When selecting an LPG boiler at City Plumbing, "size" refers to the heat output, measured in kilowatts (kW). Choosing the correct output ensures your home stays warm and your hot water remains consistent without wasting fuel.
The required output depends primarily on two factors: your heating demand (number of radiators) and your hot water demand (number of bathrooms).
LPG Boiler Sizing Guide
To ensure your home remains warm and your hot water is consistent, use these general guidelines based on your property size and demand:
Flat or Small House (1 Bathroom, 1–10 Radiators):
Combi Boiler: 24kW – 27kW
System/Regular Boiler: 12kW – 15kW
Medium Family Home (1–2 Bathrooms, 10–15 Radiators):
Combi Boiler: 28kW – 34kW
System/Regular Boiler: 18kW – 24kW
Large Detached Home (2–3+ Bathrooms, 15–20+ Radiators):
Combi Boiler: 35kW – 42kW
System/Regular Boiler: 30kW+
Understanding the Options
Combi Boilers (Higher kW): These require a higher output because they heat water instantly on demand. A 24kW combi is the entry-level for small properties, while a 42kW model is needed to provide enough flow for two simultaneous showers.
System & Regular Boilers (Lower kW): Because these boilers heat water stored in a separate cylinder, they don't need the same "instant" power as a combi. Their sizing is focused mainly on the number of radiators they need to keep hot.
The "Rule of Thumb": A common industry estimate is to allow approximately 1.5kW per radiator in your home, plus an additional 3kW for your hot water cylinder if you aren't using a combi.
Insulation Matters: Older rural properties with solid stone walls or single glazing may have higher "heat loss," meaning you might need to step up to the next available kW size to ensure the house reaches a comfortable temperature during winter.
Why Sizing is Critical
Under-sizing: Your boiler will struggle to heat the house on the coldest days, and your hot water flow may be weak or run cold if a second tap is opened.
Over-sizing: While modern boilers can "modulate" (turn their power down), a significantly oversized boiler will "cycle" on and off too frequently, increasing wear on components and wasting expensive LPG fuel.