I'll give the specifics of my set up, but I ask this in the hope of getting a more general answer too, because it's more helpful to more people.

I live in the UK, and a typical central heating set up is:

  • a boiler, with some settings - typically on, off, and perhaps a timer
  • the timer itself, it can be separate
  • the thermostat, usually located in a hallway
  • radiators, each with their own heat setting

I've just moved into a flat with another person. Bills are included so it seems they've been running the boiler a lot, but it's starting to get warm outside so there should be less need, and it's in my room and noisy!

I'd like:

  • Enough hot water for 2 hot showers in the morning (one for me, one for him).
  • Any other hot water needed, like washing up or the washing machine.
  • The radiator in the bathroom to be able to get warm, as it's where towels hang and closes are dried.
  • all the above with the boiler doing as little as possible.

What's the best combination of settings to achieve this? I'd say this is a fairly normal set of wishes, and the available settings are complex, so it'd be good to get a general idea.

If you wish to be more specific, here are some extra details of my set up:


an Ideal isar (I can't find the model number). It has an on/off button, two dials, one marked for water and one for radiators, I've set both to min.


Danfoss 103E7. It has 4 settings, on/off/auto/all-day, and 3 other buttons: a select button for the previous settings, a +1 hour, and a MAN. It's set to auto.


I've put this below 18˚C.


They all have a 0-5 on them. I've put the bathroom on 3, and the rest to 0.

Having said all that, the boiler still seems to be making a lot of effort during the day and night.

Any help or insights are much appreciated.

  • Is there a hot-water tank, or is it a combi boiler?
    – 410 gone
    May 5, 2014 at 11:58
  • This is probably a silly question, but is the bathroom towelrail heated with hot water, or is it electric?
    – Flyto
    May 5, 2014 at 12:04
  • @EnergyNumbers I don't see a tank anywhere so I'm guessing it's a combi.
    – ian
    May 5, 2014 at 12:24
  • @SimonW not silly at all, because there's no heated towel rail, just a radiator! :) Apologies if I wasn't clear.
    – ian
    May 5, 2014 at 12:25

2 Answers 2


OK, given the clarifications in comments (that there's no hot water tank, so this is presumably a combi boiler doing "instant" water heating, and that the bathroom has a radiator and no towel rail):

Hot water

There is no hot water storage - the boiler warms it at the time that you run the tap - so timings are irrelevant to this. The only variable left is the temperature that you would like hot water to be provided at, which is probably the dial marked for water on the boiler is. This is partly down to personal taste, but as a starting point I usually tune it so that the thing that I use the hottest water for - in my case, washing up - can be done with water straight from the hot tap. But, it probably makes little difference in energy terms. Be careful about setting it too high, or you may be able to scald yourselves on tapwater.

One question that I forgot to ask in comments was whether the shower actually uses hot water from the boiler, or whether it does its own electric water heating - many do - but this doesn't make much practical difference, because there's little you can do about it either way.


You have a few more controls to play with here.

Set the timer for when you would like the heating to be on; this will presumably require negotiation. If you both go out during the day then I suggest a period in the evening (for comfort, and for having a warm bedroom when you go to bed) and a period just before you get up in the morning (so you're not shivering when leaving your duvet!). There's no point in heating a house when nobody is there, and unless you've in a very cold climate (or there are health reasons), no point in heating a house when everybody is asleep. When choosing the times, bear in mind that the house will take some time to warm up from the start time that you set - perhaps anywhere between 10 and 60 minutes, depending on many factors. Some experimentation will be needed.

Note that contrary to myth, it is not more energy-efficient to keep a house warm throughout the day than to let it cool off when unoccupied.

The dial for heating on the boiler will control the temperature that water is heated to when sent to the radiators. Think of this as the rate at which the boiler can transfer heat into your rooms. The higher this is, the more "power" the system will have, the faster it will respond, and the better it will cope with cold days, but the less energy efficient it will be (because heat leaks faster from hotter pipes). You may find that having this set high causes a "cycling" behaviour where the boiler runs for many short periods with gaps inbetween, and that having it set lower means that it is on more constantly, at a lower power. This may be important when you're considering the noise in your bedroom, and so long as you don't put it so low that the system can't heat the house, you can probably tweak this without your housemate even noticing! See also this question: Setting radiator temperature to save energy

The third control that you have here is the overall house thermostat. If this instrument thinks that the house is warm enough, then the heating will be switched off - regardless of what the individual thermostats on the radiators reckon. If it's been sited well this shouldn't be a problem. You should use this to set the overall temperature that you would like your house to be at.

The fourth control that you have here is the individual thermostats on the radiators. You can use these to control the relative temperatures between different rooms, allowing some variation from the setting of the main thermostat. You'll only get these right through experimentation and (in shared areas) negotiation, but as a starting point I would have the bedrooms slightly cooler than the living areas. Sometimes these devices are labelled with temperatures (Fahrenheit or Celsius) and sometimes just with numbers. If you have temperatures, ignore them - these things aren't usually very well calibrated - and just go with trial and error :-)

Now, in summer, you could just rely on the thermostats to keep the heating off... but if the temperature dropped slightly below the hall thermostat's setting, it would fire up the boiler and burn a load of fuel to raise it back again, when you or I would know that the sun would do that in half an hour anyway. So you may choose to turn the heating off completely. If you want to do this, do it by turning the heating dial to minimum, and then it will probably never come on. Do not use the boiler's overall "off" switch, as that will probably turn off water heating as well.


This is the tricky bit. In winter, it's easy, as the bathroom radiator will be warm. In summer it's trickier. You could leave the heating on, set the house thermostat high, and set every other radiator thermostat to minimum so that only the bathroom radiator comes on. But that would be quite wasteful. Instead, remember that you don't always need heat to dry things - airflow can be helpful too. Can you hang your towel up by an open window or something :-)

  • Much appreciated.
    – ian
    May 8, 2014 at 8:07

It will vary, depending on the system.

If there's a hot water tank, then outside the heating season (i.e. this applies to May-September) normally you want the boiler on just long enough to give you enough hot water, when you need it. That might mean having the timer set to put the boiler on for an hour or two each day.

The water temperature should be set to high (above 55 °C). Maybe just set it to max, unless you've got some old-fashioned non-mixer taps, in which case, set the temp to around 55 °C.

Then set the central thermostat to its lowest value (probably 10 ° C). Then the boiler shouldn't turn on for central heating at all.

The Ideal Isar seems to have a bad reputation for circuit-board failures, so it may not behave as you expect. This should be picked up by the annual inspection - ask to see the most recent inspection log.

If you don't have a hot-water tank, experiment with the timer. Turn it off, wait 20 minutes, and then see if the boiler still kicks in when you run a tap for hot water. The timer probably only controls the central heating, so you can have it turned off for all of May to September.

Many boilers cycle - that is they'll run for a few seconds every half-hour or so - as part of their maintenance regime. It's annoying, but it uses little energy and is a sign of normal operation.

Having the bills included can lead to frivolous use of energy: I've seen many people just open windows rather than turn the heating down! That will need careful and tactful negotiation with your flatmate.


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