Why Two Identical Houses Cost Different Amounts To Heat

Two houses built to the same drawing on the same street can produce very different heating bills. The divergence comes from four factors that act independently of each other.

Heat leaves through fabric and through air

A building loses heat by conduction through walls, roof, floors and glazing, and separately by air moving out and cold air moving in to replace it.

Insulation addresses the first and airtightness addresses the second. A house can be well insulated and still expensive to heat if it leaks air continuously.

Identical designs diverge here because one owner has insulated a loft, sealed a chimney or replaced windows and the other has not, and none of that is visible from the street.

Position changes exposure

A mid-terrace house is bounded by heated neighbours on two sides, while an end-terrace of the same design has an additional external wall losing heat.

Orientation matters too. South-facing glazing collects useful heat in daylight hours, and a sheltered elevation loses less to wind than an exposed one.

Wind is significant because it drives air through gaps. The same leakage that is tolerable in a sheltered position becomes expensive on an exposed corner.

The system converts fuel at different rates

Two houses can demand identical heat and still consume different amounts of fuel, because the appliance, its controls and the distribution pipework all lose energy along the way.

An older appliance running at a temperature that prevents it operating efficiently, or a system with no thermostatic control, delivers the same warmth for more input.

Uninsulated pipes and cylinders, and hot water circulating when nobody needs it, add a constant background demand that has nothing to do with the weather.

Occupancy is a large variable

A house occupied all day is heated all day. One left empty from morning until evening has a different demand profile entirely, whatever its fabric.

Preferred internal temperature has a disproportionate effect, because the rate of heat loss depends on the difference between inside and outside.

Household size drives hot water use, and hot water is a substantial part of annual energy demand that continues through the months when no space heating is required.

Why the published rating diverges from the bill

Energy assessments model a standard household with standard patterns in order to compare buildings with each other, which is a legitimate purpose.

The model cannot know that one household works from home, keeps one room warm, or has replaced a heating system since the assessment was carried out.

Treating the rating as a description of the building and the bill as a description of the household explains most of the gap between the two.