A lot of attention goes on what temperature to set the thermostat, and far less on when the heating is allowed to run at all. Yet the timer, or programmer, decides exactly that, and a schedule that quietly heats an empty house at three in the afternoon wastes gas no matter how carefully you have set the dial. Getting the timing right is one of the cheapest improvements you can make, because it costs nothing and the controls are already on the wall. This guide explains what the timer actually does, how to build a schedule that fits your day, and the settings that catch people out.
The short answer. The timer controls when the heating is allowed to come on; the thermostat controls how warm the house gets while it is allowed to run. Set the timer so the heating switches on shortly before you wake and shortly before you get home, and off again when you leave or go to bed. The house holds warmth for a while after the boiler stops, so you can usually switch off twenty to thirty minutes before you actually need to. Avoid heating the house at full temperature while it is empty all day.
What the timer controls, and what it does not
It helps to be clear about the division of labour between your two main controls. The timer, also called the programmer, decides the hours during which the heating system is permitted to fire. The room thermostat decides whether it actually needs to, by comparing the room temperature with your target and calling for heat only when the room is below it.
The two work together. During a timed on period the boiler does not run flat out the whole time; it fires until the thermostat is satisfied, then waits, then tops up as the house cools. Outside the timed periods the boiler stays off whatever the thermostat reads. So the timer sets the window, and the thermostat governs what happens inside it. If you only ever change one of them you are leaving the other doing nothing useful. For the temperature side of the equation, our guide to thermostat settings covers what number to aim for.
This also explains a common misunderstanding. Turning the thermostat up does not extend the hours of heating, and setting a longer timer window does not make the house warmer, only warm for longer. You need both set sensibly.
Knowing which controls you have
Central heating controls vary a lot by age, and what you can do depends on which you have. It is worth identifying yours before you start.
- A basic time switch. The oldest type, often a dial with tappets or a simple on/off/timed slider. It gives the same schedule every day and controls heating and hot water together. Limited, but you can still set sensible on and off times.
- A programmer with separate channels. Very common. It lets you set heating and hot water on different schedules, and usually offers two or three on periods a day. Many allow different times for weekdays and the weekend, which is worth using.
- A digital programmable room thermostat. Combines the timer and the thermostat in one unit, so you set a target temperature for each time block. This is the most flexible of the traditional controls.
- A smart thermostat. Adds app control, learning and features like optimum start, which we come to below. See smart thermostats for whether one is worth fitting.
If your system has thermostatic valves on the radiators as well, those handle room-by-room temperatures on top of all this. They do not switch the boiler on or off, they just throttle individual radiators, which is covered under radiator valves and zoning.
Building a schedule around your day
The principle is simple: heat the house for the hours you are in it and awake, and let it drop back the rest of the time. The detail is fitting that to your actual routine rather than a generic template.
Take a household out at work on weekdays. A schedule that fits might look like this.
| Period | Heating | Why |
|---|---|---|
| 06:30 to 08:00 | On | Warm for getting up and ready |
| 08:00 to 16:30 | Off or setback | House empty during the working day |
| 16:30 to 22:00 | On | Warm for the evening at home |
| 22:00 to 06:30 | Off or setback | Asleep, bedrooms cooler anyway |
An illustrative weekday pattern. Adjust the times to your own routine, and set later starts and longer on periods at the weekend if you are home.
Two refinements make this better than a blunt on/off. First, use the boiler's residual heat: schedule the morning off-time for when you leave, not later, and let the house coast warm through the last part of the morning routine. Second, set the heating to come on a little before you need it rather than at the moment you want the house warm, because a cold house takes time to reach temperature. How long depends on your home; a well insulated house might need twenty minutes, a draughty one an hour or more. The wider question of timed heating versus leaving it on is settled in is it cheaper to leave the heating on all day, and for most homes the timer wins.
Off completely, or a gentle setback?
During the empty and sleeping hours you have a choice between switching the heating fully off and letting it idle at a lower target, known as a setback. Which is better depends on your house.
In a reasonably insulated home, switching off completely during a short absence is fine and cheapest, because the house barely cools before the next on period. For longer empty stretches or in a poorly insulated home that goes cold quickly, a modest setback of two or three degrees, say holding 15 or 16C rather than off, can be the better choice. It stops the fabric and the air getting cold enough to invite condensation and damp, and it keeps the morning reheat short because you are not starting from stone cold.
A setback needs the right kind of control to do properly, either a programmable thermostat that lets you set a lower target for those hours, or a smart system. A plain on/off time switch cannot hold a setback, so with that you are choosing between on and off only. Overnight, many people set back rather than switch off entirely, partly for the damp reasons above and partly so the house is quick to warm in the morning.
Timing the hot water separately
If you have a hot water cylinder rather than a combi boiler, your programmer almost certainly controls the hot water on its own channel, and it deserves its own thought. A combi makes hot water on demand and has no cylinder to time, so this section does not apply to it.
With a cylinder, the aim is to heat a tankful shortly before you need it and then let it coast, rather than reheating it around the clock. A common pattern is a heating period early in the morning to give a hot tankful for showers, and perhaps a shorter top-up in the evening if the household uses a lot of hot water later in the day. A well lagged cylinder holds its heat for hours, so you rarely need more than that. There is no benefit to keeping the cylinder reheating all day, and a good cylinder jacket and pipe lagging cut the standing losses so the water stays hot longer between heat-ups. For the wider set of hot water savings, see hot water savings.
Optimum start and smart scheduling
Pick an on time and you face a small dilemma: set it too early and you heat the house before you need it, set it too late and the first half hour is chilly. Optimum start solves this. It is a feature on smart thermostats and some digital programmers where you tell the system the time you want the house to be warm, and it works out for itself when to fire the boiler, starting earlier on a cold morning and later on a mild one.
That moving start time is more efficient than a fixed one, because on milder days it does not waste the early heating a fixed schedule would have run. Smart systems add other tricks: control from your phone so you can push the schedule back if you are out late, and in some cases location sensing that holds the heating off until you are heading home. None of this changes the underlying logic of heating only when you are in and awake; it just hits the target more precisely. Whether the saving justifies the kit is the subject of smart thermostats.
The settings that quietly waste gas
A few timer habits cost money without anyone noticing.
- Leaving the holiday schedule running. A timer set for a working week keeps heating an empty house if your routine changes and you are now home, or away. Review it when your life does.
- The permanent override. Many programmers have an advance or boost button for a one-off top-up. Used as a habit it defeats the schedule entirely; if you are always overriding, the schedule itself is wrong and should be changed.
- Heating and hot water locked together. If your controls allow separate hot water timing, use it. Running the central heating just to get hot water in summer is pure waste.
- One schedule for every day. If your programmer supports weekday and weekend programs, set them. Heating the house from half six on a Saturday you will sleep until nine is money for nothing.
- Starting from cold every single time. A sensible overnight setback in a leaky home can cost less overall than letting it go fully cold and paying for a long, hard reheat each morning.
The bottom line
The timer is the control that decides when your heating is even allowed to run, and a schedule shaped around your real routine is one of the easiest savings in the house. Heat for the hours you are home and awake, switch off or set back when you are out or asleep, and start the heating a little before you need the warmth rather than at the moment you want it. Time the hot water on its own schedule if you have a cylinder, lean on a setback rather than a full switch-off in a home that goes cold fast, and let optimum start handle the timing for you if you have it. Pair a good schedule with a sensible thermostat setting and you are getting both halves of the controls working, instead of just one.