Building a Stove Recess & Chimney Stack using Isokern Liner System
Published: 1st January 2022 - Updated 28/6-25
Building a Stove Recess & Chimney Stack with the Isokern Pumice Liner System
Constructing a stove recess and chimney stack for an indoor fireplace requires precision to meet safety standards, ensure efficiency, and enhance your home’s aesthetic. At Stove Saver, we recommend the Isokern pumice liner system, crafted from volcanic pumice for superior insulation, durability, and compliance with Building Regulations Document J. This comprehensive guide walks you through the process of building an indoor chimney stack and stove recess, addressing key considerations, debunking myths, and providing practical advice based on our expertise. For a visual guide, Schiedel’s installation videos on their official website are an excellent resource. Let’s build from the ground up, focusing solely on indoor applications and the Isokern pumice liner system.
Constructional Hearth: The Solid Foundation & Heat Sink
Every indoor masonry chimney breast requires a constructional hearth (or sub-hearth) to provide a stable, heat-resistant base. Per Building Regulations Document J, this must be a reinforced concrete slab at least 125mm thick, projecting 500mm in front of the chimney breast face and extending 150mm wider than the stove recess or builders’ opening on each side. This regulation, originally designed for open fires that radiate significant heat downwards, protects underlying combustible materials like timber floors.
Modern wood-burning stoves, however, are more efficient and typically keep hearth temperatures below 100°C. For freestanding stoves, many HETAS-approved models only require a superimposed hearth (at least 12mm thick) without a constructional hearth, provided the manufacturer’s specifications confirm this. Always follow the stricter of the regulatory or manufacturer requirements to ensure safety and compliance.
Superimposed Hearth: Balancing Safety and Style
The superimposed hearth, or decorative hearth, is the non-combustible surface atop the constructional hearth, adding both safety and aesthetic appeal. Building Regulations Document J and manufacturer guidelines typically require it to extend 300mm in front of the appliance and 150mm wider than the appliance on each side. For closed appliances like wood-burning stoves, this can sometimes be reduced to 225mm if the manufacturer approves. HETAS suggests the hearth should project beyond the stove’s door opening, though this is advisory. Always prioritise the most stringent requirement.
Choose non-combustible materials like slate, granite, or porcelain for the superimposed hearth. For freestanding stoves not exceeding 100°C on the hearth surface, a 12mm-thick superimposed hearth may suffice without a constructional hearth, simplifying installation. At Stove Saver, we recommend selecting a hearth material that complements your interior while meeting safety standards.
Fireplace Recess: Crafting the Core
The fireplace recess, or builders’ opening, is the cavity housing your stove. For internal walls, Document J requires a back wall at least 200mm thick. On cavity walls or back-to-back with another recess, this can be reduced to 100mm. The recess size depends on the appliance; for a 5kW wood-burning stove, a typical recess measures approximately 900mm high, 900mm wide, and 350mm deep.
To ensure durability and safety, line the recess with A1-rated non-combustible materials, such as Promat Promafour boards, rather than standard pink plasterboard, which cannot withstand high temperatures. Plastering or skimming is generally avoided, as it may crack or deteriorate under heat. Instead, finish the recess with cast iron panels, vermiculite panels, porcelain tiles, or natural stone for a heat-resistant, stylish look that enhances your indoor space.
Flue Size and Diameter: Ensuring Optimal Performance
Selecting the correct flue diameter is critical for safety and efficiency. For open fires, Document J requires the flue’s cross-sectional area to be at least 15% of the fireplace opening unless full performance calculations are conducted. For example, a 550mm x 550mm opening needs a minimum 200mm-diameter flue for solid fuel. Larger openings (e.g., 1000mm x 1000mm) may require flues up to 450mm, which are costly and often bespoke. To reduce flue size, consider installing a cassette stove, bringing the diameter down to 150mm–300mm, depending on the appliance.
For closed appliances like wood-burning stoves, the flue diameter typically matches the stove’s flue collar (commonly 5” or 6”). DEFRA-exempt wood-burning stoves maintain the collar size throughout the system, while multifuel stoves with a 5” outlet often increase to 6” when transitioning to the chimney system, per manufacturer guidance. The Isokern pumice liner system, available at Stove Saver, offers modular flue liners designed for indoor use, ensuring compliance and optimal performance starting at 150mm / 6".
Gathers and Support Blocks: Supporting the System
For open fires, a gather block—typically made of pumice in the Isokern system—is essential to funnel combustion products smoothly into the flue, improving performance and reducing spillage risks. The gather block also supports the weight of the flue liner, ensuring structural integrity. For closed appliances like wood-burning or multifuel stoves, support blocks play a critical role in anchoring the flue liner system. These blocks, typically constructed from lightweight yet durable pumice, are designed to sit on lintels within the chimney structure, providing a stable base for the flue liner.
The Isokern support block features a pumice spigot, which seamlessly connects to the flue liner, ensuring a secure and airtight fit. When paired with an Isokern stainless steel adapter, this setup creates the perfect transition from the stove’s flue pipe to the chimney system, ensuring a smooth flow of combustion gases and minimising the risk of leaks or backdrafts. The stainless steel adapter is precision-engineered to fit the Isokern system, making it an ideal choice for installing a stove, as it simplifies the connection process and enhances the overall efficiency and safety of the chimney. This combination of support block, pumice spigot, and stainless steel adapter ensures a robust, reliable, and compliant installation tailored to modern indoor heating appliances.
Flue Liners: The Isokern Pumice Advantage
The term “flue liner” can cause confusion. For new-build indoor chimneys, Document J mandates rigid liner systems, like the Isokern pumice liner system, which cannot be replaced with flexible liners. Flexible liners are only permitted for retrofitting existing masonry chimneys as a temporary repair and must be surrounded by at least 100mm of masonry. The Isokern pumice liner system, made from volcanic pumice, offers superior insulation, reducing heat loss and improving safety. Its modular design simplifies installation while meeting stringent regulatory standards, making it a cost-effective and durable choice for new constructions.
Mid-Feathers: Enhancing Safety and Stability
In chimneys with multiple flues, a mid-feather—a masonry dividing wall—must separate each flue along the entire chimney length. Mid-feathers provide a firebreak during chimney fires, add structural rigidity, and act as a barrier if a liner fails. They also support the chimney pot at the stack’s top. For multi-flue chimneys, mid-feathers are mandatory for safety and compliance. The Isokern system integrates seamlessly with mid-feather constructions, ensuring a robust and safe chimney.
Installation Tips for the Isokern Pumice Liner System
To achieve a successful indoor chimney installation with the Isokern pumice liner system, follow these tips:
Plan Thoroughly: Work with your architect and builder to align the chimney breast and recess dimensions with your appliance and Isokern specifications, referencing Document J.
Use a Reinforced Base: Ensure the constructional hearth is at least 125mm thick and reinforced to support the chimney’s weight (e.g., approximately 500kg for a standard setup, excluding masonry).
Maintain Clearances: Keep a 75mm gap between the chimney and combustible materials to prevent fire risks.
Follow Instructions: The Isokern system includes detailed guides, and Schiedel’s website offers installation videos (Schiedel). Professional installation services, available through retailers like Bonfire, may be ideal for complex projects.
Seal Properly: Use Isokern’s lip glue to seal liners, preventing leaks and ensuring efficiency.
Test the System: Conduct a smoke test post-installation to verify proper flue draw and no leaks.
Debunking Common Myths
Myth: Flexible liners are suitable for new-build chimneys.
Only rigid liners, like the Isokern pumice system, are permitted for new indoor chimneys per Document J.
Myth: A constructional hearth is always required.
Freestanding stoves keeping hearth temperatures below 100°C may only need a 12mm superimposed hearth, per manufacturer guidelines.
Myth: Fire-rated plasterboard can line a fireplace recess.
Only A1-rated non-combustible materials, like Promat Promafour, are suitable for high-temperature environments.
Why Choose the Isokern Pumice Liner System from Stove Saver?
The Isokern pumice liner system, available at Stove Saver, is a premier choice for indoor chimney construction. Its volcanic pumice composition offers exceptional insulation, durability, and ease of installation, ensuring compliance with Document J and optimal performance. Whether installing a wood-burning stove, multifuel stove, or open fire, the Isokern system delivers safety and efficiency. Customers praise its reliability, with one noting, “The Isokern system made our indoor chimney installation seamless. The stainless steel adapter and support block ensured a perfect fit for our stove, making it both safe and efficient.”
Conclusion
Building a stove recess and chimney stack with the Isokern pumice liner system ensures a safe, efficient, and stylish indoor fireplace. By carefully constructing the hearth, recess, flue, gather and support blocks, liners, and mid-feathers, you can meet regulatory standards and create a lasting heating solution. The Isokern system’s pumice components, including the support block with its pumice spigot and stainless steel adapter, provide unmatched insulation and durability, making it ideal for modern homes. Visit Stove Saver to explore the Isokern pumice liner system and start building your dream indoor fireplace today.
Old Post
Building a chimney stack should be a simple affair, it is covered well in building regulations document J and many drawing examples are available at the click of a button, You construct a masonry (brick or block) chimney breast to the dimensions to suit your design, room size and type of appliance and size the flue accordingly. However we have found over the years between the builder and the architects very rarely is it done correctly for the customers needs. Here we will run through a few key points of vital information we are frequently asked when discussing flues with customers, also hopefully cover a few myths as well. There is a useful video also.
Lets start from the ground up.
HEARTHS
CONSTRUCTIONAL HEARTH
All Masonry Chimney breasts to be built must have a constructional hearth (or sub hearth). In simple terms this is a concrete slab that is 125mm thick, project 500mm in front of the chimney breast face and be 150mm wide than the builders opening / stove recess. This is an old regulation requirement really, aimed at open fires as they put significant amounts of heat downwards and could easily set fire to any timber beneath. However modern stoves will generally not get the hearth beneath over 100°c so are mostly now approved to only stand on a superimposed hearth of at least 12mm thick, which does not require a constructional hearth when in a free standing situation..
SUPERIMPOSED HEARTH
A superimposed hearth (or decorative hearth) is the non combustible material on top of sub hearth, there are still requirements for this to be non combustible, and of suitable dimensions to suit the appliance manufacturer's requirements. Generally it is 300mm in front of the appliance and 150mm either side wider than the appliance, you can get this down to 225mm when installing a closed appliance and the stove manufacturer also states that 225mm is suitable. Hetas do recommend that the hearth should project more than the door opening size, but this is only guidance. When there is a discrepancy between regulations and manufacturers instructions You should always go with the most stringent requirement. If the stove does not get the hearth over 100° then normally in a Free Standing situation (no chimney breast) then a sub hearth is not required and often a 12mm thickness will be adequate.
FIREPLACE RECESS
The fireplace recess (builders opening or chamber) should be constructed as per the measurements and guidance below. If the fireplace is on an internal wall then it must have a back of at least 200mm thick, if on a cavity wall or back to back with another chamber this can be 100mm.
RECESS SIZE & LINING
The recess dimensions will be dictated by the appliance type to be installed. For example, a 5KW wood burning stove generally you will create a recess of approximately 900mm x 900mm x 350mm deep. Recesses that require a finished plaster look should be fully lined in A1 rated non combustible materials (not pink plasterboard!) such as Promat Promafour, normally the recess will not be skimmed/plastered as this will normally craze or fall off and look unsightly. You can finish in many materials including cast iron panels, Vermiculite panels or simply porcelain or natural stone tiles.
FLUE SIZE & DIAMETER
Flue diameters are a constant discussion with chimney construction, mainly it depends on what appliance and what fuel you will be installing whether it is a closed appliance such as a stove or and Open fire, such as a dog basket. Open fires should have a flue with a cross section of at least 15% the total opening size (unless full flue performance calculations are undertaken). A fireplace opening of 550mm x 550mm should have a flue diameter of no less than 200mm in diameter for solid fuel. We are often asked to supply flues for open fires with opening of approximately 1000mm x 1000mm, this often results in flues of around 450mm in diameter to comply with building regulations, which can be quite costly, very large and are made to order. To reduce this the only realistic way is to reduce opening size or install a large firebox or cassette stove to bring the flues down to between 150mm - 300mm in diameter (appliance dependant) . Stoves and Closed appliances, Generally you will maintain the flue collar or outlet diameter on wood burning stoves that are Defra Exempt, so most commonly now 5" or 6" in diameter, if your appliance is multifuel stove with a 5" outlet you will increase to 6" where it converts from the stove outlet pipe to chimney system. 6" diameter flues and above you will normally maintain the outlet size.
GATHERS & SUPPORT BLOCKS
When constructing a masonry chimney you need to make the decision of what type of appliance are you going to be installing. Will it be an open fire? like a dog basket, or a closed appliance? such as a free standing wood burning stove or an inbuilt stove. With an open fire you will need to incorporate into the construction a gather block, normally concrete or pumice, the gather block is designed to funnel the products of combustion smoothly into the flue system, this increases flue performance and minimises the chances of spillage from the fire. The gather block will normally support the weight of the flue liner also.
FLUE LINERS
Flue liners is a term that causes confusion within the construction industry, as it can carry dual meaning, so its critical to know which one you are talking about, as it is possible to line and lined chimney!. For all new build houses, chimneys have to be constructed with a rigid liner system and cannot be a flexible liner. So it is the most cost effective way to build your chimney stack with the most appropriate liner from the start, as this can minimise liner and construction cost and also reduce stove installation costs too. Flexible liners can only be installed in a existing masonry chimney system, as they are classed as a temporary repair. It is not permitted to use a flexible liner as part of a new construction or outside of a masonry construct of 100mm in thickness surrounding the liner.
MID-FEATHERS
A mid feather is a masonry dividing wall between two flues, within a chimney construction. When constructing a chimney with more than one flue contained within it, a midfeather must be present throughout the length of the construct. Mid feathers are present to separate flues from one another, this creates a firebreak for additional safety under chimney fire situation, gives constructional rigidity around the liner and laterally for the chimney stack, and acts as another barrier should any products of combustion be released under liner failure for any reason. Lastly a mid feather also gives your chimney port something to sit of properly at the top. You must include mid feathers in all chimney constructs with more than one flue.
