Boiler calculator
Find the right boiler size for your home
A quick estimate based on standard UK sizing guidance. Adjust the sliders and we'll suggest an output in kW, plus whether a combi or system boiler makes more sense.
Tell us about your home
Based on standard UK sizing guidance (approx. 1.5kW per radiator plus hot water demand).
Your estimate
30kW
combi boiler
- Estimated heat load
- 15 kW
- Approx. hot water flow
- 13.3 l/min
- Hot water storage
- Not required
A guide only. We always measure mains flow rate and check your pipework before confirming a boiler size and fixed price.
Confirm my quote, 0117 4054064How our boiler quotes work
1. We measure, not guess
Mains flow rate and static pressure are tested on the day of survey, the numbers that decide whether a combi will actually perform.
2. Fixed price, in writing
A clear quote covering boiler, controls, filter, flush, flue, scaffolding if needed, and building regs notification.
3. Installed and registered
Usually a one-day install. We register the warranty and Gas Safe notification for you and hand over the paperwork.
Boiler sizing FAQs
- Is a bigger boiler always better?
- No. An oversized boiler short-cycles: it fires, quickly satisfies demand and shuts off, over and over. That wastes gas, stresses components and shortens its life. Correct sizing matters more than headline output.
- What flow rate should I expect from a combi?
- Roughly 10–14 litres per minute at a comfortable temperature rise, depending on output and your incoming mains supply. If your mains can only deliver 9 l/min, no boiler can exceed that.
- Do I need a powerflush with a new boiler?
- Most manufacturers require the system to be cleaned to BS7593 for the warranty to stand. On an older system that usually means a full powerflush plus inhibitor and a magnetic filter.
- How long does an installation take?
- A straight swap is typically one day. Moving the boiler, converting from a conventional system to a combi, or adding a cylinder usually takes two.