A hot water cylinder stores heated water ready for use, and the key distinction is how it is fed. A vented cylinder is supplied by a cold tank in the loft and works on gravity. An unvented cylinder is fed at mains pressure and is a sealed pressure vessel, carrying safety controls that must be tested annually by a qualified engineer.
The short version
- Vented cylinders are gravity-fed from a loft tank. Unvented run at mains pressure, no tank.
- Unvented cylinders are pressure vessels whose safety devices need an annual service.
- Lukewarm water, slow reheat and rumbling point to the coil, immersion, thermostat or scale.
- Any work on the gas boiler heating the cylinder is Gas Safe engineer work.
- Poor mains pressure limits an unvented cylinder, so check it before fitting one.
Vented or Unvented: How Do You Tell?
The quickest test is to look in your loft. A large cold water storage tank up there, plus a smaller feed and expansion tank, means a vented system. Water falls from that tank into the cylinder under gravity, which is why pressure at your taps depends on how high the tank sits above them. That low pressure is why so many homes with vented cylinders also have a shower pump.
An unvented cylinder has no loft tank. It connects straight to the cold mains, so hot water leaves it at close to mains pressure, balanced with the cold. That gives strong showers without a pump and lets you run more than one outlet at once. Because the water is sealed under pressure, and water expands as it heats, the cylinder carries safety devices to manage that expansion and protect against overheating.
You can spot an unvented cylinder by the ironmongery attached: an expansion vessel, a pressure reducing valve on the inlet, temperature and pressure relief valves, and a discharge pipe running to a safe outside point. Those components are why the servicing requirement exists. A well designed central heating system has the cylinder, boiler and controls working together rather than fighting each other.
Vented cylinderGravity-fed from a loft tank. Lower pressure, simple, often needs a shower pump. |
Unvented cylinderMains-fed sealed vessel. Strong pressure, no tank, annual safety service required. |
Thermal storeStores heated primary water, heating mains water on demand via a coil. |
Combi boilerNo cylinder. Heats water instantly, so no stored volume to run out. |
Why Unvented Cylinders Must Be Serviced Every Year
An unvented cylinder is a sealed vessel holding a large volume of hot water under mains pressure. Handled properly that is entirely safe, and hundreds of thousands of UK homes run them without incident. The safety depends on the protective devices working, and those are mechanical parts that can seize, scale up or lose their charge. That is the reason for the annual service, and it is a Building Regulations Part G requirement rather than an optional extra.
During a service an engineer checks the expansion vessel charge, because a vessel that has lost its air cushion cannot absorb expansion and pushes water out through the relief valve instead. They exercise the temperature and pressure relief valve and the expansion relief valve to confirm they are free and reseat, inspect the discharge pipework and tundish, check the inlet pressure reducing valve, and verify the thermostat and overheat cut-out.
Skipping the service usually shows itself as water dripping from the discharge pipe outside, which is a relief valve doing its job because something upstream has failed. The Approved Document G guidance on hot water safety sets out the requirements applying to these installations.
Water dripping from the discharge pipe outside is not a leak to be capped off. It is a safety device telling you something has stopped working as designed.
The MGH engineering team
Common Hot Water Problems and What They Point To
Most cylinder complaints fall into a few patterns, and knowing which one you have makes the conversation with your engineer quicker. The commonest is water that is warm rather than properly hot. On a cylinder heated by the boiler that often means the coil has scaled up, the cylinder thermostat has drifted, or the motorised valve diverting heat to the cylinder is not opening fully. On a cylinder relying on its immersion, it is usually the element or its thermostat. Hot water running out sooner than it used to, slow reheat, and rumbling as it heats all point to scale or a partly failed heat source.
| 1 |
Note exactly what happens Warm but not hot, hot then quickly cold, or no hot water at all. These point to different faults. |
| 2 |
Check the obvious controls Confirm the programmer is calling for hot water and the cylinder thermostat has not been knocked. |
| 3 |
Look for water outside Any discharge from the outside pipe of an unvented cylinder is a fault, not a quirk. |
| 4 |
Listen near the cylinder Rumbling or kettling as it heats usually means scale on the coil or element. |
| 5 |
Call a qualified engineer Anything past a control setting, and all gas work, needs a registered engineer. |
Sizing and Mains Pressure Matter
If you are replacing a cylinder, or moving from vented to unvented, two things decide whether it works well. The first is the incoming mains flow rate and pressure, measured at the stopcock rather than guessed, because an unvented cylinder can only deliver what the mains supplies it. The second is capacity, matched to how many bathrooms you have and how the household actually uses hot water.
The boiler and cylinder also need considering together, because the boiler must have the output to reheat the cylinder in a sensible time. Where a system is being reworked, have the whole thing assessed rather than swapping one component. Our plumbing services cover cylinder work, and any part touching the gas boiler is carried out by a Gas Safe registered engineer.
Who Is Allowed to Work on Your Cylinder?
The Gas Safe Register help and advice pages set out what to expect from a registered engineer. Work on an unvented cylinder requires a specific competence because of the pressure vessel and its safety devices. Work on the gas boiler heating it requires Gas Safe registration, without exception. There is no safe do-it-yourself route into either, and capping or adjusting a dripping relief valve is genuinely dangerous. Ask any engineer what they are qualified for and check their Gas Safe registration.
If your hot water has gone off, is not getting properly hot, or your unvented cylinder has never been serviced, we can look at it. Our engineers are based in Salford, and for a total loss of hot water we can usually attend quickly through our emergency plumber service. Send us the details through our contact page.
Frequently Asked Questions
How do I know if my cylinder is vented or unvented?
Look in the loft. A large cold water storage tank and a smaller feed and expansion tank next to it means a vented system. No tanks, and a cylinder with an expansion vessel, several valves and a discharge pipe running outside, means unvented. If you are still unsure, an engineer will identify it in seconds and can tell you what servicing it needs.
How often does an unvented cylinder need servicing?
Annually. The safety devices are mechanical and can seize, scale up or lose their charge, so they need checking and exercising every year. It is a Building Regulations requirement rather than a manufacturer’s suggestion, and most cylinder warranties also depend on a documented annual service by a suitably qualified person.
Why is water dripping from the pipe outside my house?
On an unvented system that pipe is the discharge from the relief valves, so any water coming out of it means a safety device has opened. The usual cause is an expansion vessel that has lost its air charge and can no longer absorb the water as it heats. It needs an engineer, and the pipe must never be capped, extended incorrectly or blocked.
Can I change my immersion heater myself?
We would not advise it. It involves draining the cylinder, an electrical connection, and on an unvented cylinder working on a pressure vessel then refilling and repressurising it correctly. Getting the seal or thermostat wrong can cause a leak or an overheat condition. Any work on the gas boiler side is Gas Safe engineer work and cannot be done by a homeowner.
Why has my hot water gone lukewarm?
The usual suspects are scale on the coil or immersion element, a cylinder thermostat that has drifted, a motorised valve not opening properly, or a boiler fault. Because those need different fixes, and several involve the gas appliance, have it diagnosed properly rather than adjusting the thermostat and hoping.
Will poor mains pressure stop me having an unvented cylinder?
It may. An unvented cylinder delivers hot water at whatever the mains can supply, so a weak incoming supply gives disappointing results. That is why a proper installation starts by measuring flow rate and pressure at the stopcock. If the supply is not adequate, an engineer can talk you through alternatives that will work in your property.
Get Your Cylinder Looked At Properly
A hot water cylinder is easy to ignore right up to the morning it lets you down, and on an unvented system the annual service is a safety matter rather than housekeeping. If yours has never been checked, or the hot water is not what it was, book a visit. Ring Manchester Gas and Heating on 07960 042563. We serve Salford, Swinton, Worsley, Pendlebury, Seedley, Prestwich, and Kersal.
Cylinder Never Been Serviced?
Engineers based in Salford servicing unvented cylinders and diagnosing hot water faults across North Manchester. Call 07960 042563.