Replacing small electric water heater

[DartmoorHedgehog]

I recently moved into a house that has a small 10-litre electric water heater in a "shepherd's hut" type outbuilding. The heater had corroded and was leaking so at the moment the water is turned off. I thought I'd just replace it like-for-like but having looked for a replacement I'm now not sure what I should buy and after a bit of advice.

The old heater was a Gabarron GTS 10, fitted under the sink but the water pipes are on the bottom of the unit which seems to make it actually an "over-sink" heater. These don't seem to be available now, but seemingly near-identical models by various other makes (Ariston etc) do exist.

The thing that's confusing me is that the Gabarron was fitted with a one-way valve and pressure release drain directly on the cold water input to the heater. All the new heaters I've found say they should have a long length of pipe (10ft?) between the heater and the one-way valve, else they need an expansion vessel fitted. I found the installation manual for the Gabarron and my heater was definitely fitted as recommended so it wasn't just a bodge.

There is room to fit an expansion vessel in the cupboard with the heater, but obviously it would be a much easier job for me if I can find a heater that could be fitted directly without having to alter the pipework to incorporate the expansion vessel. What confuses me is that both the Gabarron and the new heaters are described as unvented, so why should one just have a pressure drain and the others need the expansion vessel? Is it that regulations have changed and it's now frowned upon to have water dripping from the pressure drain as it expands or is there another difference that I'm missing?

EDIT: Sorry, just found a couple of old threads on the subject that I missed before I posted mine. Sounds like the regulations have indeed changed and all new heaters should have an expansion vessel. Any opinions or hints would still be appreciated though. Something else that I noticed in the other threads is a couple of remarks that this isn't allowed to be done as a DIY job anymore - anybody know more about this? I may well look into renting the hut out on AirBnB or similar so obviously I don't want anything I do to be deemed dodgy.

 

[polesapart]

Unvented vessels of up to 15litre capacity are exempted in the regs.

I'd recommend, if you're going for the Ariston 10l on mains water supply, to fit the full safety kit including expansion vessel.

 

[DartmoorHedgehog]

Thanks for the reply @polesapart
So not legally required but a good idea. Probably just as well to fit an expansion vessel while I'm at it then.
Are there any legal/insurance issues if I do this as a DIY job? I'd consider myself competent but I'm certainly not a trained/certificated plumber.

 

[polesapart]

Nah, follow the manufacturers' instructions and you'll be fine.

 

[DartmoorHedgehog]

OK, thanks again. I'll put the new one in with an expansion vessel to be on the safe side. I think any expanding water from the old heater was just expected to drip from the pressure release valve, which presumably worked fine but isn't what seems to be recommended now.