Ideal ICOS Boiler

9 Mar 2007
Reaction score
United Kingdom
I have the following irritating issue with my boiler:

Even with the thermostat set to minium (zero) and the timer off, the boiler springs to life, every 10 minutes or so!! And all through the night.

The boiler heats the radiators & hot water ok but it seems to have a mind of its own.

The first sign of trouble happened when the radiators remained on and wouldn't cool or go off, even though the thermostat and timer were off. An engineer attended and reset the boiler. Now the boiler just seems to come on/off randomly. I have tried pressing the reset button on the front but this didn't seem to do anything, I have also power cycled the unit. The boilers status is "c" and remains flashing "c".

I suspect the timer or thermostat is on the blink??

Any advice greatly appreciated.

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Is there anything in the manual about the preheat function?
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C shows the boiler is being asked for heat. C will not flash. light next to C will flash slowly whilst boiler is up to temp and pump still running, or flash quickly when in an ignition attempt.

Reset button will not work unless there is a 'F' fault on the display and boiler has gone off.

I would suspect external control fault, most likely 3 port valve head stuck or micro switch broken.

Why didn't your repairer fault find whilst there. Does he know about Ideal boilers :?: :rolleyes:
Thanks for all comments.

Your right, the "C" doesn't flash & it is the LED next to it flashing.

How do I locate the 3 port valve head or micro switch broken? Is the timer fitted to the wall?
A frost stat is connected to the boiler and detects low temperature, causing the boiler to fire up so it doesn't freeze, if your boiler is in the garage or an unheated area it should have frost protection, if it's in your kitchen/utility it won't.
I seem to remember in the mists of time hearing about these delightful boilers :eek: having issues with Honeywell Yplan valves which actually can pass over 100 volts back to the boiler like a back feed.

Some boiler PCB's are so sensetive that they think its a demand and fire up.

Dont shoot me down as I cannot remember the specifics sorry.

Turn heating mains off, remove bottom plate from boiler, 2 screws, pull out mains terminal block and open. Remove switched live and make safe (on L1 or L2 I think). Put terminal block back in, turn heating on again. After about 10 secs boiler should go to standby '0' unless pre-heat 'T' shows then may take a minute.

if does above boiler ok and external control problem ;)
ideal have had a lot of problems with the pcbs on these and the isar. what colour is the display? red (old) or green (newer version). just being nosey here, the pcb probs were with both types, just wondered how new it is?
I have exactly the same problem on an M3080 boiler. From this thread it would appear that it could be several things. I have removed the live feed from the terminal block and tested it as suggest (it's L2 by the way) and it definitely goes to "0" ok with the wire out. So that suggests an external control problem right?

I have looked at the stat and it still clicks so I'm assuming it's ok. The timer seems to operate ok in all other respects but it could be that. Then there's the suggestion that it's the 3 port valve head.

I'm an electrician not a plumber, so not really sure where to go next. Don't know how to change the 3 port valve head, and don't want to buy a timer or stat without knowing that that is what it is.

By the way, even when the boiler shows "0" the water still sounds like it's going around the system in the airing cupboard. It was this going all night that first alerted me to the problem??

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