Any plumbers on here? Water heater pipework/expansion question

DartmoorHedgehog

I need to replace a little water heater and thought it would be an easy like-for-like swap but reading fitting instructions for the suitable heaters I've found has confused me.

It's a 10-litre electric heater fitted under the sink (although the pipes are at the bottom so that seems to make it technically an "over-sink" heater).

The old heater (about 10 years old) was a Gabarron GTS 10. Fitted with a non-return valve and pressure release valve (with drain tube) directly on its cold water inlet.  I've looked at its fitting instructions and that is definitely how they recommend it to be set up.

All the similar new heaters I've found say that there should be at least 6ft of pipe between the heater and and the non-return valve, otherwise an expansion vessel must be fitted, even though it still has a pressure drain valve.

So it would seem like I need to alter the pipework to include an expansion vessel on the input and move the non-return valve and pressure/drain valve to the other side of the expansion vessel.  But I'm curious as to why the old heater recommended the non-return valve and pressure valve directly on the input to the heater and seemingly pretty identical models now say they need an expansion tank or long supply pipe.  Both are described as "non-vented" heaters.  Is it just that the regulations have changed and now it's not acceptable to have water dripping from the drain tube because of expansion or is there some other difference I've missed?

Funkfingers

The mains water supply pressure may have changed since your Gabarron was current.

Regulations may very well have changed. Nanny expects everything to be ultra safe because the human race is now too stupid to operate anything unsupervised.

Regarding the water inlet pipe, what is its diameter? 

DartmoorHedgehog

Pipe connections are normal 1/2" threaded ones I think.  Both the old and new heaters specify the same range of water pressure (between 3 and 5 bar I think) so I don't think that's the issue.

All I can think of is that the regulations have changed so it's now frowned upon to lose a bit of water through the pressure release valve as it's heated.  If that's the case then I'd better fit an expansion chamber, but I was hoping somebody would be able to give a definite reason.  Possibly there was something else different about the old Gabarron but I can't see anything.