We have the central heating programmed to come on at certain times but recently the boiler has been firing up outside of the programmed times.

When I look at the programmer there is nothing in it (the line to say central heating or hot water is on) to say that the CH is on but the boiler is clearly firing away. I go down the stairs and turn the boiler off manually but I can still hear what seems to be a a pump whirring away upstairs in the airing cupboard.

We’re looking at getting a combi system fitted in the summer anyway but I’m not liking the idea of wasting heat when we’re not in so would like to figure out what’s going on.

ours makes whirry noises even when turned off etc, I assume it’s just a venting mechanism etc.

I thought that but the boiler is lit up and the house is like a sauna. Can’t touch the radiators they are so hot. Thankfully no kids to burn themselves.

My combi randomly fired last summer. It was caused by a tiny worn out o-ring, with water seeping into a switch.

External wiring fault / motorised valve / programmer / room stat (I assume you haven’t got a frost stat and if you have it is locatesd properly and wired in with a pipe stat).

Get a multi meter and trace where the live feed is coming from, pretty simple if you can use a multi meter that is.

That is assuming it is a constant fault and not intermittent.

There should be a fused switch somewhere that provides power to the boiler circuit. This will provide power to the timer, boiler and external pump. The timer will switch on at the predetermined time (or permanently is set to do so). The thermostat will decide if there is a demand for heat and switch the power on to the boiler and pump.

By turning the boiler off you are only turning off the burner, not necessarily the pump, which is being controlled by the timer/stat.

To turn the heating off completely you will need to find the fused switch.

Now, from what you’re saying, it could be a faulty programmer that is permanently on. However, if the rads are extremely hot then it could be a thermostat problem, though it doesn’t account for why the boiler doesn’t switch off.

Another thought – is your hot water programmed to stay on all day? If so, you may have the motorized valve that switches off the central heating flow stuck open. In this case the stat will have no effect as demand is being dictated by the temperature of the hot water cylinder.

Central heating systems are many and varied are the ways to wire them. It’s difficult to say without seeing it.

If you’re in Pembrokeshire I’ll come and have a look.

Rich.

I’ll dig out the fluke! Problem is it has been intermittent. It started doing it last autumn but then stopped. Just got back from a weeks holiday and its doing it again. Grrrr.

Thanks for the offer but I’m miles away in hampshire. The hot water is set to come on roughly the same time as the CH so it’s not on permanently. I’ll see if I can track down a new programmer and swap them over. That way I should be able to narrow it down.

Where is your boiler located? If its in a garage or somewhere like that then it will have a frost stat which will fire up the boiler when it gets down to a certain temperature to prevent the water within it from freezing. Boilers located inside houses don’t have or need frost stats, only if its in a garage or somewhere where there is a risk of the surrounding temperature dropping to zero.

the boiler is inside the house just off the kitchen.

I had the same intermittent problem. Turned out to be the motorised valve. The plumber told me what points to check with the multimeter so I could diagnose it for him

If you’ve got a FLuke, use it should be reasonably easy to find, dig out some wiring diagrams from internet and follow them.

Pump will continue running for a while after boiler goes off btw. (pump overrun).