Ideal icos he18 blowing fuse

10 Feb 2017
Reaction score
United Kingdom

I've just replaced the pcb on our old icos he18 boiler and now its blowing the 2amp quick blow fuse in wiring centre (not the fuse in the Pcb). We tested fan resistance before changing and harness for earth leak/cross continuity. The PCB was replaced because the boiler was completely dead (no lights or activity).

I've checked all wires to PCB for earth fault. Gas valve shows 4.2k ohms across 2 wires. Flow thermostat seems to be ok as well with resistance decreasing with heat and within specs as per manual.

It doesn't appear to be externals causing the fuse to blow - when I switch on at the spur the pump runs with demand from hw cylinder stat. Moving the mid position valve manually also doesn't cause the fuse to blow.

It's only as the boiler starts it's ignition sequence that the fuse blows. I've pulled the gas valve and the spark gen connections and it still blows.

Pulling the flow thermostat and flue thermistor gives LE error on display and doesn't blow the fuse but it never starts the ignition sequence so I'm not sure this tells me anything. If I pull the connections to the overheat thermostat, the ignition sequence runs (I can hear the gas valve and the ignition sparking), the fuse doesn't blow but the boiler doesn't run due to stat being unplugged (gives LE on display).

Could it be the overheat stat causing fuse to blow? The other part that I can't eliminate is the flame detection because the ignition sequence doesn't run at all with this disconnected.

I don't want to start changing loads of parts with this boiler, I just want to keep it running for a few more months before it's replaced in the summer. i did try to get ideal to do a fixed price repair but they don't touch anything over 10 years old :( I realise we are probably lucky for it to have lasted as long as it has.

Any suggestions are much appreciated.
Sponsored Links
We are not permitted to give DIY on gas work.

Your boiler like most is room sealed and you should not have opened it.

Nor should you have disturbed the fan as that is part of the combustion process.

The only correct advice is to call an independent boiler engineer who specialises in repairs.

Hi Tony, I have tried to get somebody to look at the boiler but they suggested replacing it (after suggesting to replace the Pcb) given the model and age of the boiler.

DP, thanks for the suggestion. I will try removing the pump, however I'm not convinced this is the issue as it happily runs with demand from the cylinder stat before the boiler fires up and the fuse doesn't blow (I.e before I turn the boiler on).
Sponsored Links
Dodso, this boiler by only a small number of repairers me included at one time. Attended makers training course to find boiler is not that bad, in fact once one starts working on it, it is quite a good boiler.

Then you have operatives who will push for a new boiler if pcb replacement (often not the faulty component) does not cure the fault

Pulling leads off the overheat stat should not sequence ignition. I may well be wrong in this instance, but the overheat stat maintains a constant short circuit to allow a sequence to run. Boiler overheats, first thing that needs shutting down is ignition process but to run cooling components like pump and sometimes a fan

Check you fuses again. If I am not mistaken, pcb fuse is greater than 2 amp that is in the wiring Center :sneaky:

What I have suggested should not need opening up the boiler:whistle:
Thanks DP, the fuse in the Pcb is a 3.15at - still intact. I believe the original Pcb was indeed dead. There was a smell of burning from the casing. The old unit was black the new one is orange, which I believe is meant to be more robust.

When I tested the oh stat, resistance was infinity at room temp so I assumed that it was meant to be open circuit until hi temp condition had been reached but if I'm wrong the stat could be faulty I suppose.
I cannot think of any boiler OH stat which is not NC.

But again I cannot advise you to try to do your own repairs.

As you have discovered, this model is not well liked by engineers.

Not helped by a faulty fan being likely to damage the PCB.

I try to avoid getting involved in repairs on that model except in a very few special cases.

its the higher voltage components that usually takes fuses out
Could it be the overheat stat causing fuse to blow? The other part that I can't eliminate is the flame detection because the ignition sequence doesn't run at all with this disconnected.
Hi, I've had an engineer try a new fan today (as they had the part available) but this made no difference - fuse still blew about 2-3 seconds into ignition sequence as electrode was sparking.

Looks like I'll have to call it a day. Thanks for the help.
Dodso. you have a two amp fuse on the spur unit and 3.15 on the pcb. Do you not think 2amp fuse might be undersized?
Call Ideal and get them to do the repair if you cannot get someone who knows this boiler

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links