No Hot Water from an Unvented Cylinder: What to Check
Does the water stay cold, run out partway through a shower, or only heat when the radiators are on? Start with that description, then say which taps are affected and whether anything was recently repaired. Those details help when booking. Steam or suspected overheating needs urgent advice before troubleshooting.
Stop if the problem includes overheating or electrical danger
Steam, unusually hot discharge or suspected overheating needs urgent advice: turn off the heat sources at safe normal controls if possible and keep clear. Do not block discharge pipes or bypass safety controls. For burning smells, repeated electrical tripping or water at electrical fittings, stop using the affected equipment and seek professional help. Do not open covers or keep resetting it to obtain another shower.
Source: Megaflo Eco Slimline manufacturer safety instructions (PDF)
Match the behaviour before choosing a repair
If safe to observe, note which description fits: the hot tap has no flow; it flows but stays cold; it starts hot then runs cold; or it works only while the heating is on. Also say whether the problem affects one shower or every outlet. Record observations from ordinary use, not repeated tests that waste water or recreate a dangerous fault. If you do not know how the cylinder is heated, provide a label photograph and describe the controls you normally use.
Hot water only works when the radiators are on
In a public DIYUK discussion, a new resident reported this exact pattern but received conflicting explanations; no final repair was confirmed. Treat it as a symptom to report, not proof of a failed valve. Note whether selecting hot water alone at the normal programmer gets any response, and provide the cylinder and boiler models. The cylinder’s heat demand and controls need checking before choosing a replacement part.
Source: Public homeowner report: hot water only with heating (Reddit DIYUK; outcome unverified)
It heats, but one shower uses the hot water
Write down what happened before it ran cold: the heating programme, which labelled boost control you used, and other hot-water use. Include whether this is a recent change or how it has behaved since moving in. A public flat-owner report described short-lived hot water and an apparently ineffective boost, but did not establish the cause. A litre label or a shower duration alone is not enough to decide that the cylinder is undersized.
What the heating method changes
Direct cylinders use an internal heat source, typically immersion heating; indirect cylinders receive heat through a coil from another appliance. OSO separates no hot water, insufficient hot water and low temperature in its troubleshooting guidance. That distinction matters when describing a visit: checking an electric heater is not the same scope as tracing a boiler-fed hot-water demand. Use the manual for your model rather than assuming every cylinder has the same controls.
No water from the hot tap is a different report
Tell the booking team if there is no flow at all rather than calling it cold water. Include whether cold taps still work and whether a building water interruption or recent plumbing work preceded it. Ariston’s fault table treats no water separately from a failure to heat it. Do not order an immersion element on the strength of “no hot water” alone.
A new timer or control has not solved it
Keep the previous job record and describe what changed: did the indicator start working, did water heat once, and does the ordinary schedule now work? A tenant on DIYUK reported hot water continuing after a timer replacement despite an off setting; the thread did not confirm why. Before closing a repair enquiry, ask how the intended control operation has been checked. A working indicator by itself does not explain a recurring complaint.
Source: Public tenant report: unexpected operation after timer replacement (Reddit DIYUK; cause unverified)
A fault report you can send to an agent or repairer
Use this format: “The cylinder is [make/model if known]. Since [date], [which taps or shower] have [specific behaviour]. The normal hot-water setting is [label or schedule]. A visit on [date] changed [part or setting, if known], but [remaining problem]. There is/is not visible discharge or another safety concern. Access is through [location].” Add your postcode and contact details privately to the booking team. Leave unknowns blank rather than guessing.
What to agree before the visit
Ask for the assessment charge and what it covers, who can approve parts, and how an unresolved fault or return visit will be handled. For rented or managed properties, identify who can authorise work and provide access. If replacement is proposed, ask what finding makes a repair unsuitable. The goal is a clear diagnosis and dependable hot water, with an explanation of the controls you will use afterwards.
Common questions about this
Should I turn the immersion on if the boiler still heats the radiators?
First establish what normally heats your cylinder and follow its user instructions. Do not switch to another heat source if there is overheating, electrical tripping or a discharge fault; obtain advice.
Does a short hot shower prove I need a bigger cylinder?
No. Describe the change, heating schedule and use before it runs cold. Capacity should be assessed alongside operation rather than inferred from one shower.
What if I cannot identify the cylinder type?
Send the accessible model label and describe the normal controls when asked. You can request help without knowing the name of each valve or heater.
Related repairs and advice
Where to go next
If this matches what you are dealing with, our water heaters pages explain the service and how to request an assessment.