Boiler Size Calculator
Use this boiler size calculator to estimate a practical starting boiler-size range in kW for combi, heat-only, and system boilers based on the Omni page's property-age and bedroom-bathroom workflow.
Starting Boiler Size Range
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Quick Answer: Boiler sizing depends on boiler type, property age or insulation level, and the number of bedrooms and bathrooms. Omni notes that a 2-bathroom, 3-bedroom modern house may need at least about 18-19 kW for a combi boiler, with a larger practical selection often preferred.
How to Calculate
- Choose the boiler type: Select combi, heat only, or system boiler based on your heating and hot-water setup.
- Choose the property age: Older homes usually need a higher allowance because they are less insulated.
- Choose the bedroom profile: Use bedrooms and bathrooms for combi boilers, or bedrooms only for heat-only and system boilers.
- Use the result as a starting range: Compare the estimate with a proper heat-loss calculation and manufacturer guidance before buying.
Formula
recommended kW = base demand + insulation adjustment + hot-water demand adjustment
| Variable | Meaning | Unit |
|---|---|---|
| base demand | Heating load from property size | kW |
| insulation adjustment | Allowance for older or less insulated homes | kW |
| hot-water adjustment | Allowance for higher domestic hot-water demand | kW |
Worked Examples
Combi boiler - 3 bedrooms and 2 bathrooms, modern house
- Boiler type: Combi
- Property age: 2000 onwards
- Profile: 3 bedrooms + 2 bathrooms
Result: Starting estimate around 18-19 kW minimum, with a larger practical selection often preferred
This mirrors Omni's FAQ wording and shows why a room-by-room heat-loss check is still useful.
Interpretation Table
| Range | Meaning | Action |
|---|---|---|
| Lower kW range | Smaller or better-insulated property | Good as a first pass, but confirm output against emitter sizing and hot-water demand. |
| Mid kW range | Average-sized home | Check whether hot-water production or heating load is the controlling demand. |
| Higher kW range | Larger home, more bathrooms, or older insulation standard | A formal heat-loss calculation is strongly recommended before purchase. |
Frequently Asked Questions
Start with your boiler type, property age or insulation level, and the number of bedrooms and bathrooms, then confirm the estimate with a proper heat-loss calculation.
Omni notes that a standard boiler is often around 24-30 kW for an average home, although the real heating requirement may be much lower.
A combi boiler is often a good fit for a small apartment because it provides heating and hot water without a separate cylinder.
Note: This calculator is an estimating tool, not a full heat-loss design. Final selection should account for insulation, emitters, occupancy, hot-water demand, and manufacturer specifications.
References
Last reviewed: March 14, 2026