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Does anyone know the difference between these two terms?

I am replacing my existing heating only boiler with a new one. Currently the plumber is quoting for a heating only boiler to replace it. The central heating pump is in the airing cupboard and the existing hot water system is independent off a range cooker using an indirect coil storage tank in the same airing cupboard. I wondered if I should fit a boiler capable of supplying hot water via the storage tank in the future. I realise that that would mean a three way valve being added to the system but is there anything else to take into consideration?

There's a vast difference and not related. You can't compare a system boiler with a open vented system.

You can compare a vented system with a sealed system.
Or a Combi Boiler with a System Boiler.

Like a conventional boiler a system boiler provides central heating and hot water from a cylinder. The difference being that a system boiler - all major components are built into the boiler like the pump, pressure vessel, safety valve, auto airvent and the programmer.

A opened vented system has an open vent - Heating causes the water within the cylinder to expand, a vent pipe allows a safe route for excess pressure, air bubbles and steam should the system overheat, it runs from the top of the cylinder back up to the cold water storage header tank where its open vent is located just above the water level.

A unvented system has a pressure/expansion vessel, which is a vessel with a diaphragm, as above the diaphragm allows for expansion.

A system boiler unlike a combi has stored hot water - for larger properties - or where a few taps could be used at a time therefore a combi flow rate wouldn't suffice.

Would a Combi boiler suit your lifestyle / family ? it reduces pipework and requires less space as there's no tanks or cylinders ?

Hope this helps

Thanks for the explanation htg engineer.

What I have is a two pipe central heating system fed by an old 16/22 Potterton Netaheat. The house is a 4 bed - 2 bathroom and the DHW is heated from an Aga via an indirect cylinder. I am having to replace the boiler because I have added 50 sq metres to the ground floor which I am going to supply with wet underfloor heating and 40 sq metres to the first floor for which I am going to use 2 radiators, one in each room.

The plumber has recommended three different boilers, a Baxi Solo 30 HE, a Valiant 38KW and a Worcester 30 or 40 CDI

All of these are described as heat only when I have checked on the internet. Whilst this is what I currently want I do think that in the future I might have wished I had fitted a boiler capable of supplying DHW as well CH. Any thoughts?

Currently the CH pump is in the cylinder cupboard. I assume that if I did want DHW off a new boiler I would use a 3 way valve to divert away from CH circuit and into tank heating coil but would this need the pump to be in a particular position in the CH pipe work, i.e. before any radiator take off points?