Under sink heater (electric)

Hi all looking at some sort of water heater for an outbuilding just for hand washing.
The supply to the building is, so I am told only good for 3-4kw or so in total. There is a heater and lighting so really that takes care of a couple of kw, leaving only about 1-1.5kw for the water. that means the easy option of fitting something like an electric instantaneous heater is out, there are small storage heaters around but they are in effect unvented heaters. Now about 30 years ago I did an unvented course and have never seen one since, totally forgotten everything other than the video of one exploding was quite impressive and only qualified engineers should work on or install them, (well that ain't me any more)

Are these under sink heaters classified the same as normal domestic sized ones and what are the current regulations please?

Any other ideas?

 

Sorted now!
Redone calcs have power for 2kw heater found an under sink storage open vented heater with a vented tap

 

Thanks Nigel nice to know, best wishes,
teabreak

 

I have not ordered it myself but the rellie who I am fitting it for was looking at this (I think there is are a 15 & 5 litre one also) comes with the vented tap.
The tap works by controlling the inlet water to the heater leaving the hot outlet open al the time like a spout. You will get dripping from the tap as the water heats up and expands.

https://www.amazon.co.uk/VENTED-Und...coding=UTF8&psc=1&refRID=D3SM816ZKZJ3E0VEDJXW