Free tool
Boiler Size Calculator
Answer four quick questions and we'll tell you what type and size of boiler your Cardiff home is likely to need — then we'll confirm it properly on a free survey.
Your home
Recommendation
Combi boiler
24–27 kW
- • 9 radiators ≈ 17 kW of heating demand (about 1.5 kW per radiator).
- • One bathroom suits a combi — instant hot water with no cylinder or tanks.
This is a guide only. Final sizing depends on a heat loss calculation, your mains water flow rate, gas supply and property insulation — all of which we check on site.
How boiler sizing works in the UK
Boiler output is measured in kilowatts (kW). Too small and the house never gets warm; too big and you waste gas cycling on and off. A rough heating load is around 1.5 kW per radiator plus a base allowance, but hot water demand usually decides the final figure.
- Combi (24–35 kW): best for one-bathroom Cardiff terraces and flats. No cylinder, no loft tanks, instant hot water.
- System boiler + unvented cylinder: the right call for two or more bathrooms, or larger homes where showers run at once.
- Regular (heat-only): older properties keeping a gravity-fed system with a cold tank in the loft.
Mains water flow rate matters just as much as kW — many older Cardiff properties have small incoming mains, which limits what a combi can realistically deliver. We measure it before we quote.