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🔧 Choosing the Right Size Boiler for Your Home: A Complete Guide
When it’s time to install a new boiler, one of the most important decisions is choosing the correct size. But boiler “size” doesn’t refer to physical dimensions — it means the boiler’s output, measured in kilowatts (kW), which determines how much heating and hot water it can provide.
A boiler that’s too small won’t meet your heating needs. One that’s too big will waste energy and cost more to run. This guide will help you choose the right size boiler for your home — whether you're in a cosy flat or a large detached house.
⚙️ What Does Boiler Size Mean?
Boiler size = Heating output, measured in kilowatts (kW)
It tells you how much energy your boiler uses to heat your radiators and hot water.
There are two key outputs:
- Central heating output (CH) – Heats your radiators
- Domestic hot water output (DHW) – Heats your taps/showers
🏡 Boiler Sizing Basics by Property Type
While a professional heating engineer will calculate your exact needs, here’s a general guide:
| Property Type | Bedrooms | Bathrooms | Recommended Boiler Size |
|---|---|---|---|
| Flat / Small House | 1–2 | 1 | 24–27 kW (Combi) |
| Medium House | 3–4 | 1–2 | 28–34 kW (Combi/System) |
| Large House | 4+ | 2+ | 35–42 kW (System or Regular) |
🔍 Factors That Affect the Right Boiler Size
1. Number of Radiators
More radiators = higher heating demand. Count yours:
- 10 or fewer → ~18–24 kW
- 10–15 → ~24–35 kW
- 15+ → 35+ kW
2. Number of Bathrooms
Multiple bathrooms used at the same time? You’ll need a larger combi or system boiler with a hot water cylinder.
3. Insulation and Energy Efficiency
Well-insulated homes (newer builds, double glazing, cavity wall insulation) require less boiler power. Poor insulation = higher kW needed.
4. Hot Water Demand
- One shower at a time? → 24–28 kW combi is fine
- Two or more simultaneous showers? → 35+ kW combi or system boiler with cylinder
5. Boiler Type
- Combi boilers supply hot water and heating on demand — sizing must factor in both
- System boilers work with a hot water cylinder — sizing is mostly for central heating
- Regular boilers (for older systems) work with tanks and cylinders — best for large or traditional setups
🔥 Quick Boiler Output Guide
| Combi Boiler Output | Suitable For |
|---|---|
| 24–27 kW | 1–2 bed home, 1 bathroom |
| 28–34 kW | 3–4 bed home, 1–2 bathrooms |
| 35–42 kW | 4+ bed home, 2+ bathrooms |
🧮 Why You Shouldn’t Oversize a Boiler
It might be tempting to “go big just in case”, but this can cause problems:
- Higher running costs
- Short cycling (on/off too frequently) → more wear and tear
- Reduced lifespan
- Less efficient performance
- Unnecessary noise
⚠️ Oversizing is a common issue with older homes that had bigger boilers fitted in the past "just to be safe". Today’s boilers are far more efficient.
🛠️ Getting a Professional Sizing Assessment
While guides like this are helpful, the most accurate way to size your boiler is through a heat loss calculation done by a qualified heating engineer.
They'll consider:
- Property size and layout
- Type and number of radiators
- Insulation and U-values
- Hot water needs
- System type and age
📦 Recommended Boiler Brands by Size
| Boiler Size | Brands & Models |
|---|---|
| 24–27 kW | Worcester Greenstar 2000, Ideal Logic+ 24, Vaillant ecoTEC Pro 24 |
| 28–34 kW | Worcester 4000 30kW, Baxi 830, Vaillant ecoTEC Plus 32 |
| 35–42 kW | Worcester 8000 Life, Viessmann Vitodens 100-W 35kW, Ideal Vogue Max 40kW |
🏁 Final Thoughts
Choosing the right size boiler ensures your home stays warm and your hot water flows — without wasting energy or money. While general size guides are helpful, it’s always best to get a professional assessment before you buy.
📞 Need Help Finding the Right Boiler Size?
At Gas-Go Heating, we offer:
✅ Boiler sizing advice tailored to your needs
✅ Gas Safe registered installation
✅ Leading brands with up to 10-year warranties
📆 Contact us today. Call 02393 782 351 to find the perfect boiler for your home — no guesswork required.