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 |
Tips for an accurate estimate
- Take measurements at floor level and at the ceiling – walls and rooms are often slightly out of square.
- Use the manufacturer’s coverage, spacing and installation data when it differs from the typical values here.
- Keep a little extra of every finish material for touch-ups and repairs later.
- Check local building codes and permits before structural, electrical, plumbing or gas work.
Common mistakes to avoid
- Sizing heating or cooling equipment by area alone when insulation, windows and climate make a big difference.
- Buying too little paint, stain or flooring and ending up with mismatched batches.
- Overlooking the extra material for steps, railings, gables and trim.
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 factorCombi: 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 kWWorked 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.
What formula does the Boiler Size Calculator use?
Heating load ≈ (4 + 4 × bedrooms) kW × insulation factor.