Never replace a 3 amp fuse for a larger size

Having just completed and commissioned another new Vaillant boiler installation, I was setting off on my way home when I received the call to go to a regular client who had no hot water and a smell of burning.

When I arrived the symptoms were simply that the house consumer unit (fuse board) kept tripping and needed to be reset when the hot water came on the timer. The boiler timer in the kitchen kept going blank  - but only when hot water was timed to come on.

Our client  worked out prior to my visit that the 3 amp fuse (that feeds the boiler/timer/airing cupboard controls) kept blowing – hence tripping the main house’s consumer unit/fuse board. Unassumingly they had replaced the 3 amp fuse for a much larger fuse (13 amp) and immediately (to themselves) they had cured the fault! The hot water on the boiler then worked fine and didn’t trip any electrics or blow any fuses for a good few days.


Later however they smelt an infrequent burning smell upstairs – they didn’t quite place it was in the airing cupboard and also didn’t place it was linked to the underlying blown fuse issue. So when I arrived, through experience, I knew exactly what to look for – and sure enough in the airing cupboard the hot water ‘zone valve’ was defective. Defective in a way that was almost beyond belief to me as the zone valve motor had clearly failed and in turn had got extremely hot. This then blew the 3 amp boiler fuse immediately as a result. This was a clear warning sign to the client that something had gone wrong.

Our client then later replaced the 3 amp fuse for a much larger household 13 amp fuse – and this was big enough to send a larger current through the broken motor – thus causing something just short of a house fire. The zone valve circuitry had clearly got so hot, it even melted the heat resistant cable that fed the motor and had made one of the 230v wires completely bare.

Gas boilers join 230v electricity, water in the system and of course gas together as one entity, so a 3 amp fuse MUST be used to prevent shock or a fire etc. 13 amp fuses are too large for the appliance and system. They will not blow quick enough in the event of a problem.

The simple long term fix by me was to isolate the electrics, drain the system and replace the entire zone valve for new. The new Drayton zone valve I installed has an earth wire (whereas the old zone valve didn’t) so I not only fixed the issue, but made the system far more safer for the long term future.

Note. Our client was first asked and later informed of the images taken and they are used with full permission. No personal belongings are pictured nor any references are ever made to clients’ names or addresses.



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