Why faults happen, what you can safely observe, and when to stop using the heater. Illustrated with photographs from our own service and repair visits.
Know what you are looking at
What is a tundish?
Look for a small open-sided fitting in the cylinder’s safety discharge pipe. The opening lets you see water released by a safety valve.
In this photograph, it is the black fitting between the upper and lower copper pipes. It provides an air break; it does not control the cylinder pressure or cure a fault.
During ordinary operation it should not be continually running. Water passing through can be a warning that the expansion system, pressure controls or temperature controls need attention.
You should be able to see it. Keep the opening visible and accessible. If you cannot find it, ask your engineer to identify the discharge route. Do not remove covers or dismantle pipework to search.
Our own service photograph: the opening in the black tundish is where discharge can be seen.
Recognise the difference
A fused spur or a water-heater switch?
They can look similar. The removable fuse drawer is the important clue on a 13 A fused connection unit.
Fused spur from one of our service visits.13 A fused spur · needs checking
No obvious burn marks
The small fuse drawer holds a cartridge fuse. An undamaged-looking face does not prove that the terminals or fuse contacts behind it are sound.
Our own repair photograph: a blackened fuse holder removed by the engineer.Heat damage · stop using
A burnt fuse holder
Scorching and melted plastic are signs of damage, not normal wear. Damage may be hidden inside the drawer before it is obvious on the faceplate. Do not pull out the fuse holder to inspect it yourself.
Our own installation photograph: switch marked “Water Heater”, without a fuse drawer.20 A double-pole switch
No fuse in the switch
This type isolates live and neutral together. Circuit protection is provided elsewhere. The faceplate alone cannot establish whether the full installation is safe.
Why a 13 A spur can be a problem
A 3 kW element rated at 240 V draws about 12.5 A at that voltage, often for a prolonged heating period. A loose connection, worn contact or poor contact at the fuse can create local resistance and heat. Replacing a fuse alone will not repair damaged contacts or wiring.
A 13 A fused spur is not suitable where the cylinder manufacturer specifies a minimum 20 A double-pole isolator. For example, the OSO SX iQ manual specifies that arrangement. If your cylinder has fused spurs, have the exact model’s instructions and the electrical installation checked.
This is not a claim that every 13 A fused connection unit is unsuitable for every water heater. Suitability depends on the appliance instructions and circuit design. Equally, fitting a 20 A switch does not upgrade the rating of the cable, programmer or protective device.
If you already have the unfused “Water Heater” type: include it in servicing checks too. Its rating, connections, cable, circuit protection and condition must be verified. Never replace a fused spur with an unfused switch as a DIY shortcut.
Burning smell, buzzing, discolouration or melting? Stop using the heater. Isolate its supply only if you can do so safely, without touching damaged equipment, and arrange an electrical inspection. Do not remove covers, fit a larger fuse or repeatedly reset a tripping circuit. If there is fire or smoke, leave the property and call 999.
Understand the symptom
Six warning signs, explained.
One symptom can have several causes. These observations help an engineer investigate; they do not confirm a diagnosis.
01 · Discharge
Water dripping or running through the tundish
Water expands as it heats. An external expansion vessel, or an internal air space on some cylinders, provides room for that expansion. If this provision is lost, pressure can rise until a relief valve opens.
A valve that does not reseat properly, or an inlet pressure-control fault, can also cause discharge. The tundish shows the symptom; it is not necessarily the faulty part.
External vessels provide expansion space. Other cylinder designs use an internal air pocket.
Useful observations
Does it drip only while heating, continuously, or after taps close? Note the pattern from a safe distance. Do not touch discharge water to check its temperature.
Continuous hot discharge, steam or unusually hot tap water needs urgent attention. Switch off the heat source if safe and seek urgent competent assistance. Never plug the tundish, cap a relief pipe or interfere with a safety valve.
02 · Heating
No hot water, or only enough for one short shower
The cylinder may not have received its scheduled heating supply. A timer with the wrong time, an off-peak supply issue, a failed element or thermostat can all leave the water cold.
If the upper boost heater works but the lower heater does not, only the upper part of the cylinder may be heated. The result can be a small amount of hot water that quickly runs out.
Check the displayed programme and normal user settings; leave internal electrical checks to an engineer.
What you can safely check
Read the programmer clock, heating schedule and any visible error message. Check your electricity tariff and expected heating period. Tell the engineer whether boost still works and when hot water last worked normally.
An indicator light shows a control state, not proof that an element is actually heating. Do not open an immersion cover or reset an internal thermal cut-out yourself.
03 · Electrical
A hot switch, buzzing or a burning smell
Heat can build at a poor terminal or fuse contact while the immersion carries its heating current. Melting, a brown patch or a hot-plastic smell can indicate developing damage.
A damaged spur may stop supplying the heater, so an apparent “no hot water” fault can originate at the switch rather than the cylinder.
Actual heat damage found during our repair work.
Stop using affected equipment and arrange an electrical inspection. Do not touch it to test how hot it is. An undamaged faceplate does not rule out a fault behind it.
04 · Water flow
Weak flow, changing pressure or a bang when taps shut
Your unvented cylinder relies on the incoming mains supply. A restriction at the inlet valve or strainer, pressure-control trouble, or demand from several outlets can reduce flow. A sudden stop in moving water can produce a pressure surge known as water hammer.
Compare outlets before assuming the cylinder is faulty
Is the problem at every hot tap, or only one shower? Are cold taps also affected? If just one shower is weak while other outlets work normally, its cartridge, hose or local filter may need investigation.
Pressure is the force behind the water; flow is how much comes out. Record which outlets are affected and whether the problem changes when another tap runs. Do not adjust the pressure-reducing valve yourself.
05 · Leaks
Moisture, staining or corrosion around the cylinder
A valve joint, immersion seal or pipe connection can leak without the cylinder shell itself having failed. Repeated dampness and deposits around a fitting can help identify a slow leak. Water can also travel along pipework, so the wettest spot is not always the source.
What helps identify the cause
Photograph the first visible wet area, nearby joints and the wider cupboard. Note whether it appears during heating, after water use or all the time. Keep stored belongings clear of the affected area.
If water is near switches, cables or electrical covers, do not touch them. Keep clear and seek urgent help. Do not tighten fittings on a hot, pressurised cylinder yourself.
06 · Protection
A circuit keeps tripping, or the heater cuts out
A circuit breaker, RCD and thermal cut-out perform different jobs. Electrical trips may involve a damaged element, moisture, wiring faults or excessive demand. A thermal cut-out responds to excessive temperature or a control fault.
The cause needs investigation before the heater is put back into normal service. Repeatedly restoring power can conceal a fault without fixing it.
Inside the immersion cover during professional work. Homeowners should leave the cover in place.
Note which device trips and whether it happens immediately or after heating has been running. Photograph any accessible label or error display without opening equipment. Never bypass a cut-out or increase a fuse rating.
Heating controls & electricity use
Why permanent boost can leave you paying more.
The lower immersion usually heats the main stored volume. An upper immersion commonly supplies a smaller top-up when the main heating period has not provided enough hot water. Your cylinder and wiring may differ, so identify your controls before changing settings.
Leaving boost selected can make it reheat whenever its thermostat calls, potentially at your daytime rate. With some controllers, continuous mode heats the main element instead. Check the controller model rather than assuming what its label means.
If you have a time-of-use tariff, check its actual cheap-rate hours and the programmer’s clock, including changes after a power cut or clock change. A suitable controller can schedule main heating and limit boost duration.
Our installation photograph: a programmer provides scheduled heating and a controlled boost.
Do not assume both immersions should run together
Two elements rated at 3 kW each can create about twice the instantaneous demand of one. A shared switch, controller or cable may not be designed for that load.
One element3 kW · about 12.5 A at 240 V
Two together6 kW · about 25 A at 240 V
These figures illustrate rated demand, not the cost of a full heating cycle. Until the manufacturer’s instructions and circuit design have been verified, avoid running both together. Correctly designed systems may permit it; the arrangement must be checked.
Prevent problems
What an annual service should help you understand.
A service should leave you knowing where your tundish is, how your heating controls work, what expansion provision your cylinder uses, and which defects need attention. Keep the service record with your cylinder instructions.
Safety & discharge
Valve operation, visible discharge and the accessible discharge route should be checked against the installation requirements.
Expansion & condition
The engineer checks the expansion arrangement, leaks and component condition. Vessel pressures and air-pocket procedures are model-specific; one setting does not suit every cylinder.
Controls & records
Heating schedules, boost operation and accessible electrical components deserve attention, with findings recorded and any required electrical testing identified.
Useful photos to send if you need help
The cylinder label, a wide view of the cupboard, tundish, switch fronts, programmer display and the visible fault. Keep covers closed and stand clear of hot discharge or damaged electrics.
Know someone with an apartment hot-water cylinder? Share this guide so they can recognise their tundish, check their heating schedule and spot warning signs early.
Read the original guidance.
Use the manual for your exact cylinder model and production year. This guide explains symptoms; repairs, electrical alterations and safety-valve work require competent people.
OSO Super Xpress SX iQ, 2017–2019 manual — model-specific electrical installation, servicing and fault guidance. Check other OSO models against their own manual.
Photographs are from Manchester Pipe and Site Services’ own work. Close-up views help identify components; they are not certificates of the installation’s condition.
