Replacing warm air heating with gas central heating means building a wet system where none existed before. The old warm air unit is disconnected and removed, pipework and radiators go into every room, and a gas boiler takes over the heating and usually the hot water. In England that work is for a Gas Safe registered engineer, and installing a new heating system is notifiable building work.
What happens to the warm air unit, the ducts and the cupboard?
The warm air unit is disconnected from the gas supply and taken out, and the ducts behind the grilles stop carrying heat. A wet system works differently from warm air. It moves heated water through pipes to radiators in each room, so the duct network has no job left.
Ducts normally stay where they are. Most runs sit inside floor voids, walls or a central shaft, and pulling them out would mean opening floors across the whole house. Where a duct route is accessible, it can sometimes carry a pipe run to an upstairs room. Open grilles are closed up when floors and walls are made good.
The cupboard that held the warm air heater is often the most useful part of the change. It usually sits near the middle of the house and close to the existing gas supply. Many homes reuse that space for a hot water cylinder, for the boiler, or simply for storage. Agree in writing before work starts what happens to the old appliance and to the cupboard.
Combi boiler or system boiler with a cylinder after warm air heating?
A house heated by warm air usually has no radiators, no cylinder and no loft tank, so the system type is an open choice rather than a like for like swap. Mains water flow and how many bathrooms run at the same time decide most of it.
| Criterion | Combi boiler | System boiler with a cylinder |
|---|---|---|
| Hot water supply | Heated on demand inside the boiler, so there is no store to run out and no reheat period to wait for. | Stored in an insulated cylinder, so a large volume is ready at once and reheats after a heavy draw-off. |
| Space in the house | No cylinder, so the old warm air cupboard can be freed up or used for something else. | A cylinder needs a cupboard or similar space, often the position the warm air unit occupied. |
| Mains water | Flow at the tap follows the incoming mains flow rate, which is measured before the design is fixed. | An unvented cylinder also needs a mains flow rate the supply can hold; a vented one needs a loft tank. |
| Two outlets at once | Flow is shared between outlets, so a second shower noticeably weakens the first. | Stored water serves several outlets while the cylinder still holds heat. |
| Sign-off and servicing | The boiler is notified under the Building Regulations and serviced to the manufacturer schedule. | The unvented cylinder is separately notifiable and has its own annual service requirement. |
Mains flow rate is measured during the survey, because it sets what a combi or an unvented cylinder can realistically deliver. Radiator sizes and boiler output come from the same exercise. Our guide to combi, system or regular boiler compares how the three types behave in a London home.
Who is allowed to replace warm air heating with a gas boiler?
Only a Gas Safe registered engineer may carry out the gas work. In England, regulation 3 of the Gas Safety (Installation and Use) Regulations 1998 as amended says nobody may work on a gas fitting unless they are competent to do so. Businesses and self-employed engineers must belong to a class of persons approved by HSE, which is the Gas Safe Register. HSE states the same point for owner occupiers: gas work in a home must be done by a registered engineer qualified for that particular job.
Taking the old warm air heater out counts as gas work too. Disconnecting the appliance and capping its gas connection is not a stripping-out task for a general builder. The radiator pipework itself is plumbing, but the system is only usable once the boiler is connected, commissioned and notified.
The legal position on doing any of it yourself is set out in our guide on whether you can install your own boiler.
Two duties sit behind this whole job in England: gas fittings may only be worked on by a competent, Gas Safe registered engineer under GSIUR 1998 as amended, and a new or replacement heating system is notifiable building work. Nothing on this page describes how the work is carried out.
Does the change from warm air to central heating need building control approval?
Yes. GOV.UK states that installing or replacing a heating appliance or heating system is building work that must be notified to building control, whatever the fuel. The exception is where the installer is registered with a competent person scheme and can self-certify the work. In that case a certificate of compliance follows the installation instead of a separate building control application.
An unvented hot water cylinder is treated as its own piece of notifiable work. Approved Document G requires installation by a person competent for unvented hot water, evidenced by a competent person scheme or a valid qualification. Without scheme registration, building control has to be told before the cylinder goes in.
Keep every certificate with the house paperwork, because a buyer's solicitor will ask for it later. If you are unsure whether a notification was ever made on a previous job, our guide explains how to check a boiler was registered with building control.
Which controls, efficiency rules and alarms apply to the new wet system?
A new boiler and radiators must meet efficiency and control rules that never applied to a warm air unit. Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments) sets a minimum efficiency of 92% ErP for wet heating systems in dwellings in England, including existing homes. A gas combi in an existing dwelling must also have at least one further energy efficiency measure. The options are flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation.
Those requirements grew out of Boiler Plus, which has applied in England since April 2018 and introduced the 92% efficiency floor, a boiler interlock, and time and temperature control. Our guide on the controls a new boiler must come with under Boiler Plus goes through each option.
A carbon monoxide alarm is required when a new or replacement gas appliance other than a cooker is installed. Approved Document J (2022 edition) sets this out for England and has applied since 1 October 2022. The alarm must meet BS EN 50291 and sit in the same room as the appliance. On a ceiling it belongs at least 300 mm from any wall, and on a wall it sits high but not within 150 mm of the ceiling.
Could the old warm air heater or its ducting contain asbestos?
Old warm air heaters, their casings and the materials around them can contain asbestos. HSE lists asbestos lagging on heating systems, including boilers, calorifiers and pipework, and names rope seals and gaskets inside gas and electric heating appliances. Asbestos cement flue pipes also turn up in domestic heating and ventilation systems.
Do not open, drill or strip out the old unit, its cupboard lining or the duct runs. HSE treats work on asbestos lagging as mostly high risk, which means a contractor licensed by HSE. Asbestos work is a separate trade from plumbing and heating and is arranged on its own, before any removal begins.
What you already know about the house is worth passing on. The age of the warm air system, and whether anyone has worked on it before, affects the order in which the heating job can be planned.
How much pipework and disruption does a house with no radiators need?
Pipework is the biggest part of converting a warm air house, because nothing of the wet system exists yet. Flow and return pipes have to reach every radiator position, which usually means lifting floorboards, running pipes through joists, and chasing or boxing where a wall has to be crossed. Carpets come up in the rooms affected. The route is planned room by room before anything is cut.
Sizing is done by calculation, not by rule of thumb. A heat loss calculation looks at room dimensions, windows and doors, and insulation quality, and works to the coldest design day. That gives the boiler output in kW and the output each radiator has to deliver. Manufacturers warn against the old habit of adding a safety margin, because an oversized boiler simply burns more gas.
The existing gas supply also gets checked, since it may only ever have fed a warm air unit and a cooker. Pipe size from the meter, and the gas rate the new appliance needs, are part of the design work. Our guide on what happens during a free boiler survey describes what is measured on the visit. If you want the conversion priced properly, a free home survey and a written fixed price for a new boiler installation set out the full scope before anything is agreed.
Sources
- The Gas Safety (Installation and Use) Regulations 1998, regulation 3
- HSE — Gas safety: home owners
- GOV.UK — Building work, replacements and repairs to your home
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- GOV.UK — Boiler Plus factsheet
- Approved Document J (2022 edition)
- HSE — Locations of asbestos and taking the right action
- Worcester Bosch — Understanding various boiler types