Boiler size depends on two jobs: heating the house and producing hot water. A combi boiler heats water on demand, so its size is set by how many showers or taps may run at once, usually 24–35 kW. System and heat-only boilers store hot water in a cylinder, so they are sized for the heating load plus a little for reheating the cylinder. Older, poorly insulated homes need more heat per room.
How to use the Boiler Size Calculator
- Boiler type
- Bedrooms
- Bathrooms
- Age / insulation of property
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the boiler type, enter the number of bedrooms and bathrooms and pick the age or insulation level of the property. The calculator estimates the heating load and the hot-water load and returns the minimum output to look for. A heat-loss survey by an installer gives the definitive figure.
Typical sizes
| Home | Combi | System / heat only |
|---|---|---|
| 1–2 bed, 1 bath | 24–27 kW | 12–15 kW |
| 3–4 bed, 1–2 bath | 28–34 kW | 18–24 kW |
| 4+ bed, 2+ bath | 35–42 kW | 30–40 kW |
Inputs explained
| Field | What to enter |
|---|---|
| Boiler type | Choose one: Combi, System, Heat only (regular) |
| Bedrooms | A number |
| Bathrooms | A number |
| Age / insulation of property | Choose one: Built after 2000 or well insulated, 1976–2000, average insulation, 1930–1975, some upgrades, Before 1930, solid walls |
What each result means
- Heating load = (4 + 4×[Bedrooms])×[Age ÷ insulation of property]×1000
- Hot-water demand (combi) = (24 + 6×([Bathrooms] – 1))×1000
Formula
Heating load ≈ (4 + 4 × bedrooms) kW × insulation factor
Combi: hot-water demand ≈ 24 kW + 6 kW per extra bathroom; size = max(heating, hot water)
System: heating + 3 kW for cylinder reheat; heat-only: heating + 3 kW
Worked example
Inputs: Boiler type: Combi; Bedrooms: 3; Bathrooms: 2; Age / insulation of property: 1976–2000, average insulation.
Results: Heating load = 16 kW; Hot-water demand (combi) = 30 kW; Minimum boiler output = 30 kW.
Frequently asked questions
What size combi boiler do I need for a 3-bed house with 2 bathrooms?
Typically 28–35 kW, mainly to supply two showers at once.
Is a bigger boiler better?
No. Modern condensing boilers are most efficient when they are not oversized; they modulate down but short-cycle if far too big.
What is the difference between combi, system and regular boilers?
A combi heats water on demand; a system boiler heats a sealed cylinder; a regular (heat-only) boiler works with a cylinder and a loft tank.