If your unvented hot water cylinder suddenly starts misbehaving, it can be worrying, especially when you hear strange noises or see water dripping where it should not. With a G3 compliant system, safety is critical, so it is important to know what you can safely check yourself and what must be left to a qualified engineer.
Safety first with unvented cylinders
Unvented hot water cylinders work under high pressure and store a large volume of hot water. They have several built-in safety devices designed to prevent overheating or over-pressurising.
Only engineers with G3 certification should adjust or replace safety components on an unvented cylinder. As a homeowner, you can do basic visual checks and simple controls, but you must not attempt to strip the unit down or isolate safety valves.
Never cap, block or plug any discharge pipe or tundish outlet. These are there to safely release excess pressure and temperature. Blocking them can create a serious risk.
Quick checks you can safely do at home
Before you call an engineer, there are a few simple checks you can carry out without tools. These can often rule out basic control or power issues and will also give your engineer useful information.
Check the programmer and thermostat settings
Confirm the immersion heater switch and fuses are on
Look for any obvious leaks around the cylinder and pipework
Note whether the tundish is dry, dripping or running
Listen for unusual hissing or gurgling at the discharge pipe
If anything looks unsafe, such as water pouring from the tundish or scorching hot pipework, turn the system off at the main isolator if safe to do so and contact a G3 qualified engineer immediately.
Common unvented cylinder problems and safe checks
No hot water at the taps
First, check the programmer or timeclock. Make sure the hot water is actually set to ‘on’ and that you have not accidentally put it into holiday or off mode. Also confirm room and cylinder thermostats are not set too low.
Next, check the immersion heater switch or spur near the cylinder. It should be on, with any neon indicator lit. If the switch is on but there is still no hot water, do not open the electrical cover or reset internal stats yourself. At this point, you need an engineer to test the immersion, thermostats and internal safety cut-outs.
Water too hot at the taps
Water that is uncomfortably hot can point to thermostat or mixing valve problems. First, check any visible cylinder thermostat on the side of the unit has not been turned up too high by mistake. It is normally set around 60 °C.
If the thermostat is at a sensible setting but you still get scalding water, this might involve the cylinder thermostat, thermal cut-out, or blending/mixing valves. These are safety-critical controls, so adjustment and replacement must be carried out by a G3 qualified engineer.
Dripping or running water from the tundish
The tundish is the small viewing window in the discharge pipe, usually close to the cylinder. It allows you to see if water is being discharged from safety valves. A quick drip during heat-up can be normal on some systems, especially when the water first starts to warm.
If the tundish drips frequently when the system is not heating, or runs with a steady trickle or flow, turn off the heat source and call an engineer. The issue could be a faulty pressure relief valve, temperature relief valve, expansion vessel or pressure reducing valve. Do not attempt to tighten or plug the tundish or discharge pipe.
Low pressure or weak flow at taps
Check other cold taps in the property first. If both hot and cold taps are weak, there may be a mains water pressure issue in your area. You can contact your water supplier to confirm.
If the cold taps are normal but the hot flow is poor, it could be a partially closed isolation valve, blocked filter, scale build-up or a fault with the pressure reducing valve on the cylinder. These involve pressurised components, so a qualified engineer must diagnose and fix the problem.
Noisy discharge or banging sounds
Occasional gentle gurgling as the system heats can be normal, but loud hissing, banging or hammering from the discharge pipe or around the cylinder needs checking. Make a note of when the noise happens and how long it lasts.
These symptoms can relate to high pressure, air in the system, poor pipe support or faulting safety valves. Because they are directly connected to pressure control, do not attempt to adjust any valves yourself. Report the noises clearly to your engineer.
Intermittent hot water or heating
If your hot water or heating comes and goes, start with the basics. Check the programmer is set correctly for each day of the week and that there are not oddly timed programmes. Ensure any wireless room thermostats or receivers have fresh batteries and good signal.
Intermittent issues that are not linked to controls can involve internal motorised valves, sensors or wiring centres. These all sit within the safety system for the cylinder and should be tested and repaired only by a competent professional.
Tasks only a qualified G3 engineer should handle
Certain parts of an unvented cylinder system must not be touched by homeowners. This includes pressure and temperature relief valves, expansion vessels, combined control groups and internal cylinder components.
These devices are designed to relieve excess pressure and temperature safely. Incorrect adjustment or isolation can stop them working when they are needed most, significantly increasing risk. Always use a G3 certified engineer for any repairs, replacements or system changes.
Servicing unvented cylinders and recognising overdue maintenance
Regular servicing keeps your unvented cylinder working efficiently and, more importantly, safely. In most cases, manufacturers recommend a full service every 12 months, and this is often a condition of the warranty.
During a service, a G3 engineer will typically check and test safety valves, examine the expansion vessel and recharge it if needed, confirm correct operating pressures and inspect filters and strainers. They will also check for signs of corrosion, leaks and scale, and verify that discharge pipework and the tundish are correctly installed and unobstructed.
Signs your cylinder service is overdue include frequent tundish dripping, inconsistent hot water temperatures, increasing noise from the cylinder, unexplained trips of the immersion or boiler supply and more than a year since the last professional inspection. Booking a service promptly can prevent more serious problems and reduce the risk of emergency breakdowns.
FAQs about unvented cylinder issues
Why is my tundish dripping?
A dripping tundish can be caused by normal expansion during heat-up, but regular or constant dripping usually indicates a pressure problem or a safety valve that is not sealing properly. The underlying cause could be an expansion vessel losing charge, a faulty pressure reducing valve or a worn pressure relief valve.
You can safely observe when it drips and how much, but do not touch the valves. If the dripping is frequent, runs in a steady stream or suddenly gets worse, turn off the heat source and arrange for a G3 qualified engineer to inspect the system.
Is it an emergency?
If water is flowing heavily through the tundish, if pipework or the cylinder is extremely hot to touch, or if you hear loud hissing from the discharge pipe, treat it as urgent. Turn off the heat source immediately if you can do so safely.
Then contact a qualified engineer as soon as possible and avoid using hot water until the system has been checked. For minor intermittent dripping with no other symptoms, it may not be an immediate emergency, but it should still be looked at promptly during a service visit.
Do unvented cylinders need servicing?
Yes, unvented cylinders absolutely require regular servicing. They are pressurised systems with built-in safety devices that must be tested and kept in good working order.
Annual servicing by a G3 certified engineer helps ensure safe operation, maintains efficiency and supports manufacturer warranty conditions. Skipping services can allow small issues to develop into major faults and may compromise safety features.
Book expert help for your unvented cylinder
If you are concerned about your unvented cylinder, or it is due a service, it is always safer to get it checked by a qualified professional. You can read more about installation and system options on our unvented cylinder service page.
To arrange a service or repair with Wood & Williams Ltd, call 01202028037 or send us a message via our contact page. We will assess your system, explain any issues in plain language and help keep your hot water both reliable and safe.