When a hot water cylinder is replaced, the old cylinder is isolated, drained and removed, and the new one is set in place and connected to its water feed, the boiler coil and the controls. In England the work is notifiable under the Building Regulations 2010, so it also involves commissioning, notification and a compliance certificate for your records. An unvented cylinder adds safety valves, a tundish and a discharge pipe, and it needs an installer qualified for unvented hot water systems. Any part of the job touching the gas boiler or gas pipework is separate work for a Gas Safe registered engineer.
What happens when a hot water cylinder is replaced, step by step
A hot water cylinder replacement starts with a survey and ends with testing, notification and handover of the paperwork. The order of work is similar for a vented and an unvented cylinder. An unvented swap adds safety valves, a tundish and a discharge pipe.
- The engineer measures incoming mains pressure and flow, checks the space and plans a route for the safety discharge pipe.
- Water, boiler and immersion supplies to the cylinder are isolated, and the old cylinder is drained down.
- The old cylinder and any redundant pipework come out, and the floor or platform is checked for support.
- The new cylinder is set in place, levelled and supported, because Part G3 requires it to be adequately supported.
- Cold feed, hot draw-off, the heating coil from the boiler and the controls are connected.
- On an unvented cylinder the relief valves, tundish and discharge pipe are fitted.
- The system is filled, vented, checked for leaks and commissioned, and the time and temperature controls are set.
- The work is notified under the Building Regulations, the warranty is registered and the manual and certificate are handed over.
Vented or unvented: what differs when a hot water cylinder is replaced
A like-for-like vented replacement keeps the loft tanks and the gravity feed, so little else in the house changes. Swapping a hot water cylinder to an unvented one changes where the pressure comes from, what safety gear is fitted and what has to be notified.
| Point of difference | Vented replacement | Unvented replacement |
|---|---|---|
| Water supply | Gravity feed from a cold water storage tank; outlet pressure depends on the height of the tank above the tap | Fed straight from the mains, so hot water pressure follows the incoming main |
| Loft tanks | Cold water storage tank and open vent pipe stay in use | The tank that fed the cylinder is no longer needed for hot water, and the pipework is reworked |
| Safety devices | Expansion is relieved through the open vent pipe into the loft tank | At least two independent safety devices plus the control thermostat, as Approved Document G sets out |
| Discharge route | No tundish or discharge pipe to an outside point is needed | Tundish and discharge pipe run to a safe, visible place, with set pipe sizes and falls |
| Competence needed | Plumbing competence for the cylinder; work on the boiler or gas pipework stays with a Gas Safe registered engineer | Qualification for unvented hot water systems, plus Gas Safe registration for the gas side |
Both routes count as notifiable building work in England under the Building Regulations 2010. An unvented cylinder only suits a home where the mains can deliver enough flow at the times the house uses hot water.
Why the survey measures mains pressure and flow before a hot water cylinder is chosen
Cylinder choice follows what your incoming water main can deliver, so a survey for a hot water cylinder replacement measures pressure and flow before anything is ordered. A remote guess from a photograph cannot do that.
Heatrae Sadia's Megaflo installation manual gives a typical minimum of 1.5 bar running pressure and 20 litres a minute for domestic sizes. Larger models ask for more, up to 50 litres a minute. High pressure on its own does not give good flow, and several outlets running together thin it further. The manual asks for a 28 mm supply pipe where that is possible.
Size follows the household: the number of bathrooms, how many showers run at the same time, and the space at the chosen location. The figures your mains has to meet are set out in our guide to the minimum flow rate for an unvented cylinder. FM Power surveys the home free of charge and gives a written fixed price before any work starts.
Who is allowed to replace a hot water cylinder in England, and why two trades can be involved
Replacing an unvented hot water cylinder in England is work for a person competent in unvented hot water systems. Approved Document G accepts registration with a competent person scheme, or a current registered operative card for unvented hot water systems, as evidence of that competence.
Gas work is a separate matter. In Great Britain, anyone working on gas appliances or gas pipework must be Gas Safe registered and qualified for that specific work. HSE states that it is illegal for anyone else to do it. The legal basis is the Gas Safety (Installation and Use) Regulations 1998, as amended.
A cylinder heated indirectly by a gas boiler touches both areas at once. Ask to see the unvented qualification and the Gas Safe card, whose qualifications are listed on the back. Our guide on who can install an unvented hot water cylinder goes through those checks. FM Power is Gas Safe registered.
Is replacing a hot water cylinder notifiable building work in England?
Installing or replacing a hot water cylinder in England is covered by the Building Regulations 2010, for vented and unvented cylinders alike. GOV.UK frames the requirement for a vented cylinder mainly around energy efficiency. For an unvented cylinder it adds safe storage of water under pressure.
Where the installer is registered with a competent person scheme, the business certifies and notifies the work itself. You then do not have to approach building control at all. Where no such registration applies, the work has to go to building control before it starts.
Ask which competent person scheme your installer notifies through, and keep the compliance certificate for the hot water cylinder with your property paperwork. It is the record that notifiable work under the Building Regulations 2010 in England was declared, and it is the document a buyer's solicitor looks for later.
What the discharge pipe and tundish on a new unvented cylinder have to do
An unvented hot water cylinder discharges through a temperature or combined temperature and pressure relief valve, a tundish and a pipe to a safe, visible place. Part G3 of Schedule 1 to the Building Regulations 2010 requires stored water in England never to exceed 100 °C, and any discharge from safety devices to be visible and to cause no danger. A tundish is the small open funnel that lets you see a discharge.
Heatrae Sadia's manual sets the pipe geometry. A short metal pipe runs from the valve to the tundish, no smaller than the valve outlet, with no more than 600 mm between them. Below the tundish, at least 300 mm of vertical pipe runs before the first bend. After that the pipe falls continuously, at least 1 in 200.
The discharge route often decides where the cylinder can stand. If the pipe cannot reach a safe outside point from a cupboard, that cupboard is out. The tundish also has to sit in the same room as the cylinder and away from electrical parts. Our guide to unvented cylinder discharge pipe rules covers D1 and D2 in detail.
After a hot water cylinder replacement: temperature, servicing and paperwork
A replaced hot water cylinder needs three things settled afterwards: storage temperature, servicing and documents. HSE advises storing hot water above 60 °C and distributing it above 50 °C, because legionella bacteria multiply between 20 °C and 45 °C and do not survive above 60 °C. Part G3 in England caps hot water reaching a fixed bath at 48 °C, which is the work of a thermostatic mixing valve at the outlet. Hot in the cylinder, limited at the tap.
Servicing an unvented cylinder is engineer work. Manufacturer manuals ask for a yearly check by a competent person: relief valves opened manually to prove they are clear, the strainer cleaned and the expansion vessel charge checked. Nothing behind the cylinder casing is a householder job. Your own checks stay with the boiler pressure gauge, the thermostat and programmer settings, the reset button and bleeding radiators.
Heatrae Sadia ties its warranty to installation in line with the instructions, registration within 30 days and that annual service. FM Power registers the manufacturer warranty for the customer. If a cylinder change comes up alongside a boiler change, the new boiler installation page explains the free survey and the written fixed price.
Sources
- legislation.gov.uk — Building Regulations 2010, Schedule 1 (Part G3)
- GOV.UK — Approved Document G with 2024 amendments
- GOV.UK — Building work, replacements and repairs to your home
- GOV.UK — Competent person schemes
- HSE — Gas safety: home owners
- legislation.gov.uk — Gas Safety (Installation and Use) Regulations 1998
- HSE — Hot and cold water systems
- Heatrae Sadia — Megaflo Eco SystemFit installation manual