Updated 20 October 2026. Originally published in 2016.
The right boiler size depends on your home's heat loss and hot-water needs. Bedroom count or the number printed on your old boiler is not enough. A heating survey should assess both requirements before an installer recommends an appliance.
Heating output and hot-water output are different
Boiler output is measured in kilowatts (kW). A combi boiler heats radiators and produces hot water on demand, but its maximum heating output may differ from its hot-water rating. Check the exact model's technical data rather than assuming that all 24 kW or 30 kW boilers perform alike.
Ask your installer to explain the property's calculated heat loss, the proposed heating settings and the boiler's minimum output. A boiler that can reduce its output appropriately can better match lower heating demand; simply choosing the biggest model is not a complete design strategy.
What should a boiler sizing survey include?
- Room sizes, insulation, glazing and ventilation.
- Radiator or underfloor heating requirements and intended operating temperatures.
- Bathrooms, occupants and likely simultaneous hot-water use.
- Incoming water flow and pressure under realistic demand.
- Existing pipework, controls, flue position, condensate drainage and installation space.
Combi or a boiler with a hot-water cylinder?
A combi can free up cylinder space and provide hot water on demand. However, simultaneous showers and taps share the available hot-water delivery. A system or heat-only boiler with a correctly designed cylinder may suit a home with greater simultaneous demand.
Keeping a cylinder is not automatically inefficient, and converting to a combi is not automatically an upgrade. Compare insulation, usable storage, recovery time and any existing backup water heating. Consider future heating plans before removing useful cylinder space. Read the Energy Saving Trust's explanation of boiler types.
Water flow is not the same as pressure
Flow is the amount of water delivered, usually in litres per minute; pressure is measured separately. A simple timed tap test can indicate flow, but does not replace an installer measuring the supply and checking performance when outlets run together.
Manufacturer hot-water flow figures are stated at a particular temperature rise. Compare like-for-like figures: colder incoming water or a higher delivery temperature changes the available flow. A larger boiler cannot overcome an inadequate incoming water supply.
Check the whole installation, not just the boiler
Controls and radiator valves
Suitable heating controls and thermostatic radiator valves help avoid heating rooms unnecessarily. The controls must work with the proposed appliance and system. Savings depend on the home and how the controls are used; there is no universal percentage saving for every installation.
System cleaning and protection
Ask how system water quality will be assessed and treated. The appropriate cleaning method depends on condition and manufacturer requirements. A power flush is not automatically needed on every replacement or at a fixed interval. See our power flushing service.
Gas supply, flue and condensate
Your engineer must assess the gas supply against the appliance requirements and installation layout. The boiler connection size alone does not determine the whole gas pipe size. Flue and condensate arrangements must follow the exact appliance instructions. No single condensate pipe diameter can guarantee protection against freezing in every location.
Questions to ask before accepting a quote
- What heat-loss and hot-water assessments support this model?
- Can it meet our simultaneous shower and tap requirements?
- What controls, cleaning and pipework changes are included?
- What warranty applies to this exact model, and what registration and servicing conditions apply?
- Does the written total include VAT, commissioning and applicable certification?
For advice on boiler installation in Huddersfield and West Yorkshire, contact Union Plumbing & Heating to discuss a survey. Gas boiler installation should be carried out by an appropriately qualified Gas Safe registered engineer.
Further reading: Energy Saving Trust: heating controls. Guidance checked 20 October 2026.
