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.

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 a bygone era.


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

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.



See our recent traditional steel window replacement projects:

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 mullion windows



Document Downloads

File Type Document Description Download Document
Adobe PDF Alitherm Heritage Brochure
Download (11.3 MB)
Adobe PDF Alitherm Products
Download (2.2 MB)
Adobe PDF Alitherm Heritage
Section identification Guide
Download (32 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 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