion or relief valve | G3 engineer |
| Rumbling as it heats | Limescale on element or coil | Plumber |
The motorised valve deserves its top billing. On most systems a two- or three-port valve directs boiler water between the radiators and the cylinder coil, and when it sticks you get heating without hot water, hot water without heating, or a tank that only warms when the radiators run. It sits on the pipework near the cylinder, it is a common failure, and it is not a DIY part: the head contains mains wiring.
Water that arrives as a brief warm slug then turns cold suggests only the top of the tank is heated: a coil problem, a stat clicking off early, or an immersion doing its short best. Feel down the cylinder, carefully, and the warm-to-cold boundary tells you how much stored heat you actually have.
Faults on the boiler itself, from pressure drops to lockouts, are a separate guide, and everything inside the boiler casing is for a Gas Safe registered engineer.
The 60°C line
The cylinder thermostat, strapped to the tank about a third of the way up, should sit at 60 to 65°C. That is hot enough to stop legionella bacteria multiplying in stored water, which is why 60 is a floor and not a suggestion. If scalding taps worry you, the fix is blending valves at the outlets, never a cooler tank.
Calibrate reheat expectations too. A boiler-fed coil brings a typical tank back in well under an hour, while a 3kW immersion element doing the same job takes several times longer and usually costs more per unit of heat. The immersion is a backup and a boost, not the daily driver.
And if the airing cupboard is the warmest room in the house, heat is escaping: older vented tanks take an insulating jacket cheaply, and lagging the first metre of pipework finishes the job.
How urgent is this?
Book-a-pro this month, mostly
Most cylinder complaints are wear items on a sensible schedule. Two exceptions move to this week: a dripping tundish, and any bulging, creaking or weeping from the cylinder itself.
Unvented rules
60–65°C
the cylinder thermostat sweet spot
G3
the ticket unvented work legally requires
Unvented cylinders also appreciate an annual once-over of their own, checking the expansion vessel charge and exercising the relief valves. It is often bundled with the boiler service, and it is exactly the sort of small habit that stops the tundish ever having something to say.
Weak flow rather than cold water is a different investigation. Unvented performance leans on incoming mains pressure, and low water pressure has the checklist for whether the problem is your pipework or the street. Vented homes with gentle upstairs taps are mostly feeling gravity, not a fault: the plumbing guide explains the anatomy.
No tank anywhere, and hot water made on demand by the boiler? You are a combi household, and that guide is yours instead.
Cylinder acting its age?
Answer a few questions about tank type, symptoms and timings. Self-Diagnose names the likely part, rates the urgency and briefs a vetted local engineer for you.
Quick answers
Why does my hot water run out so quickly?
Either the tank is not fully heating or the household has outgrown it. A cylinder stat set low, a scaled coil or a failing element heats only the top of the tank, so check the stat setting first and feel how far down the warmth reaches. If the whole tank heats and still runs out, the tank is undersized for current demand.
What temperature should a hot water cylinder be set to?
Between 60 and 65°C at the cylinder thermostat. Stored water below 60 gives legionella bacteria a chance to multiply, so resist the temptation to turn it down for savings. Manage scald risk at the taps with thermostatic blending valves instead, and insulate the tank rather than cooling it.
Why is water dripping from the little funnel near my cylinder?
That funnel is the tundish, and a drip through it means a relief valve on an unvented cylinder is discharging: often a tired expansion vessel or a valve that is not reseating. It is a safety system doing its job, not a leak to plug. Book a G3-qualified engineer this week and never block the pipe.
Can I replace an immersion heater myself?
No. Swapping an element means draining the cylinder and working on a mains-wired circuit, and getting either wrong is expensive or dangerous. It is a routine job for a plumber or electrician with the right immersion spanner. Worth asking them to check the element’s separate thermostat too, since that is a cheaper and common failure.
This guide is general information for UK homes, not advice about your specific property. Anything involving gas must be done by a Gas Safe registered engineer, and notifiable electrical work by a qualified electrician. If you smell gas, call the National Gas Emergency line on 0800 111 999. If you’re ever unsure, stop and get a professional in — that’s exactly what Tickover is for.
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