Can an Unvented Cylinder Explode? The Real Risk, Explained

Why a cylinder full of hot water can be dangerous at all

An unvented cylinder holds a large volume of water at mains pressure, typically around 3 bar once it has passed the pressure reducing valve. Water under pressure can be heated above 100 degrees without boiling, because the pressure holds it as a liquid. That is the whole hazard. If a vessel full of water at, say, 120 degrees were to split, the pressure would fall to atmospheric in an instant and a large part of that water would turn to steam at once. Steam takes up roughly 1,600 times the volume of the water it came from, and that sudden expansion is what does the damage.

So the entire safety design of an unvented cylinder is about one thing: making sure the stored water can never get hot enough, or pressurised enough, for that to happen. The devices are layered so that no single failure can get you there. A vented cylinder, fed from a tank in the loft with an open vent pipe over it, cannot build pressure in the same way, which is why the rules for unvented systems are so much stricter. If you are not sure which type you have, our guide to unvented vs vented cylinder types shows how to tell in a minute.

The safety devices that stand in the way

Every unvented cylinder is supplied as a package with its safety controls, and Building Regulations require it to be installed by someone qualified for unvented hot water work, with the discharge arrangement meeting the regulations. The protection works in this order:

• Control thermostat. Switches the heat off when the stored water reaches its set temperature, usually around 60 degrees. On a boiler heated cylinder it closes the motorised valve; on an immersion it switches the element off.
• Overheat cut-out. A separate, higher set thermostat that trips if the control thermostat fails. On the boiler side it is wired to shut the motorised valve; on each immersion it is built into the thermostat, set at roughly 85 degrees. It does not reset itself, so somebody has to notice and ask why.
• Temperature and pressure relief valve. Screwed into the cylinder with a probe in the water. If the stored water reaches around 90 to 95 degrees, or the pressure climbs too high, it opens and dumps hot water out of the cylinder, which lets cold mains water in and cools it.
• Expansion relief valve and expansion vessel. Water expands as it heats. The vessel, or an internal air bubble on some cylinders, absorbs that expansion; if it cannot, the expansion relief valve on the cold inlet opens at around 6 bar.
• Tundish and discharge pipe. Everything the valves release goes through a visible tundish and then a pipe to a safe place outside, so you can see it happening and nobody gets scalded.

Never cap, plug, bend or tie up a relief valve or its discharge pipe because it is dripping. A dripping valve is a warning. A blocked one removes the last protection the cylinder has.

How cylinders actually end up dangerous

When we are called to a cylinder that has seriously overheated, the cause is nearly always something a person did or failed to do, rather than the cylinder itself. The patterns repeat.

• An overheat cut-out that has been bridged or wired out, often because it kept tripping and somebody wanted the hot water back without finding out why.
• A relief valve that dripped, so it was capped, or the discharge pipe that was blocked, crushed or run into a sealed space where nobody could see it.
• A relief valve seized with scale after years without a service. London water is hard, and a valve that has never been manually operated or inspected may not open when it needs to.
• A motorised valve stuck open with the boiler still firing into the coil, on a cylinder whose cut-out was never connected to it.
• An immersion fitted to an unvented cylinder that is not the correct type, with no built in thermal cut-out.
• A cylinder installed by someone without the unvented qualification, with the safety package incomplete or wired wrongly from day one.

Warning signs that your cylinder is overheating

A cylinder that is getting too hot gives you plenty of notice if you know what to look for. Any one of these is a reason to switch the heat off and get it checked the same day:

• Hot water running from the tundish, or steam coming from it or from the end of the discharge pipe outside.
• Every hot tap in the property suddenly scalding at the same time.
• Water spitting, surging or arriving with a hiss at the taps.
• Banging, knocking or a rumbling kettle sound from the cylinder or the pipes around it.
• The cylinder cupboard far warmer than usual, or the pressure gauge on the inlet group, if you have one, reading higher than normal.
• An immersion or boiler side reset that keeps tripping. The cut-out is doing its job, and the fault behind it is still there.

What to do right now if you see those signs

Stop putting heat into the cylinder. Switch the immersion off at its fused spur on the wall, set the hot water to off at the programmer, and if you are not sure of the controls, turn the boiler off at its own switch. With no heat going in, the situation cannot get worse, and it will slowly improve.

Leave the tundish, the relief valves and the discharge pipe completely alone. Do not try to lever a valve open or shut. Keep people away from the tundish and from the outside end of the discharge pipe, because what comes out of them can scald. Do not turn off the cold water supply to the cylinder, because the relief valve relies on cold water coming in to replace what it dumps.

If the hot taps are scalding, our page on a hot water cylinder overheating explains how to draw hot water off safely to bring the temperature down, and why a bath tap with the door shut is the place to do it. Then call us. A cylinder discharging hot water is one of the jobs we treat as urgent.

Is a dripping relief valve a sign it is about to explode?

Usually not. A small, steady drip from the tundish when the cylinder is cold or only warm is most often a relief valve that has a bit of debris on its seat, an expansion vessel that has lost its charge, or a pressure reducing valve letting the pressure creep up. Those are faults to fix promptly rather than emergencies, and our guide to a pressure relief valve leaking goes through them in order. The same applies to tundish dripping in small amounts after the heating has been on.

What changes the picture is heat. Hot water or steam from the tundish means the temperature side has operated, and that tells you the control thermostat and the cut-out have both failed to stop the heating. That is the one to treat as urgent.

Keeping the risk where it should be: close to zero

The safety devices only protect you if they work, and the only way to know they work is to test them. That is what the annual service is for. On an unvented cylinder service the engineer operates both relief valves, checks the discharge pipe runs clear, checks the expansion vessel charge and the inlet pressure, tests the thermostats and the cut-outs, and looks for signs of overheating you might not have noticed. Our article on what happens in an annual unvented cylinder service lists every check.

Just as important is who touches it. Work on the safety side of an unvented cylinder should only be done by a G3 qualified engineer, and it takes two minutes to find out whether someone is. Our guide on how to check a plumber is qualified for unvented work shows you what to ask to see. Cheap fixes on these cylinders are where the dangerous ones come from.

If your cylinder is showing any of the warning signs, or it has not been serviced for years and you are not sure anything on it still works, we can look at it. We are 20 engineers across London and unvented cylinder repair is the core of what we do. Our rate is on the pricing page and is agreed before anyone attends. Call 020 4652 5111 or send an enquiry.