Troubleshooting “No Hot Water” in Unvented Cylinders: Step-by-Step Checks Before Calling an Engineer

Waking up to no hot water is frustrating, especially when your unvented cylinder has always been reliable. The good news is that many “no hot water” problems have simple explanations, and there are a few safe checks you can make before calling an engineer.

This guide focuses on unvented hot water cylinders commonly installed across the UK, such as Megaflo, Range Tribune, Heatrae Sadia, Telford, and others. The steps below will help you diagnose the likely cause and decide whether a professional visit is necessary.

1. Check Whether Your Immersion Heater Is Actually Powered

It sounds obvious, but it’s the most common cause. The immersion heater may just not be receiving power.

You can safely check:

• Is the switch turned on?
• Is the fuse OK?
• Is the timer/programmer set correctly?

If you’re on an Economy 7 tariff, remember that most of the heating occurs overnight. Running out of hot water in the evening might just mean the tank wasn’t set to reheat during the day.

2. Try Resetting the Thermal Cut-Out

Every UK unvented cylinder has a safety thermostat and a thermal cut-out. If the system overheats for any reason, the cut-out will trip and shut off the immersion heater to prevent damage.

You can reset this yourself:

• Switch off the immersion heater.
• Locate the small reset button on the thermostat (often under a removable cover).
• Press the reset button. You should feel a click.
• Turn the immersion back on.

If the cut-out keeps tripping, that’s a sign of a deeper problem, such as a failed thermostat or wiring issue. This requires an engineer.

3. Has the Immersion Element Failed?

Immersion elements eventually burn out. When they do, your cylinder won’t heat at all.

Common signs of a failed immersion element include:

• Water stays completely cold despite being left on for hours.
• No sound or vibration from the immersion when running.
• The fuse trips repeatedly.

Many UK cylinders have two immersion heaters (top and bottom). If you still get a small amount of hot water, your top element may be working while the bottom element has failed.

4. Check the Pressure and Expansion System

Unvented cylinders rely on controlled expansion and stable pressure. If pressure drops too low or the expansion vessel fails, the system may not heat properly.

Look for:

• Low pressure on the gauge (if your system has one).
• Water discharging through the tundish.
• The combination valve making continuous noise or hissing.

5. Is the Cylinder Actually Emptying Too Quickly?

Sometimes you do have hot water, just not enough. Possible causes include:

• Guests increasing hot water demand
• Long baths or multiple showers in a short period
• A timer not programmed for peak usage
• A cylinder size too small for your current household needs

6. When You Should Call a Professional Immediately

Unvented cylinders operate under high pressure and are regulated under UK Building Regulations (G3). If you’re experiencing any of the following, don’t try to handle it yourself:

• Continuous discharge from the tundish
• Persistent low pressure
• A leaking combination valve
• Burning or electrical smells
• Repeated tripping of the thermal cut-out
• Signs of overheating

Say Goodbye to No Hot Water

Many “no hot water” problems are simple fixes, like a tripped thermostat, incorrect timer settings, using too much hot water at a time, or a single failed immersion element. Taking a few minutes to run through the checks above can save time and unnecessary call-outs. But if anything seems unsafe or if the problem keeps occurring, contact a G3-qualified unvented cylinder engineer.

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There’s nothing worse than stepping into the shower and discovering you’ve got no hot water, especially when your unvented cylinder has always been reliable. Fortunately, most hot water failures are caused by simple faults that can be identified quickly with a few safe checks. Before calling out an engineer, it’s worth confirming whether your immersion heater is actually receiving power, whether the timer is set correctly, or if the thermal cut-out has tripped.

In other cases, the problem may be a failed immersion element or even a pressure issue. UK unvented cylinders have multiple safety features designed to shut the system down if something isn’t right, so a sudden lack of hot water doesn’t always mean major repairs are needed.

This guide walks you through all the essential homeowner checks, explains the most common causes of hot water loss, and highlights the situations where you must call a G3-qualified engineer straight away, especially if there’s discharge from the tundish or signs of overheating.

Read the full post for the complete step-by-step troubleshooting guide.

Source:

https://plumbers365.uk/hot-water-problems/

https://www.boilerguide.co.uk/common-hot-water-cylinder-problems