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The Alitherm Heritage steel replacement window system has been specifically designed as an ultra slim, thermally efficient and secure aluminium replacement for traditional steel windows.

Read our Buyers Guide To Steel Window Alternatives

It is particularly suitable for openings with through stone work or where slim stone mullions and transoms require the fame work to be as narrow as possible. The Alitherm Heritage window system is also the perfect solution for timber sub frame applications and is available with a choice of outer frame profiles to accommodate different timber rebates.

With an authentic stepped aluminium profile and classic hardware options our Alitherm Heritage aluminium window system is the perfect choice to sympathetically replicate the style and timeless appearance of old steel windows from Edwardian, Georgian and period bygone eras.


Steel replacement mullion windows

Key Features

  • Aluminium that looks like steel
  • Approved steel replacement windows for listed buildings by many council authorities
  • Ultra slim framework ideal for fitting between stone mullions
  • Can achieve Grade 'A' thermal rating
  • A fraction of the cost of steel
  • Fully compliant with latest building regulations

Compare traditional & new steel with Alitherm Heritage aluminium

Comparison at a glance

Original Steel Windows New Steel Windows Alitherm Heritage
Aluminium Windows
Lowest cost

very high*
Double Glazed
Thermally efficient
Slim frame
Original Appearance
Traditional fittings
Factory painted
Textured finish
Dual colour
Rust proof
100% recycled
*new steel is typically 3 to 4 times more expensive than Alitherm Heritage aluminium
Read our Buyers Guide To Steel Alternatives

A combination of the latest inline polyamide thermal break, krypton gas filled sealed units ensures that our Alitherm Heritage aluminium window range meets the highest standards of thermal performance.

A dedicated external door system with matching frame profiles and exclusive hardware is also available to compliment this window range.





Rated 5.0 out of 5 Maureen Brett

We had first class service from Marlin Windows. They are professional and efficient. We are delighted with our windows and would highly recommend them.


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Product Information

Inspired by the aesthetics of traditional steel windows, Alitherm Heritage features signature slim sightlines and attractive contours making it the ideal choice for heritage building conservation areas where building regulations demand a like-for-like replacement.

Alitherm Heritage slim systems have already been approved as replacement windows for listed buildings by many councils.

All main profiles feature polyamide thermal break technology that enhances the thermal performance of the product, allowing Alitherm Heritage windows to achieve a window energy rating ‘A’ grade when fabricated with the correct specification and glazed unit. Profiles have a high quality powder coated paint finish at Smart’s state of the art paint facility in a choice of finishes, including single or dual colour, in a range of standard and non-standard colours.

Design Applications

Windows: fixed windows, side hung & top hung open out casement
Doors: single or double open out balcony doors


  • Window Energy Rating ‘A’ – when using correct profile, glass unit & spacer bar
  • 1.6 W/m²K with 1.2 centre pane U value
  • 1.4 W/m²K with 0.8 centre pane U value
  • Method of fabrication: corner crimping
  • Glazing options 24mm to 32mm (opening windows accept 24mm units only)
  • Gasket glazed
  • High security multi point locking
  • Traditional Art Deco handles available
Steel replacement window profile


Matching heritage steel style doors

Heritage Doors

Smart Visofold 6000 bi-fold doors

Steel replacement heritage doors

NEW - Heritage Bi-Folds

crittal style heritage bi-fold doors

NEW - Steel style heritage bifolding doors.

Glazing and frame colour choice


Our aluminium profiles are polyester powder coated by our systems suppliers using the very latest paint application techniques guaranteed for 25 years.

Fully compliant to the latest June 2022 building regulations our standard double glazing is the exceptional light transmitting, energy saving ClimaGuard® A+ low emissivity (Low-E) glass manufactured by Guardian Glass.

Read More

View Our Featured Steel Replacement Window projects:



Rated 5.0 out of 5 Anne Corrigan

Marlin Windows were reassuringly professional, calm & helpful from first contact which made the decision to use them so right. The pre-sales team were full of advice - not in the least bit pushy contrary to stereotype - & the advice was spot-on because they'd done it all before.


The product looks amazing too... I suppose the windows might be much the same elsewhere & maybe even of similar price or cheaper, but the customer support, reliability, professionalism & interest at Marlin is second to none.


I honestly think it's a rarity these days to come across a company with values like Marlin so 5*s well-deserved.


Read the rest of this and other 5 star reviews of Marlin Windows left on Google


Technical advisors are on hand at the aluminium window and door showroom in Keighley in West Yorkshire

Unrivalled Quality Customer Service

Our high quality window and door products are backed by the same high quality customer service. Technical advisors are just a phone call away to answer all your questions no matter how technical.

Typical help provided includes:

  • Choosing the right product
  • Getting the right made to measure size
  • Help with ordering
  • Threshold advice

We provide a FREE sample service to assist with surveying decisions, design considerations, or planning applications – just pick up the phone and ask.

Click To Call



Rated 5.0 out of 5 Dom B

Super customer service, they actually answer the phone and call back promptly if they’re not available... very helpful throughout with technical advice.


Read the full reviews and our other 5 star reviews left on Google


Document Downloads

File Type Document Description Download Document
Adobe PDF Alitherm Heritage Brochure
Download (11.3 MB)
Adobe PDF Alitherm Products
Download (5.6 MB)
Adobe PDF Alitherm Heritage
Section identification Guide
Download (33.5 MB)
Adobe PDF Alitherm Heritage
Product Data Sheet IMS123AH.201410
Download (508 KB)
Adobe PDF Alitherm Heritage
Supply only – Guide to installation
Download (403kb)

Technical Specification


Product Type Windows and Doors
Material Aluminium Polyamide
Dual Colour
Thermally broken


Max width 900
max height 2100
Max weight 60
Thermal suitability ★★★★★
Min Glazing 24
Max glazing 32

Locking options


Building suitability

light duty


Rated 5.0 out of 5 Myer Craig Brown

"We ordered Steel Replacement French Patio doors and Windows. They are absolutely beautiful. And they were an absolute pleasure to deal with. Thanks Marlin!"


Read the rest of this and other 5 star reviews of Marlin Windows left on Google


Replacing Windows In A Listed Building

Read our guide to replacing windows in listed buildings

Read our guide to listed building window replacement

Replacing Windows In Conservation Area Properties

Read our guide to replacing windows in conservation area buildings

Read our guide to conservation area property window replacement

Alternatives To Steel - Buyers Guide

Read our guide to Crittall® window and door alternatives

Read our buyers guide to the best Alternatives To Steel Windows and Doors


Why Choose Marlin Windows?

We care...

about the quality of finish
about you, your project and your property
about your 100% satisfaction

And we are rated ★★★★★ on Google

need more reasons? click here

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"our builder said they were the best supplier and fitter they had ever used"
Mrs Nellist Haworth, Yorkshire


Call 01535 603909