Heating works but no hot water, or only lukewarm water? The usual causes for combi and cylinder systems, safe checks you can do and when to call an engineer.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
If the heating works but there is no hot water, check the hot water settings and timer first. On a combi boiler, a stuck diverter valve is a common cause; on a system with a hot water cylinder, check the timer, cylinder thermostat and immersion heater. Diverter valve and motorised valve faults need a Gas Safe registered engineer.
Smell gas, or a carbon monoxide alarm is sounding? Do not reset the boiler. Open windows, turn off the gas at the meter if it is safe to do so, leave the building and call the free gas emergency line on 0800 111 999. What to do →
What it could be and what to do
Hot water settings or timer You may be able to fix this
Likely cause: The hot water may be switched off, set too low or scheduled off. Timers can reset after a power cut.
What to do: Check the hot water setting on the boiler and the programmer, and set the time again after a power cut.
Likely cause: Vaillant says a faulty diverter valve, which sends hot water either to the radiators or the taps, can leave the heating working while the taps stay cold.
What to do: Not a DIY repair. Vaillant recommends a Gas Safe registered engineer clean or replace the valve.
Lukewarm water, or hot water only when the heating is on Engineer needed
Likely cause: Vaillant says a stuck diverter valve can let some hot water pass to the radiators, giving lukewarm taps, or only send hot water to the taps while the heating is on.
What to do: Note when it happens and call a Gas Safe engineer.
Boiler not firing at all You may be able to fix this
Likely cause: Low pressure, a frozen condensate pipe, no gas supply or a lockout.
What to do: Check the pressure, the condensate pipe in freezing weather, your gas supply and meter credit, then reset once.
Cylinder system: no hot water from the tank You may be able to fix this
Likely cause: The hot water timer, cylinder thermostat or motorised valve on a system or regular boiler.
What to do: Check the hot water timer and the cylinder thermostat setting. If you have an immersion heater, it can give you hot water meanwhile. Valve faults need an engineer.
Safe checks you can do for most codes
These are the only checks we suggest without an engineer. Never remove the boiler casing.
Pressure: look at the gauge. Most boilers need about 1 to 1.5 bar when cold. If it is low, top up using the filling loop as shown in your manual.
Gas supply: check your cooker or hob works, and that a prepayment meter has credit.
Frozen condensate pipe: in freezing weather, warm the white plastic pipe outside with warm (not boiling) water or a hot-water bottle, only where you can reach it from the ground. How to thaw it safely →
One reset: press reset once, as your manual describes. If the code comes back, stop and call an engineer; repeated resets can lock the boiler out.
Controls: check the programmer, thermostat and any smart controls are calling for heat.
Code still showing? Our Gas Safe engineers can fix it.
Boiler repair labour is £90–£350 in standard hours, plus the callout from £75 (out of hours £100–£180). The price is agreed before work starts. Tell us the make, model and code when you call.
Our emergency line is open 24 hours a day, 7 days a week.
Every boiler is different. Check your boiler’s own manual. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why do I have heating but no hot water on my combi?
Often the diverter valve, which switches the boiler's hot water between radiators and taps. Check the hot water settings first; a valve fault needs a Gas Safe engineer.
Why is my hot water only lukewarm?
On a combi, a stuck diverter valve can send some of the hot water to the radiators. It needs an engineer to clean or replace it.
Is no hot water an emergency?
Not usually, unless someone vulnerable is in the home. Call 0131 210 0505 and we will advise whether it can wait for a daytime visit.
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