System boiler or conventional when replacing combi

Large extension added to my 3 bed property. Combi has been a pain in the arse and now won't cut it with 2 main bathrooms.
My question is should I go for a system boiler with an unvented cylinder or get a conventional boiler and have pressurised ch and again an unvented hot water cylinder?
Plumbing in the extension hasn't started yet and I plan to put the unvented cylinder in the gurage.
My plans were for the system boiler but I met a plumber who said to go conventional.
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On 11/02/2019 14:44, Darksyphon wrote:

A conventional boiler with a cold water tank and a hot water cylinder allows the use of an immersion heater to provide hot water if the boiler fails. Simpler boiler with less to go wrong.
Ultimately it is up to you to decide which alternative you prefer. I am just about to go to a property which has a system combi boiler that is not working, so I will be cold and have no hot water (kettles apart) until I fix the problem.
--
Michael Chare

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On 11/02/2019 14:44, Darksyphon wrote:

A system boiler implies a conventional system. You have a choice of unvented, vented or thermal store.
I went for the thermal store as the cylinder isn't pressurised so doesn't need inspection and testing while still having high pressure hot water.
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Wrote in message:

A bigger combi might be all you need (and probably the cheapest option), but a conventional boiler with an unvented tank and immersion heater back up is the "Rolls Royce" of systems from the point of view of HW supply with back-up IMO.
Tim
--
Please don't feed the trolls

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replying to Tim+, Darksyphon wrote: Thanks for the replys. The only thing I'm wondering is if I need the central heating side pressurised too? I like the idea of not having any tanks. But that means more gear....another pressure vessel. Just wondering if the extra reliability of a conventional boiler is worth having to add all the things seperatly that are now inside system boilers. I'm not bothered about the cost just want the best system that is reliable. Not looked into Thermal stores. Will have a read
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On 11/02/2019 19:44, Darksyphon wrote:

reliability is more about implementation than design.
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a car with the cramped public exposure of 
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On 11/02/2019 19:44, Darksyphon wrote:

Ultimately you will have the same major components, its just the location of them that changes.
Note however you may be able to implement more sophisticated controls with the system boiler, to give things like split temperature operation[1] or weather compensation[2], which are harder to do with heating only boiler usually.

http://wiki.diyfaq.org.uk/index.php/Thermal_Stores_and_Heat_Banks
http://wiki.diyfaq.org.uk/index.php/DIY_Heat_Bank
[1] Split temp allows you to run lower flow temps through the heating (for greater condensing efficiency) when its not desperately cold outside, but still run high temperatures when reheating the cylinder.
[2] Weather compensation makes the flow temperature automatically adjust to the outside temperature, and sometimes also the heat loss characteristics of the building and sometimes also the size of the difference between the actual internal temperature and the target temp set on the room stat. So it runs hotter water through the rads on colder days, or when it needs to heat the place more quickly.
--
Cheers,

John.
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On 11/02/2019 14:44, Darksyphon wrote:

If converting from a combi, system boiler makes sense since your won't already have a separate pump or the separate paraphernalia associated with a sealed system (since they are all in the combi).
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Cheers,

John.
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