Why generic UK online boiler sizing tools undersize boilers for Glasgow’s historic sandstone housing stock, and how to calculate the correct kilowatt output for your flat.
Most online boiler calculators produced by national aggregators assume modern British room heights of 2.4 metres. A traditional Glasgow tenement flat in Partick, Shawlands, or Dennistoun features ceiling heights between 3.0 and 3.4 metres.
This creates 30% to 40% more cubic air volume that must be heated, combined with uninsulated solid sandstone external walls (U-values often exceeding 1.7 W/m²K) and large single-glazed sash windows.
| Property Size | Radiator Count | Domestic Hot Water (DHW) | Recommended kW Rating | Example Package |
|---|---|---|---|---|
| 1-Bed Tenement | 4–6 Radiators | 1 Bathroom / Shower | 28kW – 30kW | Ideal Logic+ C30 / Worcester 4000 30kW |
| 2-Bed Tenement | 6–9 Radiators | 1 Bathroom / High Flow | 30kW – 32kW | Worcester Bosch 4000 30kW |
| 3-Bed Tenement | 8–12 Radiators | 1-2 Bathrooms | 32kW – 35kW | Ideal Vogue Max C32 / Worcester 8000 35kW |
| 4+ Bed Tenement / Villa | 12+ Radiators | 2+ Bathrooms | 35kW – 40kW (or System Boiler) | Worcester Bosch Greenstar 8000 Life 40kW |
With modern combi boilers, the kilowatt rating primarily dictates your hot water flow rate rather than central heating output. For instance, a 30kW combi boiler delivers approximately 12.3 litres of hot water per minute at a 35°C temperature rise, ensuring powerful shower pressure.
Modern combi boilers feature wide modulation ratios (e.g., 1:10), meaning that when heating mild Scottish autumn rooms, the boiler can automatically turn down to 3kW without short-cycling, maximizing fuel efficiency.
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