Converting an open vented heating system to a sealed system means taking the feed and expansion tank out of the loft and holding the circuit under pressure instead. An expansion vessel, a filling loop, a pressure gauge and a safety valve set at 3 bar take over the job that loft tank used to do. Work on the gas boiler itself is for a Gas Safe registered engineer, and in England the installation is notifiable under the Building Regulations. Below you will find the parts the conversion needs, the cleaning the old pipework should have first, and what you look after once the system is sealed.
What changes when you convert an open vented heating system to a sealed system?
Converting an open vented heating system to a sealed system removes the loft tank and seals the water in the pipework under pressure. An open vented system has a feed and expansion tank sitting at atmospheric pressure in the loft. A vent pipe lets heated water expand upwards into it, and a feed pipe tops the circuit up on its own.
A sealed heating system does the same job with an expansion vessel, which is a sealed steel vessel with a rubber diaphragm and a cushion of air behind it. On a system or combi boiler the vessel is often built into the casing. Filling and topping up then happen through a filling loop instead of automatically from the loft. A pressure gauge and a safety valve set at 3 bar complete the set.
The conversion is normally carried out alongside a new boiler, because the instructions for sealing an existing system sit in the boiler manufacturer's installation manual.
Open vented versus sealed heating system: what changes day to day
An open vented heating system keeps itself full from a tank in the loft. A sealed system holds the same water under pressure with no tank, so the differences show up in how it is topped up and in what you look at when something seems wrong.
| What differs | Open vented system | Sealed system |
|---|---|---|
| Where the water expands | Up the vent pipe into the feed and expansion tank in the loft | Into an expansion vessel, often inside the boiler, close to the appliance |
| Topping up | The loft tank refills itself through a float valve, with no action from you | You top up by hand through a filling loop and watch the pressure gauge |
| Normal pressure | Set by the height of the loft tank above the radiators, with no gauge to read | Around 1.0 to 1.5 bar on the gauge when the system is cold |
| After bleeding a radiator | The loft tank replaces the water that came out of the radiator | Check the gauge and top up through the filling loop if the pressure dropped |
| If water is being lost | No gauge to watch; the tank keeps the circuit full as water goes | Pressure on the gauge falls, which points to a leak that needs tracing |
Once the heating is sealed, the gauge becomes the thing you glance at. A cold reading of roughly 1.0 to 1.5 bar is normal, and many gauges mark that range in green. Pressure that keeps falling is a sign of a leak, so topping up again and again only buys time. Our guide on low boiler pressure explains what the gauge is telling you.
How do you tell whether your heating system is open vented or sealed?
Two tanks in the loft point to an open vented heating system. The large one stores cold water for taps and the cylinder. The small one, with a float valve inside, is the feed and expansion tank for the heating. A sealed system has neither.
- A small tank in the loft with a float valve, separate from the bigger cold water cistern: open vented heating.
- A pipe hooked over the edge of that small tank and pointing down into it: the open vent pipe.
- A pressure gauge on the boiler or on the pipework beside it, reading in bar: sealed heating system.
- A filling loop under the boiler, either a braided silver hose or a rigid horseshoe-shaped pipe with one or two valves: sealed heating system.
- An expansion vessel inside the boiler case or fitted on the return pipe near it: sealed heating system.
A house can be part way between the two. An older heat-only boiler sometimes sits on an open vented heating circuit while the hot water comes from a mains-pressure cylinder, so look at the heating side specifically.
Which parts does a sealed heating system conversion need?
A sealed heating system conversion needs five things fitted, plus the removal of the loft tank and the vent pipe. Boiler manufacturers list them in the conversion section of their installation manuals.
- A WRAS-approved filling loop, used to fill the system and to top it up later.
- An expansion vessel sized for the system, fitted on the heating return as close to the boiler as possible.
- A pressure gauge, so the running pressure can be read at a glance.
- A safety valve set at 3 bar, with a discharge pipe of at least 15 mm that falls continuously to the outside.
- An isolating valve, so the system can be worked on without draining the lot.
Sizing and charge pressure are the installer's decision, not a detail to guess at. Worcester Bosch charges its built-in vessels to 0.75 bar, which corresponds to 7.5 metres of static height, and Vaillant states the charge should not sit below 0.75 bar when measured at the vessel valve. Worcester Bosch also calls for an extra vessel on the return near the boiler where the system holds more than 100 litres, or where 2.65 bar would be exceeded at maximum heating temperature. An older house with cast iron radiators and long pipe runs can easily reach that volume.
Does an open vented system need flushing before it is sealed?
Worcester Bosch recommends cleansing an existing heating system before a new boiler is fitted to it. Old open vented circuits collect magnetite, the black sludge that forms as radiators corrode, and a sealed system pushes that debris through a narrower heat exchanger. The cleaning follows BS 7593, the code of practice for treating water in domestic heating systems, together with the boiler manual and the flushing chemical maker's instructions.
Where the old boiler and pump still run, they are usually left working during the flush. Where they cannot, a power flush moves water and chemical through the system under pressure. The cleaner must be fully removed, and every low point drained, before inhibitor goes in. The inhibitor then has to suit the metals in the system, including aluminium heat exchangers. Our guide comparing a power flush with a chemical flush sets out which one a system tends to need.
Who may carry out the conversion, and must it be notified in England?
Work on the gas boiler must be done by a Gas Safe registered engineer. The Gas Safety (Installation and Use) Regulations 1998 as amended state that nobody may work on a gas fitting unless competent, and that such work is reserved to a class of person approved by the HSE. The HSE adds a useful dividing line for a conversion: pipework and radiators may be installed by someone who is not registered, but work on the boiler and the final connection of the water pipes to it may not. You can check an engineer's ID card, which carries a photo, registration number and expiry date on the front and the work categories on the back.
Removing the feed and expansion tank and vent pipe, setting the expansion vessel charge, fitting the safety valve and its discharge pipe, and connecting the boiler are installer tasks on a gas system, never householder jobs.
Installing or replacing a boiler or heating system in England is notifiable under the Building Regulations. A business registered with a competent person scheme can certify its own work, notify the local authority on the owner's behalf, and issue a certificate of compliance within 8 weeks of completion. Approved Document L also expects accessible pipework in the home to be insulated when a boiler or hot water cylinder is replaced, and the replacement appliance to be no less efficient than the one it replaces.
What happens to the loft tanks and the hot water cylinder?
The feed and expansion tank goes as soon as the heating is sealed, along with its vent pipe. The larger cold water storage cistern is a separate question. It stays if you keep a vented hot water cylinder fed from it, and only comes out if the hot water side moves to mains pressure as well.
Moving the cylinder to an unvented one brings its own rules. Requirement G3 of the Building Regulations in England covers unvented hot water storage, and the safety discharges must reach a safe place outside. A tundish gives the air break in that discharge pipe and makes any discharge visible. The installer needs registration with a competent person scheme or a valid operative skills card for unvented hot water systems; without scheme registration, the building control body has to be told before the work starts. Our guide on converting a vented system to an unvented cylinder goes through that side in detail.
If the hot water is changing at the same time as the heating, our page on Megaflo and unvented cylinders shows what that part of the job involves, and a free home survey gives you a written fixed price before anything is agreed.
Sources
- The Gas Safety (Installation and Use) Regulations 1998, regulation 3
- HSE – Domestic gas: frequently asked questions
- HSE – Gas safety check: who can do it?
- GOV.UK – Building regulations approval: use a competent person scheme
- Approved Document L, Volume 1: Dwellings
- Approved Document G: Sanitation, hot water safety and water efficiency (2024 amendments)
- Worcester Bosch – 24CDi BF installation manual (converting to a sealed system)
- Worcester Bosch – Greenstar 12i/24i System installation instructions (expansion vessel charge)
- Worcester Bosch – Should I have my heating system treated or cleansed?
- Vaillant – What pressure should my boiler be?
- Ideal Heating – How to repressurise your boiler
- Vaillant – Open vent boiler (glossary)