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Gas boiler regulations. Help with boiler flues, cupboards, gas pipes & efficiency


Jo Alsop

Heating Hero

The Heating Hub

Lots of new building regulations will apply since you last changed your boiler. We guide you on all aspects.


Which building regulations apply to my new boiler?

Building regulations cover many aspects of a new boiler installation, including flue position, efficiency, boiler location, gas pipe size and condense pipe drainage. Some of these regulations will have come in since your last boiler was fitted and you will have to comply with them, the most recent uplift was 15th June 2022. If you are also moving your boiler then it is likely that more regulations will need to considered. We guide you here. Note, this guidance is no substitute for getting a Gas Safe registered engineer out to survey.


  1. Flues
  2. Boiler location
  3. Gas pipe
  4. Condense pipe
  5. Heating controls and TRVs
  6. Boiler efficiency and flow temperatures
  7. Boiler sizing
  8. Heating system zones
  9. Pipe insulation
  10. Boiler filters and cleaning

1. Flues

Building regulations provide detailed guidance on where a flue can safely discharge its gases. Burning gas creates heat, water and CO2. The heat is needed for the home and the remaining combustion gases (made up of water vapour, carbon dioxide, particulates, heavy metals and acidic gases) need to be removed via the flue. The position of the flue is important as the combustion gases must not re-enter a property via a close-by window or cause a nuisance by discharging into your neighbours walkway for example.

Horizontal flue

A horizontal flue is the simplest and cheapest option as it goes straight out the back of the boiler. The boiler must be on an external wall and the flue must be 300mm from an opening window or door or from the eaves of the roof. If it is closer the engineer can use a plume management kit to direct the flue away from the window or door.

A horizontal flue cannot discharge into another closed space, such as a garage, or into a public alley way or shared access walkway between two houses.

Flues must also not discharge too close to your neighbour's boundary. Flues that point in the same direction as your neighbour's boundary (i.e. are parallel) must still be 300mm away from the boundary. Again a plume management kit can assist if the flue is closer. 

Horizontal flues for condensing boilers must rise slightly in order for the acid water vapour to trickle back down the flue and be safely removed via the condense pipe.

Vertical flue

A vertical flue rises off the top of the boiler and can go through a flat or pitched roof or through the wall at a higher point. Vertical flues have a limit for the number of metres then can travel. Each manufacturer will specify this for each of their boilers.

For every bend used in the flue, the maximum length it can travel is reduced by 1m. For example, a boiler that can work with up to six metres of flue can use six straight metres of flue or four metres with two bends, or five metres with one bend and so on.

Like horizontal flues, the exit point must be 300mm from opening windows but distances rapidly increase to upto 2000mm for 'Velux' windows and neighbouring Velux windows.

2. Boiler location


New gas boilers can be located anywhere in the home, including a bedroom, as they are ‘sealed’ appliances, i.e. they do not draw in air from the room. Older boilers drew in air from the room and this increased the risk of carbon monoxide poisoning. They could still be located in a bedroom, but the cupboard had to be well ventilated.


Boilers can also be located in bathrooms and are commonly found in the airing cupboard, however they cannot be located above the bath or anywhere inside the non-electrical ‘zones’ (the area around sinks, baths and WCs).

In the roof space

Boilers are commonly fitted into loft spaces, but it does mean equipping the loft for easy future access and this adds to the cost. The boiler manufacturer’s engineer will not attend under warranty if these provisions are not in place.

The boiler must be hung on a brick wall (e.g. the gable end of a roof or party wall) or a new timber/ply wall if this is not available. If it is hung on a new ply wall it must be able to take the weight of the boiler.

If the boiler is hung on the gable end of the roof it can flue directly to outside. If it is hung on the party wall or on a new timber wall it will need to flue through the roof. This will add to costs particularly if scaffolding is needed.

The loft must be boarded from the hatch to the boiler, have a permanent light in place, a fixed ladder and a grab rail around the hatch. If you have 250mm rock wall or similar insulation in your roof space, the boarding for access should be raised so as to not squash the insulation, as this reduces its effectiveness. Alternatively replace this area of insulation with a foam board of equivalent effectiveness.

Be mindful that, if you fit a combi boiler in the roof space or locate the hot water cylinder in the roof, it may take longer for your hot water to reach your kitchen tap as it has further to travel.

Also be mindful that you will have a long gas pipe run from the ground floor to the roof space. ‘Tracpipe’ is often used in these circumstances as it offers a continuous pipe run rather than copper that has joins.

In the garage

Boilers can also be fitted into garages. They need to be fitted with a frost stat to prevent pipe freezing in cold weather and the pipes should be insulated with foil coated pipe insulation. The boiler should be high enough to avoid a collision.

Cupboard clearances

A boiler can be located in a cupboard providing the minimum clearances given in the manufacturer’s instructions are met. The boiler must be fully accessible with at least 60mm of clearance in front of the boiler. An opening cupboard will provide 60mm of clearance when the door is open.

Kitchens and utility rooms

Boilers ideally need to be on an outside wall and close to an internal drain, which is why they are so often located in kitchens and utility rooms.

3. Gas pipe

Now this is a complicated area of regulation and a dangerous one to get wrong. Which is why you must use a Gas Safe registered engineer as they are trained specifically in this area. See our Guide to Gas Safety Accreditation for how to check your engineer's qualifications.

Gas pipe can be run in 15mm, 22mm, 28mm or 35mm pipe and it depends entirely on 1) the size of the boiler and 2) on the length of the gas pipe run. A large boiler at the end of a long gas run, for example a 36kW combi in the loft, will need a larger gas pipe (perhaps a mix of 28mm and 35mm) than a 12kW heat only boiler close to the gas meter.

Copper is expensive and the longer the gas run the more joins in the pipework, which are weak spot for leaks. There are a few ways to mitigate the cost of the gas pipe and better still to try to keep your existing gas pipe in place. The first is to accurately size the boiler. If the boiler is over 18kW then it will need to be run in 22mm pipe.

For heat-only and system boilers, it is sensible to keep to a boiler under 18kW. As the average heat requirement for UK homes is 6-8kW then this is fairly straight forward. Of course a long gas pipe run may still mean that you need 22mm pipe.

For combi boilers you need to avoid going from 22mm to 35mm! Keep to a sensible sized combi. If you only have one bathroom then the smallest combi boiler will comfortably meet your needs.

4. Condense pipe

If you are replacing a non-condensing boiler with a condensing boiler you will be fitting a condense pipe for the first time. Similarly if you are moving your existing condensing boiler to a new location, your installer will need to consider where to run the condense pipe.

Condensing boilers produce acid water vapour that needs to be discharged via a drain. The ideal and best place to discharge the water vapour is via an internal waste pipe, often via the kitchen sink.

If this is not available, pipes can be run externally to a drain, however the pipe must be bigger to prevent freezing. The same applies for boilers in garages. The boiler will stop working if your external condense pipe freezes - TOP TIP poor hot water over the pipe to get it working again.

5. Heating controls and TRVs

New building regulations were introduced in April 2018 called Boiler Plus. These regulations require an 'advanced energy saving measure' for all new combi boilers and 'time and temperature' controls for system and heat only boilers AKA a programmable room stat.

Advanced energy measures include load and weather compensation controls, but these only work if they speak the same language as your boiler, and the lessor automation and optimisation controls (most smart controls). For more detail on these controls and compatibility see our Guide to Boiler Plus.

From 15th June 2022, all radiators must have TRVs when a new boiler is fitted (except where the room thermostat is located).

6. Boiler efficiency and flow temperatures

All new boilers must be 'A-rated' on the label, which is equivalent to 92% efficient. However unlike other white goods, boilers do not have a fixed efficiency. They are only A-rated if they are fitted with a compatible modulation control and setup correctly by an installer who understands the requirements of condensing boilers. Less than 1% of engineers have had training in condensing boiler setup. For more see our guide to Why condensing boilers do not condense.

From 15th June 2022, for full heating systems (where all new radiators and pipework are also fitted), the radiators must be sized to run at no higher than 55degC. Where only the boiler is replaced there is no requirement to reduce the flow temperature. However it is vital to efficiency that the installer does reduce the flow temperature. The building regulations do at least acknowledge boiler efficiency is linked to a lower flow temperature. You can read more on how a lower flow temperature can off set rising energy prices for you today.

7. Boiler sizing

From 15th June 2022, a heat loss calculation must be undertaken on the property when a new boiler is fitted to ensure the boiler is not oversized. For combi boilers, the boiler must be able to reduce it's output down to the 'typical heat load of the dwelling'. In other words, its minimum kW output cannot exceed the maximum kW demand for the home. Many boilers have a minimum output of 8kW when the home never needs more than 6-7kW even in very cold weather. Boiler oversizing is a massive problem in the UK and this uplift if building regulations is an attempt to prevent it.

8. Heating system zones

Houses above 150SQM (typically 4 bed homes and upwards) must be split into two radiator zones (e.g. upstairs and downstairs). This can be achieved via pipework or grouped together smart TRVs. When only the boiler is replaced there are no requirements to create separate zones for a home over 150SQM. For homes under 150SQM only one zone is required.

9. Pipe insulation

When only the boiler is replaced, all accessible pipes must be insulated.

When a whole system is fitted, all primary pipework for heating and hot water outside of the living space (eg in loft spaces and beneath floors) must be insulated as well as all pipes connected to the cylinder for at least 1m.

10. Boiler filters and cleaning

Corrosion has been a problem since we started 'sealing' our heating systems. Corrosion is caused by air getting to the water, often when we top up the pressure on our boilers. Rather than treat the problem - remove the air - the industry has moved to treat the cause. This includes magnetic filters that remove corroded metal before it can enter the boiler and chemical treatments which many argue are unnecessary and come at a huge cost to the environment.

From 2019, BS7593 came into force that requires an in-line filter and chemical inhibitor with new boilers. The filter should be serviced annually and the water tested every 5 years for the correct levels of inhibitor.

These methods are not used widely in Europe. In Germany a process know as VDI 2035 is used which naturally treats the water to prevent corrosion and uses an deaerator to remove air from the system on an on-going basis. Installers need to buy the equipment and test the PH levels of your water to get it right. Not all boiler manufacturers will accept VDI 2035 and it is likely that if your installer uses this method then he or she fits boilers that do accept it. NB they are usually the German boiler manufacturers who understand the benefits of this well-tested process.


Jo & Caroline - Heating Heroes

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