Switching from gravity to system boiler/heating system? - mains inlet width & more

Our regular boiler (25 years) seems to have started coming to the end of its life (suddenly having problems, last servicer emphatic that getting spare parts is now/would be a serious pain) etc. so we're looking to replace it. Two tanks in the roof, a vented cylinder upstairs, 17 radiators, and boiler on the ground floor. Mains pressure is around 20l, and the gas pressure has always been fine too. So far so good.

Several survey visits on, everyone has sucked their teeth at some/much of the piping and the water cylinder (neither of which was unexpected - it's a late 60s central heating installation with a medley of changes over the years, think the boiler just got replaced like for like last time with no changes) - and been very unenthusiatic about doing like-for-like this time, instead recommending one or other variant of a 32-5kw system boiler, plus associated piping changes. (It's a reasonably big detached house, central heating runs constantly in the winter, and hot water can get used fairly heavily.)

I've got no problem with doing whatever's needed, and indeed with changing from gravity to system if that's a good idea, but opinions seem to be split as to the details of making it work. There's an old water mains inlet (1910s build house) - apparently narrower than it would be today. Some have suggested fitting a large (200l) accumulator before the boiler (to counteract the narrow mains) and an 250l unvented cylinder; the others have said that's not a good option, instead they'd fit a (smaller) expansion vessel after the boiler and fit a new 250 vented cylinder (retaining gravity water tank for hot water only accordingly). And comments have varied completely as to whether we should be preparing for leaks/old joints/pipes/radiators suddenly needing replacement or not, if/when we switch to a pressurised system. (The radiators are the original 60s/70s ones (though they were flushed 25 years ago, apparently).)

Frankly, I have no idea at this point. Everyone who's been to visit seemed good (and I'm not telepathic, but it's been fairly difficult to pull wool over my eyes in the past), and yes, is local and well reviewed. Any thoughts as to positives/negatives of these diffierent options? Or is this going to uncover so many problems that we should instead just be trying to get a new conventional boiler? Thanks in advance!

What doesn't your existing set up provide you with ? What exactly do you want to achieve by changing your existing set up ,other than the boiler being over the hill ?

It's largely fine...waiting for the gravity-fed hot water cylinder to refill [EDIT meant reheat] is a bit of a pain if there's lots of hot water use, but other than that not much is wrong. That said, that cylinder and the loft tanks are also ancient, so presumably heading for EOL, so if boiler/tanks/cylinder are all going to need to be replaced in fairly quick succession, does it then make more sense to change the setup simultaneously? But, long story short, it's mostly fine as is. Was fairly clear on this to people who came to look - what would they recommend? - wasn't that I pushed switching away from the present setup, but that that was the suggestion made by all of them thus far...

Waiting for hot water cylinder to refill ??
Are you referring to the time it takes to re heat the water ? That has nowt to do with the fact that it's gravity fed. You may benefit from a larger cylinder that can store more hot water ,but the time it takes to re heat is largely down to the boiler.
If you have the cash ,by all means have new cylinder / radiators/ loft tanks etc. But the additional costs of unvented cylinder/ accumulator etc ,and risks associated with a pressurised system on old pipework ,without you having a specific reason for wanting them is questionable.

terryplumb said:

Waiting for hot water cylinder to refill ??

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Sorry, meant reheat, my bad!

terryplumb said:

But the additional costs of unvented cylinder/ accumulator etc ,and risks associated with a pressurised system on old pipework ,without you having a specific reason for wanting them is questionable.

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And that's really what I wanted to know about. Thanks!

If the design of the system does what you need then keep the same design but re-new the items that are close to end of life.

terryplumb said:

and risks associated with a pressurised system on old pipework

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Very real risks as a person I spoke to on Saturday discovered in July. He had an ancient gravity system ( heating and hot water ) replaced by a modern pressurised system in his three bed bungalow. Several leaks developed almost immediately and now a plumber ( not the one who advised and installed a pressurised system ) is about to re-pipe the bungalow.

On a gravity system, the flow to the outlets will be determined by the height of the Cwsc(s) above the highest outlet and the bore and layout of the feeds.
How many showers do you have and baths?
28mm feed to the hwc would be advisable and 22mm off it to as far as you can to showers... All the way to a bath.
What's the flow rate at the ball valve to the tanks? If you pump the hot from the tanks, you can getter better performance but will need to ensure that the tanks can refill to keep up and the heating of the water can also... Fast recovery cylinders are available.