When the old cylinder goes, you get one cheap opportunity to change system type. Whether you should take it comes down to your mains supply.
The decision only comes round when the cylinder does
Most households never actively choose a hot water system — they inherit one. The cylinder failing is the one moment when changing it costs comparatively little extra, because the cupboard is already open and the pipework is already being disturbed. So it is worth spending an hour on the decision rather than defaulting to whatever came out. The general strengths of each type are covered in the article on unvented cylinder pros and cons; this is about the replacement itself.
Route one: like-for-like vented
A new vented cylinder fed by the existing cold water storage tank is the simplest and usually the cheapest route. The system stays gravity-fed and low pressure, the loft tank stays where it is, and there are no unvented safety components or discharge arrangements to install. It is still notifiable building work — replacing a hot water storage vessel of more than 15 litres is notifiable whether it is vented or not — but a registered installer self-certifies it as a matter of course. A modern vented cylinder is still a real upgrade on a 1980s one — bonded foam insulation instead of a jacket means noticeably less standing heat loss.
The limitation is unchanged: hot water pressure is determined by the height of the loft tank above the tap, so upstairs showers stay weak unless a pump is added, and you keep the tank, its feed and vent pipes, and the loft space they occupy.
Route two: converting to unvented
An unvented cylinder is sealed and fed directly from the mains, so hot water arrives at the same pressure as cold. Upstairs showers improve, the loft tank and its pipework can be removed, and the space is reclaimed. That is the appeal, and for most London households with more than one bathroom it is the reason conversions happen.
Conversion is a bigger job than a swap. It adds a mains-pressure cold feed to the cylinder, a set of safety components — expansion vessel, pressure control, expansion relief and a temperature and pressure relief valve — a tundish and a discharge pipe routed somewhere hot water can be dumped safely, removal of the redundant tank and feed pipes, and often replacement of hot pipework and fittings that were only ever intended for gravity pressure. It is also notifiable building work, so it must be certified by an installer holding the unvented hot water qualification.
Your mains supply decides whether route two exists
This is the gate everything else passes through, and it is why a conversion cannot be quoted over the phone. An unvented cylinder can only deliver what the incoming mains delivers. If the flow rate and pressure at the stopcock are poor — common in older London properties with long, narrow or shared supply pipes, and in upper-floor flats — an unvented cylinder will disappoint no matter how good it is, and a vented system with a pump may genuinely serve the house better. A surveyor measures the flow rate before recommending unvented, and an installer who does not measure it is guessing.
Space and access are the second gate. The cylinder needs a position where the safety valves and expansion vessel can be reached for the annual service, and the discharge has to reach a safe termination. Internal cupboards in flats with no nearby outside wall are where conversions most often become impractical.
The third option worth pricing: no cylinder at all
If the household is one bathroom and the mains flow is strong, removing the cylinder entirely and fitting a combi boiler frees the cupboard as well as the loft. It is not automatically the right answer — a combi's hot water performance is capped by mains flow, and two simultaneous showers are where it struggles — but it belongs on the list, and the combi, system and regular boiler comparison sets out the trade-off. If the boiler is old anyway, price it as one project via a combined boiler and cylinder replacement rather than as two.
Comparing the quotes fairly
A vented swap and an unvented conversion are not comparable on headline price, because they are not the same purchase. Compare them on what you get for the gap: mains-pressure hot water at every outlet, the loft cleared, and a certificated installation — against a lower figure, an unchanged shower and a tank still in the roof. Ask each installer to state the measured mains flow rate, what happens to the loft tank, where the discharge terminates and who notifies the work. Those four answers separate a real conversion quote from an optimistic one. A cylinder replacement survey covers all four before any figure is put in writing.
Frequently asked questions
Can I convert from a vented cylinder to an unvented one?
Usually, provided the incoming mains flow rate and pressure are good enough and there is a safe route for the discharge pipe. The flow rate has to be measured at the property first, because an unvented cylinder can only deliver what the mains delivers.
Will an unvented cylinder improve my shower pressure?
It removes the limitation of a gravity-fed system, so upstairs outlets run at mains pressure rather than at whatever the loft tank height provides. How good that is still depends on the incoming mains supply, which is why it is measured before an unvented cylinder is recommended.
Do I have to remove the loft tank if I go unvented?
The cold water storage tank and its feed pipes become redundant once the cylinder is fed from the mains, and removing them is normally part of the conversion. Check the quote says so, along with capping off the pipework properly rather than leaving it live.
Is a vented cylinder replacement notifiable building work?
Yes. Replacing a hot water storage vessel of more than 15 litres is notifiable whether it is vented or unvented, and a registered installer normally self-certifies it. What changes with unvented is the competence required: the installer must hold the unvented hot water qualification, and the alternative to a competent person scheme is notifying local authority building control before the work starts.