You do not automatically need a new hot water cylinder when the boiler is replaced. If you stay with a system or heat-only boiler, the existing cylinder can often remain, once a Gas Safe registered engineer has checked it against the new appliance. Moving to a combi is the different case. A combi heats water from the mains as you draw it, so the cylinder and the loft tanks are taken out. This guide sets out what makes a cylinder worth keeping, what rules apply in England, and what the survey looks at.
Does a new boiler always mean a new hot water cylinder?
A new boiler does not always mean a new hot water cylinder. The boiler type decides it. Worcester Bosch explains that a combi heats water directly from the mains when a tap is opened, so no stored cylinder and no cold tank in the loft are part of the design. System and heat-only boilers work the other way round. They heat a separate cylinder, so the cylinder stays in the plan.
Ideal Heating describes a heat-only replacement as replacing the boiler and keeping the rest of the equipment in place. On a like-for-like swap, your existing cylinder is often retained. The engineer still has to confirm that it suits the output and controls of the new boiler before anything is ordered.
When does the hot water cylinder have to come out with a new boiler?
The cylinder comes out when you move from stored hot water to a combi. A combi has no cylinder, so the cylinder, the feed and expansion tanks in the loft and the pipework serving them are removed. The airing cupboard is freed up. Hot water is then limited to what one appliance can heat at that moment, which matters in a home with two bathrooms. Our guide to system to combi conversion sets out what you gain and what you lose.
A second case is the change from vented to unvented. Worcester Bosch states that a vented cylinder cannot be used on an unvented, sealed system. An unvented cylinder is built for mains pressure and carries its own safety equipment. So mains-pressure hot water means a new cylinder, not an adapted old one.
What decides whether your existing cylinder can stay with the new boiler?
Five things decide whether your existing cylinder stays: heat input, capacity, recovery, pipework and the condition of the old system. These are measured on site, not guessed from the age of the cylinder.
- Capacity against the household. Baxi works to roughly 60 litres per person, with the number of bathrooms and peak demand driving the sizing. Our guide on what size hot water cylinder you need goes into this.
- Recovery time. Litres set the store, kilowatts set how quickly it refills. Heatrae Sadia points out that a higher heat input reheats a cylinder sooner, so a change in boiler output changes recovery.
- Pipework. Ideal Heating notes that the bore of existing pipes can be too narrow or too wide for a particular boiler type.
- Cleanliness of the old system. Ideal Heating names the state of an old system as a risk to a new boiler connected to existing pipes and radiators.
- Controls. A stored system programmes heating and hot water separately, so the timer, cylinder thermostat and valves are checked with the cylinder.
A cylinder is kept or replaced on the strength of a survey, not a rule of thumb. No manufacturer or official source supports a blanket claim that an old cylinder always suits a new boiler, or never does.
Vented or unvented, direct or indirect: which cylinder do you have?
A vented cylinder is fed by a cold water tank above it, usually in the loft, and works on the head of water from that tank. An unvented cylinder is connected straight to the mains and runs at mains pressure, with safety equipment such as pressure relief fitted to it. Viessmann describes that split, and notes that unvented systems need that extra safety equipment and belong to a qualified person for installation and servicing. If there is no tank in the loft and the cylinder has valves discharging to a tundish, you have an unvented cylinder.
Direct and indirect describe how the water is heated. Baxi explains that an indirect cylinder is heated by a coil fed from an outside source, such as your boiler. A direct cylinder heats the water inside it, for example with an immersion heater. A boiler heats water through a coil, so a boiler system uses an indirect cylinder.
What do the rules in England require when the boiler and cylinder are worked on?
In England, regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 as amended bars anyone from working on a gas fitting unless they are competent, and requires businesses to belong to a class of persons approved by the HSE. In practice that means a Gas Safe registered engineer. Anything on the gas supply, the flue or inside the boiler casing is their work, never a task for you.
In England, Requirement G3 of the Building Regulations covers hot water storage systems with no vent pipe open to the atmosphere. It requires installation by a competent person, protection against the stored water exceeding 100°C, and safe discharge of hot water from the safety devices. Our guide on who can install an unvented hot water cylinder covers the G3 qualification.
Replacing a boiler or a heating system is notifiable work under the Building Regulations in England. GOV.UK states that an installer registered with a competent person scheme can self-certify the work, which removes the need to notify the building control body in advance. Approved Document L in England also sets requirements when a boiler or hot water cylinder in an existing system is replaced, including insulation of accessible pipework and self-regulating devices such as thermostatic radiator valves.
How should the old system be prepared before the new boiler is connected?
The heating circuit should be cleaned before the new boiler goes on, and the cylinder primary circuit with it. Worcester Bosch recommends that a heating system is cleansed and then treated, because debris from old pipes and radiators ends up in the new appliance. A suitable inhibitor goes into the primary circuit afterwards to slow corrosion and scale.
Worcester Bosch also points to the Benchmark checklist, which the installer completes at the end of the job and which records the water treatment. Manufacturers treat that record as part of their warranty conditions, so keep the paperwork with the boiler manual. If you are unsure which clean your system needs, read our guide on whether a power flush is needed with a new boiler.
What temperature should a retained cylinder run at, and who services it?
HSE guidance on hot and cold water systems puts stored hot water at 60°C or above, with water reaching 50°C at the outlets within a minute. Legionella bacteria multiply between 20°C and 45°C and do not survive above 60°C. Water stored above 60°C brings a scalding risk, which is why thermostatic mixing valves are used at taps and showers to balance the two.
An unvented cylinder needs an annual service by an engineer holding the G3 qualification. Heatrae Sadia lists the usual checks, including the expansion vessel or air gap, the strainer on the pressure reducing cartridge and the immersion heater. A relief valve that drips again and again points to a lost air cushion. Expansion vessels, safety valves, the tundish and its discharge pipe are all engineer work, not householder work.
If you want the cylinder question settled for your own home, a free home survey before a new boiler installation is where it gets looked at, with a written fixed price before any work starts.
Sources
- The Gas Safety (Installation and Use) Regulations 1998, regulation 3
- Sanitation, hot water safety and water efficiency: Approved Document G — GOV.UK
- Building regulations approval: use a competent person scheme — GOV.UK
- Conservation of fuel and power: Approved Document L — GOV.UK
- Hot and cold water systems — HSE
- Boilers Explained — Worcester Bosch
- Can I use Worcester unvented cylinders on a vented system? — Worcester Bosch
- Should I have my heating system treated or cleansed? — Worcester Bosch
- Replacing an Old Boiler — Ideal Heating
- The Different Types of Cylinders Explained — Baxi
- Hot water cylinder for a two or three bedroom house — Baxi
- A guide to vented vs. unvented cylinders — Viessmann UK
- Recovery Time vs Size of a Cylinder — Heatrae Sadia
- Get servicing savvy about cylinders — Heatrae Sadia