Hi I am hoping to fit a tankless inline water heater of about 4kw under the kitchen sink. I know it cannot run off the ring main and, as the kitchen is fully tiled, I cannot feed another cable in. There is a cooker control unit with socket fitted, which is fed with a 6mm cable. At present there is a gas cooker and gas hob though I appreciate future occupants could change over to electric cooking. My question is, can I have a fused spur running down from the cooker control unit to behind the units, then wire the heater into the fused spur. I will be having a qualified electrician do the work, just hoping to find out if it can be done before paying him to say yes or no. Many thanks
UK fused connection units take 13A fuses so can't be used for 4kW you can have boxes which contain fuses or MCB's but they are much larger than the standard FCU.
Cooker supply is likely 32A which is ample for 4kW so yes the supply can likely be used if the unit needs less than 32A protection the MCB in consumer unit would likely be changed and in the kitchen a cooker supply point fitted to wire the cable into.
4kW is rather small for a hot water supply direct. I have fitted many 3kW units which have a small tank with 7 litres of water in them under sink with special tap the tap actually controls the water into the heater but looks as if controlling the outlet these work very well and give hot water the same as having a hot water supply as far as kitchen use goes. I took a damaged one home and used it in my caravan with a 1kW element instead again worked very well 7 litres is ample for a kitchen. They were very well insulated and did not heat up the cupboard.
The 7 litres reservoir means full bore hot water. The tap was a mixer tap so use wise the users had no idea the water heater was local except no delay in hot water reaching taps. Only down side was as water reheated and expanded there was often a drip from the tap which would stop once reheated.
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