When heating works and hot water does not, the boiler is firing and the gas or oil side is fine. The fault sits in whatever decides where that heat goes. On a combi that is usually the diverter valve. On a cylinder system it is usually a motorised valve, the cylinder coil or the thermostat calling incorrectly.
Knowing which system you have changes the likely repair and the likely cost, so start there.
Combi boilers: the diverter valve
A combi diverts flow between the radiators and the plate heat exchanger. When the diverter sticks in the heating position, radiators stay hot and the tap runs cold or lukewarm. Sometimes you get a brief burst of hot water then nothing — that is the classic stuck-diverter signature.
Debris from an unfiltered system is the usual culprit, which is why diverter failures cluster on systems with no magnetic filter fitted.
Cylinder systems: valves, coils and thermostats
- A failed motorised (mid-position or two-port) valve head will not open the hot water circuit.
- The cylinder thermostat can fail closed, so the boiler is never asked for hot water.
- A scaled or blocked coil transfers heat far too slowly — water is warm, never hot.
- On S-plan systems, a wiring-centre fault can drop just the hot water leg.
Unvented cylinders need a G3-qualified engineer
Unvented cylinders are pressurised and carry safety components — expansion vessel, tundish, temperature and pressure relief. Work on them is restricted to engineers holding the unvented (G3) qualification. If your hot water fault is on an unvented cylinder, that is not general-plumber territory.
What to check before you call
- Is the programmer set to hot water on, and is the timer actually in a hot-water window?
- Does the cylinder thermostat dial sit at 60°C or above?
- Does hot water improve at all after 20 minutes of the boiler running?
- Any gurgling, hammering or a warm cylinder that never gets properly hot?
Frequently asked questions
- Is a diverter valve repair or a replacement better?
- On most modern combis the cartridge or the whole valve body is replaced rather than repaired. It is usually a same-visit job when the part is common; less common models sometimes need a next-day part.
- Why is my hot water only lukewarm rather than cold?
- Lukewarm normally means heat is transferring, but poorly — a scaled plate exchanger on a combi, or a sludged cylinder coil. Cold usually means the circuit is not opening at all.
- Could it just be the thermostat setting?
- It can be, and it costs nothing to check. Cylinder thermostats set below 60°C also raise a Legionella risk, so that setting matters for more than comfort.
- Do you carry common parts on the van?
- We carry the common diverter cartridges, valve heads, thermostats and seals for the boiler ranges we see most often locally. Anything unusual we order and return.
Get hot water back without guessing at parts
Tell us the boiler make and whether you have a cylinder — we will bring the likely part with us.