Level of pipes in primary HW circuit

Agreed, but there are lot of them about - as evidenced by many of the questions in this NG!

When I had one in my previous house, I at least had a Cytrol valve (a bit like a TRV) in the gravity return, to stop the flow when the HW was hot enough. [Only problem was that the flow would occasionally decide to go the other way round - whereupon the Cyltrol found itself on the *input* side of the cylinder - and I got no hot water!]

illegal on new systems.

Yes - but on the basis of thermal efficiency rather than safety.

I believe safety was factored into their decision.

Do you know of any legislation which explicitly forbids gravity systems running at boiler temperature? As far as I am aware, such systems would be implicitly - but not explicitly - outlawed simply because they fail to meet the boiler interlock requirement. My understanding is that boiler interlocks are only required on fuel efficiency grounds.

Having a boiler interlock there is another level of temperature control and safety.

That is certainly my understanding too. However, there are rumours of a safety requirement coming in as well, which will mandate TMVs on the output of conventional cylinders, bringing the temperature down to some ridiculous level like 40C, which effectively bans hot baths and prevents the topping up of the BS tepid bath with hot water to extend its life.

Christian.

user adjustable.

SNIP

Despite having "plan" systems installed I very often find cylinder stats wound right up so as to be ineffective. Asking the householder often elicits the reply "we like our hot water to be really hot". Makes all this well meaning efficiency legislation totally useless doesn't it?

You can please some of the people some of the time...................................