The Boiler Works, So Why Is It Cold?
This is one of the most common calls we get. You can hear the boiler fire up, the pilot light is on, but the radiators stay cold or the hot water isn't heating. It's frustrating - and it's almost always a controls problem, not a boiler problem.
Here are the most common causes, roughly in order of how often we see them.
1. Faulty Zone Valve
The motorised zone valve directs hot water from the boiler to either your radiators or your hot water cylinder. If it's stuck closed, the water simply can't get through.
Signs: You can hear the boiler running, but the pipes after the valve are cold. You might hear the valve motor clicking or humming.
Fix: Replace the valve head or the entire valve body. Usually a 1-2 hour job.
2. Thermostat Not Calling for Heat
If your room thermostat isn't sending a signal to the boiler, the boiler may fire but the pump and valves won't activate.
Signs: The thermostat display seems fine, but nothing happens when you turn it up. Or the thermostat is blank/unresponsive.
Fix: Could be dead batteries (wireless stats), a wiring fault, or the thermostat itself needs replacing.
3. Programmer Not Switching
The programmer controls when your heating and hot water come on. If it's stuck or the clock is wrong, your system may not be in an "on" period.
Signs: The programmer display shows the wrong time, or the heating/hot water indicator lights don't come on when they should.
Fix: Try resetting the clock and checking the programme. If it's faulty, we can replace it.
4. Wiring Fault
Loose connections, corroded terminals, or damaged cables in the wiring centre can break the circuit between your controls and the boiler.
Signs: Intermittent faults - works sometimes, not others. Or a specific circuit (heating or hot water) doesn't work at all.
Fix: Systematic fault-finding to trace the break in the circuit, then repair or replace the affected wiring.
5. Pump Not Running
The circulation pump pushes hot water around the system. If it's stuck or has failed, the water heats in the boiler but doesn't circulate.
Signs: Boiler fires, pipes near the boiler get hot, but radiators stay cold. You can't hear or feel the pump vibrating.
Fix: Sometimes a stuck pump can be freed by gently tapping it or turning the manual spindle. Otherwise, replacement is needed (usually a plumber's job, but we can diagnose and confirm).
6. Cylinder Thermostat Fault
If your hot water cylinder thermostat has failed, it may not call for heat even when the water is cold.
Signs: Heating works fine, but hot water doesn't heat up.
Fix: Replace the cylinder thermostat - a quick and inexpensive job.
What Should You Do?
Before calling anyone, check these basics:
- Is the programmer set to the correct time and in an "on" period?
- Is the room thermostat turned up above the current room temperature?
- Are the thermostat batteries fresh (if wireless)?
If those are fine and you're still cold, it's time to call a controls specialist. That's us - get in touch or call for a free phone consultation. We'll talk through the symptoms and often narrow it down before we even visit.