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Gas boiler flue regulations are very important for ensuring that your home’s central heating systems work safely and effectively. These rules say what kinds of flue systems can be installed, what materials and parts are needed, and where and how far away from walls the flue must be placed. Building owners can increase the safety of their heating systems and lower the risk of accidents by following boiler flue regulations.

In this blog, we’ll discuss gas boiler flue rules in general, covering things like the various types of flue systems, how they should be installed, what should come out of a gas boiler flue, and more. Let’s start!

Which Building Regulations Apply to My New Boiler?

Building rules cover a lot of factors when installing a new boiler, like where the flue goes, how efficient it is, where it goes, the size of the gas pipes, and how the condensed pipes drain. Some of these rules will have changed since your last boiler was installed, and you will need to follow them. The most recent change happened on June 15, 2022. There are more rules to think about if you are also moving your boiler. We will show you the way here. This advice is not a replacement for having a Gas Safe registered engineer to survey.


  • Flues
  • Boiler location
  • Gas pipe
  • Condense pipe
  • Heating controls and TRVs
  • Boiler efficiency and flow temperatures
  • Boiler sizing
  • Heating system zones
  • Pipe insulation
  • Boiler filters and cleaning

Flues In Gas Boiler 

Building regulations give specific instructions on where a flue can safely emit its gases. Gas burning makes CO2, heat, and water. The house needs heat, and the flue is the only way to get rid of the leftover combustion gases, which are made up of water vapour, carbon dioxide, particles, heavy metals, and acidic gases. The flue’s location is very important because the combustion gases shouldn’t come back into the house through a nearby window or make your neighbours’ driveway smell bad, for example.

Horizontal Flue

The easiest and least expensive choice is a horizontal pipe, which goes straight out the back of the gas boiler. The flue has to be 300 mm away from an open window, door, or roof eave, and the boiler has to be on an outside wall. If the flue is close by, the engineer can use a plume control kit to move the smoke away from the window or door.

A horizontal flue can’t vent into a garage, a public alleyway, or a path between two homes that both people can use.

Also, the flues must only be empty a short distance from the edge of your neighbour’s property. Even if the flues are parallel to the boundary line with your friend, they must still be 300mm away from it. Also, if the flue is closed, a plume control kit can help. 

The horizontal flues of condensing boilers need to rise slightly so that the acid water vapour can safely escape through the condensing pipe.

Vertical Flue

A vertical flue can pass through a flat or sloping roof or wall above the boiler. Each manufacturer provides boiler travel metres.

For every flue curve, its maximum length decreases by 1m. A boiler with up to six metres of the flue can employ six straight metres, four with two bends, five with one curve, and so on.

For horizontal flues, the exit point must be 300mm from the opening windows. For “Velux” and adjacent Velux windows, this distance quickly grows to 2000mm.

Gas Boiler Location


New gas boilers are “sealed,” which means they don’t pull air from the room so that they can be put anywhere in the house, even in a bedroom. However, older boilers did draw air from the room, which raised the risk of carbon monoxide poisoning. Even though they were in a bedroom, the cupboard had to be well-ventilated.


Additionally, boilers can be found in bathrooms and the airing cupboard. However, they are not allowed above the bathtub or anywhere in the “zones” that are not electrical, which include areas around sinks, bathtubs, and toilets.

In the Roof Space

Gas boilers are often installed in lofts, although outfitting the loft for future access increases the cost. If these criteria are not met, the boiler manufacturer’s engineer will not attend under warranty.

If unavailable, the boiler must be hung on a brick wall or a new timber/ply wall. The new ply wall must support the boiler.

The boiler can flow outside if installed on the roof gable. It must be flued through the roof if hung on a party wall or new timber wall. If scaffolding is needed, this will increase expenditures.

Board the loft from the hatch to the boiler and install a permanent light, ladder, and grab rail around the hatch. The roof space’s access boards should be raised to avoid squashing a 250mm rock wall or comparable insulation, which lowers its efficiency. Alternatively, use an effective foam board instead of insulation.

If you install a combi boiler or hot water cylinder on the roof, hot water may take longer to reach your kitchen tap.

Be aware that the ground-floor to-roof gas pipe will be long. Since ‘Tracpipe’ is continuous, it is typically utilised instead of copper with joins.

In the Garage

Garages are another place where boilers can be installed. There should be a frost stat on them and foil-coated pipe protection around the pipes to keep them from freezing in cold weather. The boiler should be high enough so that it doesn’t hit anything.

Cupboard Clearances in gas boiler

A boiler can be put in a cupboard as long as the minimum clearances required by the manufacturers are met. There needs to be at least 60mm of space in front of the boiler so that it can be fully reached. If the door of a cupboard opens, there will be 60mm of space between the door and the cabinet.

Kitchens and Utility Rooms

Ideally, boilers should be on an outside wall and close to an inside drain, which is why they are often found in utility rooms and kitchens.

Gas Pipe

This regulation is complicated and dangerous to get wrong. You must choose a Gas Safe registered engineer because they are skilled in this field. 

The boiler size and gas pipe length determine whether the gas pipe is 15mm, 22mm, 28mm or 35mm. A 12kW heat-only boiler near the gas metre needs a smaller gas pipe than a 36kW combi in the loft at the end of an extended gas line.

Copper is expensive, and longer gas runs mean more pipe connections, which are leak points. You can keep your gas pipe or reduce its expense in several ways. First, precisely size the boiler. Boilers exceeding 18kW must use a 22mm pipe.

It is wise to keep heat-only and system boilers under 18kW. The average UK household heat demand is 6-8kW, making this easy. Of course, a long gas pipe run may require a 22mm pipe.

For combi boilers, avoid 22mm-35mm! Choose size wisely. If you have one bathroom, the small combi boiler will be sufficient.

Condense Pipe

The first condensing pipe is installed when you replace a non-condensing boiler with one. If you move a condensing boiler, your installer must decide where to run the pipe.

Acid water vapour from condensing boilers must be drained. Water vapour should be discharged by an internal waste pipe, usually the kitchen sink.

If this is not possible, pipes can be run externally to a drain, but they must be larger to avoid freezing. Same with garage boilers. If your external condensate line freezes, pour hot water over it to restart the boiler.

Heating Controls and TRVs

New building regulations named Boiler Plus were adopted in April 2018. New combi boilers must have an ‘advanced energy saving measure’ system, and heat-only boilers must have ‘time and temperature’ controls or programmed room stats.

Advanced energy measures include load and weather compensation controls, which only operate if they speak the same language as your boiler, and lessor automation and optimisation controls. The Guide to Boiler Plus has more information on these controls and their compatibility.

From 15 June 2022, new boiler radiators must feature TRVs (save for room thermostats).

Boiler Efficiency and Flow Temperatures

All new gas boilers must be ‘A-rated’ (92% efficient). As compared to other white goods, gas boiler has no set efficiency. A-rated boilers must have a compatible modulation control and be set properly by a condensing boiler expert. Fewer than 1% of engineers possess condensing boiler setup training. Visit our guide about why condensing boilers don’t condense.

From June 15, 2022, full heating systems (with all new radiators and pipes) must have radiators sized to run at 55°C. No flow temperature reduction is needed when replacing simply the boiler. To maximise efficiency, the installer must lower the flow temperature. Building codes recognise that lower flow temperatures improve boiler efficiency. Learn how lowering flow temperature can counteract growing energy prices now.

Boiler Sizing

A heat loss calculation must be done on the property when installing a new boiler from 15 June 2022 to ensure it is not oversized. Combi boilers must reduce output to the ‘average heat load of the dwelling.’ Thus, its minimum kW output cannot surpass the home’s maximum kW consumption. Many boilers produce 8kW, while the residence only needs 6-7kW in frigid conditions. This building regulation increase aims to reduce boiler oversizing in the UK, a major issue.

Heating System Zones

Four-bedroom residences over 150SQM must have two radiator zones (upstairs and downstairs). Pipework or smart TRV grouping can do this. When replacing merely the gas boiler, distinct zones are not needed for homes beyond 150SQM. One zone is needed for homes under 150SQM.

Pipe Insulation

The boiler must be updated to insulate all accessible pipes. All principal pipework for heating and hot water outside the living space (e.g., in lofts and beneath floors) and all pipes linked to the cylinder for at least 1m must be insulated when an entire system is installed.

Boiler Filters and Cleaning

We’ve had corrosion issues since ‘ sealing’ our heating systems. Air entering the water sometimes occurs when gas boiler pressure is topped up. Instead of removing air, the business is treating the cause. Magnetic filters that remove corroded metal before it enters the gas boiler and chemical treatments, which many say are unneeded and environmentally harmful.

BS 7593:2019 code of practice mandated in-line filters and chemical inhibitors be permanently installed for new boilers. Service the filter annually and test the water for inhibitors every five years.

These gas boiler regulations are followed by the people in the UK. In the UK, a standard called VDI 2035 is used. This naturally treats the water to stop rust and constantly removes air from the system with a deaerator. To get it right, installers have to buy tools and check the PH of the water. Not every gas boiler maker supports VDI 2035, and if your provider does this, they probably only install boilers that do.


Can a Boiler be 100% efficient?

Unfortunately, because of the way boilers work, no boiler is ever completely energy efficient; nonetheless, modern gas boilers can attain high energy efficiency ratings.

What is a G-rated Boiler?

An older boiler’s energy efficiency might be as low as 60% (G rating). With a G-rated boiler, 40% of the energy you are paying for is wasted rather than being used to heat your home. That means replacing an old boiler with a newer, more efficient model is a good way to save money.

What is the most energy efficient boiler?

The Baxi 800 Heat has a maximum efficiency rating of 93%, making it one of the most efficient conventional boilers available.

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