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TLDR; Why are modern primatic cylinders unsuitable for use with an uncontrolled heat source?

The scenario: I have a friend (really) that needs a new HW cylinder fitting. Currently there is a 40ish year old 36x18 primatic type cylinder in place (I assume that’s what it is as it is ‘self priming’, has a single feed, a single outlet, flow and return pipes, and vents back to the cistern). This just provides hot water - there is no central heating in the house, (only electric storage heaters). In addition to the immersion heater the flow and return pipes connect to a small solid fuel fire in the kitchen, and ‘possibly’ to some kind of chimney-breast fitted heat exchanger in the living room.

I figured that the simple fix would be to just fit/get fitted a new primatic cylinder, but all the ones I’ve seen say that they are "not for use with an uncontrolled heat source" – which I’m led to believe that the solid fuel fire is.

So:
a) are they made differently these days, or is it b) just a regs change, or are c) ones suitable for this setup still available? I have done much reading but can't find why they are unsuitable. My guess is that it's just a regs change, but am here for the wisdom of people that know.

Whilst I may well have a go at putting a like-for-like back in, replacing with a different type will need the services of a tradesman, which would normally be the preferred way to go, but currently having strangers in the house is not something that the resident is happy about as they have been isolating since April, but if it was the only option then a way would have to be found.

The common “chuck a combi in” route is not viable, as there is no gas in the village, and replacing it with a simple direct cylinder (which would be my choice) is also not an option as the ‘free’ hot water is something that they value very much.

So will a modern primatic cylinder do, or would it end in tears?

Luv n hugs etc.
 

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I'm afraid primatic on solid fuel big no way.
They have a air bubble inside that separates primary and domestic hot water.
If water boils bubble fails primary water through the taps!
Needs a indirect cylinder with new expansion tank simple as that
 
I'm afraid primatic on solid fuel big no way.
They have a air bubble inside that separates primary and domestic hot water.
If water boils bubble fails primary water through the taps!
Needs a indirect cylinder with new expansion tank simple as that
Cheers for that. Yes, I understand the separation method, I'm just curious as to why there has been no problem with it like that for the last 40 years if it's such a failure prone setup - and why it would have been fitted in the first place if it was a big no-no with solid fuel. Perhaps I should just spend less time trying to understand :)
 
I would suspect that there is insufficient demand to justify testing for use on solid fuel nowadays.

They may work O.K, but Cylinder designs have changed over the years to increase efficiency so the manufacturers would need to comprehensively test both in factory and in a variety of installed situations in homes.
 

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