I'm sure this can't be right. The fused switch that controls my immersion heater is very hot. I know that some heat will be generated in the cabling as the element gets hotter, but the switch itself gets too hot to touch. I've checked the connections inside the switch, and they all seem secure. Does anyone have any other ideas why this switch is so hot?
Also, is it usual for an immersion heater circuit to be spurred off a ring main, or should it be on it's own dedicated circuit wired with a higher rated cable?
In order to generate heat within the switch, the current flowing to the immersion heater must be meeting some resistance. If the wires are all secure, the resistance must be coming either from the switch contacts or from the fuse not being a tight enough fit in the fuse holder. Either way, if it's got seriously hot, it will never be the same again - and must be replaced.
I'm not sure about the legality. It's generally considered to be good practice to have a dedicated circuit - with its own breaker - for the immersion heater. However, I suspect that a lot *are* spurred of a ring circuit. Mine certainly is - but I'm not too worried, because my immersion heater is for emergency use only, with all hot water normally being heated by gas throughout the year.
Most likely IMHO the fuse and fuse contacts. Replace the fuse with a new one and pinch up the contacts slightly if you can, checking they are clean and not corroded.
But you might just change the entire FCU - they're not expensive.
Either there's some arcing going on (can you hear any fizzing sounds from the switch?) or there is some contact resistance which is generating heat. Try taking the switch off and using some switch cleaner on it. Make sure the terminals are screwed up onto copper, not insulation. If that doesn't work replace the switch.
I've always had the immersion heater on it's own dedicated circuit with a 15amp fuse and 1.5 mm^2 cable. I don't see why you can't spur it off a ring main as long as you're not running other powerful equipment off it at the same time. For example immersion heater + electric kettle +
3kW fire would overload the ring main and blow the 30amp fuse.
The Regs are pretty explicit about immersions and similar water heaters needing their own dedicated final circuit. The reasoning is, AFAIUI, that when you do use that immersion, it's pulling a beefy 15A for a sustained period (more than half an hour from cold, 20mins plus to refill after a bath or similar). That's a substantial enough part of the design load for the ring that it shouldn't be sharing with other appliances: other water-heating appliances (kettles, washing machines, dishenwashen) pull similar amperages but for a notably shorter time. Only electric heaters pull similar long-lasting loads, and even those are more often thermostatically controlled (not so many 3-bar electric heaters sold these days!). And the "worst case" for a ring, safety-wise, is a long, sustained overload - your 30A MCB will pass 40A for A Very Long Time Indeed without disconnecting, but currents that big, especially if unevenly distributed in the ring, can cause heating in the cables and at joints which go beyond the realms of the Safe And Sensible.
As others have mentioned, these days our houses typically have lots of small loads on the ring, and use of the immersion is more of a backup/emergency thing than the usual way of heating hot water; so you can often get away with sharing the immersion with the upstairs ring (and it's what the Professional did in our current place when putting in a new ring for an extension, with no spare ways in the CU). But it does violate a direct and clearly-stated provision of the Regs, so shouldn't be done lightly - or at all, depending on how smart you think you are...
I'm not convinced MK is worth the considerable extra cost - unless to match an existing installation, looks wise. All these things conform to the appropriate BS and should be equally capable of carrying their stated load.
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