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Magnum Firechest Kits
Isokern Magnum firechest kits in four sizes, from the compact Magnum 500 to the 1246mm-wide Magnum 1200, plus the Isokern Standard Firechest. Each is a flat-packed Schiedel pumice open fire chamber, tested to EN 13229, with the gather included. Size and flue details are below the products.
5 Products
Which Magnum firechest size do I need?
Isokern Magnum firechests are flat-packed pumice fire chambers for building a real open fire with a matched Isokern chimney. The specially shaped chamber burns wood logs at 41% to 45% efficiency depending on model, and the range is tested to EN 13229. Start with the fire opening you want, then check the flue it needs:
Isokern Standard Firechest
690 × 660mm opening, 850 × 660 × 450mm external, 100kg.Used with a separate gather; the pumice gathers have a matching 690mm internal width.
ViewWider openings need more combustion air (see the table), a larger flue and a stronger hearth and foundation. Not sure which to choose? Call 0113 322 5187 or email [email protected].
Magnum firechest dimensions and weights
| Model | Fire opening W × H | External W × H × D | Weight | Inside flue diameter | Combustion air (free area) |
|---|---|---|---|---|---|
| Magnum 500 | 510 × 540mm | 685 × 1540 × 440mm | 390kg | 200mm | 200cm² |
| Magnum 950 | 990 × 960mm | 1090 × 1761 × 710mm | 715kg | 350mm | 248cm² |
| Magnum 1100 | 1130 × 960mm | 1230 × 1761 × 710mm | 790kg | 350mm | 338cm² |
| Magnum 1200 | 1246 × 960mm | 1346 × 1761 × 710mm | 845kg | 350mm | 385cm² |
Source: Schiedel Isokern Chimney Systems brochure (firechest table) and Magnum Firechest installation manual. Always design from the current manufacturer drawings.
What's in a Magnum firechest kit?
Each Magnum firechest is packed flat on a pallet with assembly instructions. It contains the pumice base slabs, side and back elements, the gather sections and the top plate. Every Magnum kit we sell includes Isokern lip glue for assembly:
- Magnum 500 kit: 2 bags of lip glue, buff firebricks and firebrick grout.
- Magnum 950: 5 bags of lip glue. Firebricks and grout are optional extras.
- Magnum 1100 and 1200: 6 bags of lip glue. Firebricks and grout are optional extras.
You will also need a flue above the firechest: Isokern pumice flue liners in your own masonry, or an Isokern DM54 system chimney. An optional flue damper is available in the Magnum range.
Foundations, hearth and surrounding masonry
A Magnum firechest is heavy and load-bearing, so it must start on a suitable foundation and constructional hearth. Schiedel states it will carry up to 2,600kg of chimney above, and more with additional lintels.
The installation manual requires 100mm of brick or blockwork around each side and behind the back of the firechest to comply with Building Regulations. The fire also needs a permanent supply of combustion air: the free-air area for each model is in the table above. Open fires are notifiable building work, so use a competent installer and check with Building Control. See our guide to Approved Document J for chimneys.
How a Magnum firechest goes together
Schiedel says that, from a prepared foundation and constructional hearth, the firechest and gather could be assembled in less than an hour. In outline, from the Magnum Firechest installation manual:
- Bed the base slabs onto the foundation with mortar and check they are plumb and level.
- Mix the lip glue and use the supplied application bag to bed the side and back elements, layer by layer. Keep the joints 2–3mm thick and wipe away the excess.
- Bed the gather layers on top. If you are using a damper, fit it loose onto the gather elements, with no mortar or glue.
- Bed the top plate slab and check that everything is plumb and level.
- Lay 50mm firebricks loose on the base (glue only the front row), glue 50mm firebricks to the back and 25mm firebricks to the sides with firebrick mortar, then point the joints.
- Build the 100mm surrounding masonry and connect the flue system above.
Read our Magnum firechest buying guide and installation tips.
Damper and efficiency
The Magnum was tested at the Fraunhofer Institut, Stuttgart (cert no. P8-094/2006). The optional flue damper lets you control the draught and avoid heat loss when the fire is not in use. Schiedel notes that including the damper halves the chimney ventilation rate in the SAP calculation for Approved Document L. The damper must not be installed with gas fires.
Planning the chimney above your firechest
The Isokern system is designed as a whole, avoiding many of the variable factors that lead to draught problems and smoky fireplaces. The Magnum 500 needs a 200mm internal diameter flue; the 950, 1100 and 1200 need 350mm. Key rules from the Pumice Liner installation guide:
- Install the liners socket uppermost and seal each joint with Isokern lip glue, wiping away the excess.
- Clad the liners with a minimum of 100mm of brick or blockwork and backfill with a LECA mix.
- Use a steel collar as well as lip glue on any cut joint.
- Allow no more than two complete offsets (four bends) per chimney.
Check the flue against your opening with our flue size calculator.
First fires and sweeping
Leave the chimney for at least 72 hours before use, then light only small fires for the first week and build up gradually. Have the chimney swept at least twice a year, before and after the heating season, with a medium-density polypropylene brush the same diameter as the flue. Steel brushes must never be used on Isokern pumice flues.
Accessories and parts
- Firebricks and firebacks and Isokern firebrick grout 10kg.
- Isokern lip glue 5kg.
- Pumice flue liners: 200mm for the Magnum 500, 350mm for the 950–1200.
- Gathers for the Isokern Standard Firechest.
Technical downloads
- Isokern Magnum Firechest installation manual (PDF)
- Isokern Chimney Systems brochure, with firechest dimensions and weights (PDF)
- Isokern gather technical drawings (PDF)
- Pumice flue liner installation guide (PDF)
- DM double module chimney installation guide (PDF)
- Magnum firechests CAD drawings (ZIP)
- All documents: Isokern manuals and guides
Magnum firechest FAQs
What is an Isokern firechest?
A flat-packed fire chamber cast from lightweight, highly insulating pumice. The parts interlock like a three-dimensional jigsaw to form a fireplace recess and gather, ready for a surround.
Which Magnum sizes are available?
Four: the 500 (510 × 540mm opening), 950 (990 × 960mm), 1100 (1130 × 960mm) and 1200 (1246 × 960mm).
Is the gather included?
Yes. The Magnum installation manual builds the gather layers and top plate as part of the firechest, so you do not need a separate gather. The Isokern Standard Firechest is different: it is used with a separate gather.
Are firebricks included?
The Magnum 500 kit includes buff firebricks and firebrick grout. For the Magnum 950, 1100 and 1200, firebricks and grout are optional extras; each product page lists the quantities.
What flue size does a Magnum firechest need?
The Magnum 500 uses a 200mm internal diameter flue. The 950, 1100 and 1200 use a 350mm flue, built from pumice liners or DM blocks.
How much does a Magnum firechest weigh?
Schiedel lists 390kg (500), 715kg (950), 790kg (1100) and 845kg (1200).
How much chimney can a firechest carry?
Schiedel states the firechest is load-bearing up to 2,600kg of chimney above, and more with additional lintels.
Do I need a damper?
It is optional. A flue damper controls the draught and reduces heat loss when the fire is not in use, and it halves the chimney ventilation rate in the SAP calculation. It must not be installed with gas fires.
What goes around the firechest?
The manual requires 100mm of brick or blockwork around each side and behind the firechest, plus a permanent combustion air supply sized for the model.
Can I use a Magnum firechest outdoors?
For outdoor fires use the purpose-made Isokern garden fireplace kits, which include chimney blocks and render.
Need help choosing your Magnum firechest?
Speak to our Isokern team on 0113 322 5187 or email [email protected]. We can check sizes, flue requirements and what to order with your kit.
Isokern is made by Schiedel; we are not the manufacturer.