Radiators cold but hot water working? Check the thermostat, timer and trapped air first. Vaillant's advice on the usual causes and when to call an engineer.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
If you have hot water but no heating, the cause is usually the thermostat or timer, trapped air in the radiators, a frozen condensate pipe, or on a combi a stuck diverter valve. The first three you can usually check yourself; a diverter valve needs 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
Thermostat You may be able to fix this
Likely cause: Vaillant says the thermostat is a common reason the heating will not come on while hot water works.
What to do: Make sure the thermostat is on, set above the room temperature (British Gas suggests above 21°C to test it), and has fresh batteries.
Timer or programmer You may be able to fix this
Likely cause: The heating is not scheduled on, or the timer reset after a power cut.
What to do: Check the heating schedule and the clock on the programmer.
Radiators cold at the top You may be able to fix this
Likely cause: Air trapped in the system.
What to do: Bleed the radiators, then recheck the boiler pressure and top up if it has dropped below 1 bar.
Cold weather You may be able to fix this
Likely cause: Vaillant notes that a frozen condensate pipe can stop the boiler working properly in colder months.
What to do: Thaw an accessible pipe with warm (not boiling) water, then reset the boiler.
Combi: hot water fine, radiators never warm Engineer needed
Likely cause: A stuck diverter valve. Vaillant says this is not something you can fix yourself.
What to do: Call a Gas Safe registered 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.
Most often the thermostat or timer. Check it is on, set above room temperature and has batteries. Trapped air, a frozen condensate pipe or a stuck diverter valve are the other usual causes.
How do I know if my diverter valve is broken?
Hot water works but the radiators never heat, or the hot water is lukewarm. It needs a Gas Safe engineer.
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