A heating timer that has failed, or that simply sits outside an ON period, will keep the boiler off even when the boiler itself is in good order. Start with the clock and the programme on the timer, then the room thermostat set point, because both can hold the heating back without any fault at the appliance. If those settings are right and nothing fires, the problem usually sits in the switching chain between the controls and the boiler, which is work for a Gas Safe registered engineer.
Heating timer not working: why the boiler stays off even though it is fine
Your boiler only fires when every control in the chain asks it to. The programmer or timer decides the hours, the room thermostat decides the temperature, and the boiler does as it is told. If the timer has stopped, lost its time or sits outside an ON period, the boiler stays silent and looks broken when it is not.
A programmer overrides the room thermostat, not the other way round. Worcester Bosch user instructions state that with the programmer off, the heating will not run whatever the thermostat is set to. On a combi boiler the hot taps usually still work, because a tap calls the boiler directly and bypasses the heating programme. That combination of warm water and cold radiators is one of the most common ways a control fault shows itself.
A boiler that heats water but never fires for the radiators is usually being told to stay off. The clock, the programme and the room thermostat come first. Only then does the wiring and valve chain between the controls and the boiler come into question.
Check the time and the programme on the heating timer first
The time shown on the timer is the first thing to look at when the heating will not come on. Viessmann points out that a digital timer runs every switching time from its own clock, so a wrong time shifts the whole programme. A mechanical timer works the same way: the dial has to sit at the current time of day for the pins or tappets to mean anything. Setting the clock and the programme is one of the few jobs on a heating system that belongs to you rather than to an engineer.
- The current time of day, and the day of the week on a seven day programmer
- Whether an ON period actually covers the time you are standing there
- The selector position: ON, Auto or Timed rather than Off
- The clock change in spring and autumn, which many timers do not follow on their own
- A recent power cut, after which Vaillant notes that timers can drop back to midnight and run at odd hours
- Switching times that someone has nudged by accident, which Ideal Heating lists as a common cause
Timer models differ a great deal in how they display and accept a programme. Viessmann recommends following the instructions for your own control rather than a general method, and that holds for every brand.
Timer set to ON but the radiators stay cold: is the room thermostat satisfied?
A room thermostat that is already satisfied will stop the heating even with the timer on. The thermostat measures air temperature in the room where it sits, not the temperature of your radiators or the boiler. Ideal Heating makes the point that on a mild day the set point can be reached before the heating ever runs. Turning the set point up to around 20 degrees is a fair test of whether the thermostat is the reason nothing happens.
A second clue comes from the boost or override button on the programmer. If the boiler fires on boost but never on the programme, the appliance and the valves are doing their job and the timed side is not. That points at the clock, the stored programme or the programmer itself rather than at the boiler.
Programmer or room thermostat: which control is holding the heating off?
The programmer sets when the heating may run and the room thermostat sets how warm the room gets. Telling them apart narrows down why the boiler will not come on.
| What to compare | Programmer or timer | Room thermostat |
|---|---|---|
| What it controls | The hours when heating, and hot water on a cylinder system, are allowed to run | The temperature a room is held at while a heating period is running |
| Typical symptom when it fails | Heating never starts, starts at odd hours, or only runs when you press boost | Heating runs on and on, or never starts although the clock shows an ON period |
| What you can check yourself | Current time, day, ON periods and the ON or Auto selector | Set point above the room temperature, around 20 degrees as a test |
| Where you find it | On the boiler case or on a wall plate beside it, with a dial or a display | Usually on a hall wall, mains wired or wireless with its own batteries |
| How a flat battery shows up | A digital timer with a spent backup cell can lose its programme after a power cut | A wireless thermostat with tired cells stops calling for heat at the boiler |
Glow-worm lists the same three points on a timer that refuses to bring the heating on: correct time and date, batteries with charge, and a programme that is actually set. Replacing a battery inside a control housing is not a householder check, so an engineer confirms it during a diagnosis.
What else you can check yourself before calling an engineer about cold radiators
Four checks are safe for a householder when the heating will not come on, and the timer is only one of them. The others are the boiler pressure gauge, the reset button and the radiators themselves. Anything that means opening the boiler case, touching gas, the flue or the wiring is work for a Gas Safe registered engineer.
- Pressure gauge: Ideal Heating gives roughly 1 to 2 bar in operation, and Baxi puts a cold system at about 1.0 to 1.5 bar
- Reset button: one press clears a lockout, and a boiler that locks out again has a fault behind it
- Radiators: cold at the top and warm at the bottom points to trapped air, which you can bleed once the system has cooled
- Radiator valves: a closed hand wheel or TRV keeps that one radiator cold while the rest of the house warms up
Our guide on what to check when the boiler has stopped sets out those four checks in order, with the limits of each one. If the needle on the gauge sits well below the normal band, low boiler pressure and what the gauge means explains what that reading does to the heating. Where the boiler fires and then shuts down again and again, the guide on a boiler that keeps locking out and needs resetting explains why a reset is not a repair.
What a Gas Safe registered engineer checks when the heating controls are suspect
An engineer looking at a failed timer starts with the switching chain, not with the boiler. On a system with a hot water cylinder, the mains supply, the programmer, the boiler and the motorised valve meet at a wiring centre, often fitted near the cylinder. Heatrae Sadia installation manuals for Megaflo cylinders show that layout, with the room thermostat wired back to the programmer. A dead programmer, a motorised valve that no longer moves or a loose connection in that centre all leave the boiler with no call for heat.
On a combi boiler the same symptom can come from the diverter valve, the part that sends hot water either to the radiators or to the taps. Vaillant states that a faulty diverter valve is work for a Gas Safe registered engineer and not something to tackle at home. Glow-worm describes the identical pattern of hot water without heating when that valve sticks. Our guide on hot water that works while the radiators stay cold goes through that symptom in detail.
A two channel programmer adds one more possibility on a cylinder system. Set to heating only, the hot water circuit stays off; set to hot water only, the radiators stay cold. Heatrae Sadia notes that straight swap of channels in its cylinder documentation, and it catches people out regularly.
Can you replace a heating programmer or timer yourself in England?
Replacing a heating programmer is not a job to take on yourself where it is wired into the control circuit of a gas boiler. Regulation 3(1) of the Gas Safety (Installation and Use) Regulations 1998, as amended, says no person may carry out work on a gas fitting unless they are competent to do so. HSE states that work on gas fittings in a home must be carried out by a Gas Safe registered engineer with the right qualifications, and that it is illegal for anyone else. That position applies in England.
The same applies when a smart thermostat takes the place of an old timer. The receiver has to be wired and paired to the boiler correctly, or the heating will behave worse than before. An engineer can also tell you whether the existing backplate suits a replacement control, which decides how much work the change really is.
Boiler Plus in England: a new boiler must come with working time and temperature control
Boiler Plus has required time and temperature control with every boiler installation in England since April 2018. Where a new boiler goes into an existing heating system, the installation includes a timer or programmer and a room thermostat, unless working controls are already in place. The standard also requires boiler interlock, which means the controls shut the boiler down once no room or cylinder is calling for heat. It sits in statutory guidance to Part L of the Building Regulations and applies in England only.
A dead programmer is therefore more than a comfort problem when a boiler change is on the cards. Our guide on the Boiler Plus regulations and the controls a new boiler must come with explains what is expected at installation. FM Power works from a free home survey and a written fixed price, so you know what the control work covers before anything starts. If the cold radiators turn out to sit alongside a tired cylinder and its controls, the page on Megaflo and unvented cylinders in London sets out what that work involves.
Sources
- Worcester Bosch - User instructions, wall hung gas fired condensing combination boiler
- Viessmann - How to set a timer on a boiler
- Vaillant - How to set a boiler timer
- Ideal Heating - Why is my boiler not working?
- Glow-worm - Why is my boiler giving me hot water but no heating?
- Vaillant - How to fix a boiler diverter valve
- Heatrae Sadia - MEGAFLO Eco SystemFit installation manual (wiring centre and motorised valve)
- Baxi - How to bleed a radiator in 8 easy steps
- The Gas Safety (Installation and Use) Regulations 1998
- HSE - Domestic gas: frequently asked questions
- GOV.UK - Boiler Plus: new standards for domestic boiler installations from April 2018