Your hot water comes from one of three arrangements. A combi boiler heats water on demand with no storage, a vented cylinder is fed by a loft tank at low pressure, and an unvented cylinder stores water at mains pressure. The right choice depends on how many bathrooms you run at once, your incoming mains flow rate, and whether you have space for a cylinder.
Key Takeaways
- Combi boilers suit smaller homes with one bathroom and no simultaneous demand.
- Unvented cylinders give mains pressure hot water to several outlets at once.
- Incoming mains flow rate, measured in litres per minute, decides what is viable.
- Unvented systems are covered by building regulations and need annual attention.
- A vented system remains a sound choice where mains pressure is genuinely poor.
How Do the Three Systems Actually Differ?
A combi boiler has no cylinder. When you open a tap it fires and heats water as it passes through, which means unlimited hot water but only at the rate the boiler can heat it. Everything hangs on flow rate, and if two showers run together each gets roughly half the flow.
A vented cylinder stores hot water fed from a cold tank in the loft. Pressure comes from the height difference between tank and tap, so it is inherently low, which is why these systems often need a shower pump. The upside is tolerance of poor mains supply and simple maintenance.
An unvented cylinder stores hot water but is fed directly from the mains, so it delivers stored hot water at mains pressure. That gives you strong flow at several outlets simultaneously without a pump and frees up the loft. It is the arrangement most often specified when a home gains a second bathroom, and our heating and gas engineers install and commission them across Merseyside.
Which One Suits Your Home?
The honest answer depends on three measurements and one preference, and the measurements come first. You need your incoming mains flow rate, your static mains pressure, and the number of outlets you might realistically run at the same time. The preference is whether you can give up a cupboard for a cylinder.
| System | Works well when | Struggles when |
|---|---|---|
| Combi boiler | One bathroom, one or two occupants, good mains flow, no cylinder space | Two outlets run together, or mains flow is below about twelve litres a minute |
| Unvented cylinder | Two or more bathrooms, simultaneous demand, mains flow above roughly twenty litres a minute | Mains flow is weak, or there is genuinely no space for cylinder and safety devices |
| Vented cylinder | Mains supply is poor, loft space exists, simple maintenance is preferred | Strong shower pressure is wanted without pumps, or loft tanks are unwelcome |
| Cylinder with a system boiler | Larger homes with high peak demand across several bathrooms | Household is small and stored heat would mostly go unused |
Matching system type to household demand and supply.
Why Does Mains Flow Rate Matter So Much?
Because no system can deliver more water than the mains brings into the house. Flow rate is measured in litres per minute at a fully open outlet, and it is the single most useful number in any hot water discussion. A combi rated to deliver plenty of hot water will still disappoint if the incoming supply cannot feed it.
Testing takes minutes with a weir gauge or a measured container and a timer, and it should happen before any system is specified. Where flow is low, the answer may be an upgraded supply pipe from the boundary, or an accumulator that buffers peak demand. Guidance on domestic water supply is published across the government water management pages.
Pressure and flow are not the same thing
Pressure is the force behind the water and flow is the volume that arrives per minute. A property can have healthy static pressure but poor flow if the supply pipe is narrow, partly blocked or shared. Only measuring both tells you what the property can support, and quoting a system on pressure alone is a common cause of disappointing showers after installation.
What Does Installing an Unvented Cylinder Involve?
More than swapping one cylinder for another. An unvented system stores water above 60 degrees under mains pressure, so it carries a set of safety devices and a discharge route that a vented cylinder does not need. Getting that discharge arrangement right is the part that dictates where the cylinder can physically go.
The installation sequence
1 Survey and measurement
Mains flow and pressure are tested, peak demand is calculated, and cylinder capacity is sized to the number of bathrooms and occupants.
2 Location and discharge planning
A position is chosen that allows a safe tundish and discharge pipe route to a suitable termination point outside or to a soil stack.
3 Removal and pipework alteration
The old cylinder and any loft tanks come out, and primary and secondary pipework is altered to suit the new arrangement.
4 Cylinder and safety devices
The cylinder is set, then the expansion vessel, pressure reducing valve, temperature and pressure relief valve and tundish are fitted and routed.
5 Commissioning and handover
The system is filled, vented, brought up to temperature and checked for correct operation, then documented and notified as required.
Are There Regulations to Meet?
Yes. Unvented hot water storage is covered by the building regulations requirement for hot water supply and safety, and installation is notifiable work. It must be carried out by someone holding the appropriate unvented hot water qualification, and the installation must be notified to building control or self certified through a competent person scheme.
Practically, that means the safety devices must be present and correctly set, the discharge pipe must run to a safe visible termination, and the whole thing must be commissioned and recorded. Keep the paperwork with your other property documents, because it is exactly what gets asked for at sale and by insurers. The same applies to any annual servicing and safety checks you have carried out afterwards.
Where the cylinder is heated by a gas appliance, you can confirm any engineer working on it is properly registered through the Gas Safe Register.
What About Maintenance?
Unvented cylinders need annual attention, which is the one genuine downside against a vented system. The expansion vessel loses its air charge over time and needs recharging, the pressure relief valves should be exercised and checked, and any immersion heater and thermostat should be tested. Neglect it and you get relief valves discharging, or a loss of pressure performance.
The good news is that it slots into an annual boiler service visit rather than being a separate appointment. Scale is the other factor in harder water areas, accumulating on immersion elements and reducing efficiency.
What Drives the Cost?
Cylinder capacity and quality set the starting point, and capacity follows from bathroom count rather than preference. Beyond that, the biggest variable is how much pipework has to change. Converting a vented system to unvented means removing loft tanks, altering primaries, and running a new mains feed of adequate diameter to the cylinder, which is considerably more work than a like for like cylinder replacement.
The discharge route is the other significant factor. Where the cylinder sits against an external wall, the tundish discharge is short and simple. Where it sits in an internal airing cupboard on a first floor landing, that pipe has to be routed to a safe termination, and the length and complexity of that run can change the job noticeably. If the work forms part of a wider bathroom installation project, much of the access work is shared and the incremental cost falls.
Frequently Asked Questions
Can I fit an unvented cylinder with low mains flow?
It is possible but often unsatisfactory. An unvented cylinder can only deliver what the mains supplies, so weak flow produces weak showers regardless of the cylinder. The options are upgrading the incoming supply pipe, fitting an accumulator to buffer peak demand, or retaining a vented system with pumped showers. Measurement first, specification second.
How big a cylinder do I need?
Capacity is driven by peak simultaneous demand rather than household size alone. As a rough guide, a two bathroom home with three or four occupants commonly suits something in the 180 to 250 litre range, but the correct figure comes from calculating how many outlets could run together and for how long. Oversizing wastes standing heat, undersizing runs out.
Is a combi boiler always cheaper to run?
Not necessarily. A combi avoids standing heat losses from a cylinder, which is an advantage in a low demand household. In a home with high hot water use, a well insulated cylinder heated efficiently can compare favourably, particularly where a heat pump or solar input is involved. Usage pattern matters more than the technology label.
Can I keep my existing cylinder and just improve the pressure?
Sometimes. A vented cylinder with poor performance can often be improved with a correctly specified shower pump, or by raising the cold tank if headroom allows. That is a much smaller job than converting to unvented, so it is worth assessing before committing to a full system change, particularly if mains flow is limited anyway.
Does an unvented cylinder need servicing every year?
Yes, and it genuinely matters. The expansion vessel charge falls over time, and once it is low the relief valve starts discharging and performance drops. An annual check that recharges the vessel and tests the safety devices takes a modest amount of time and prevents both nuisance discharge and premature cylinder failure.
Where does the discharge pipe go?
To a safe, visible termination where a discharge of very hot water cannot scald anyone, typically low down on an external wall or into a suitable internal soil stack arrangement. The route has specific requirements on diameter, length and number of bends, which is why the discharge position often determines where the cylinder can be sited.
Getting the Hot Water Right
Hot water problems are usually specification problems rather than equipment failures, and they are avoidable with the right measurements taken before anything is bought. We test the supply, calculate your real peak demand and recommend the system that suits the property rather than the one that suits a catalogue. Full details of our heating services are on the Omnis gas and heating page.
Quality Building and Contracting Services
Omnis Developments, Liverpool and the North West