Estimating a building's peak daily fuel demand

The peak daily fuel demand is of considerable interest if you suspect that your building's heating plant is oversized. Overcapacity is often designed-in (thanks to excessive safety margins) and sometimes is made worse by the building having been subsequently insulated and draught-proofed.

Oversized plant is wasteful because of higher-than-necessary standing losses, a problem which can be exacerbated by idle duty cycling of boilers (especially when the burner startup sequence includes a pre-ignition purge).

With the benefit of actual operating experience, the true plant load factor can be calculated exactly. This is the procedure:

1. Work out the building's baseline performance relative to degree days, yielding values for k₀ and k₁
2. Note the degree-day base temperature used, T_base
3. Choose an outside-air temperature T_extreme representative of the coldest day
4. The weather-dependent demand on the coldest day is then given by

   (T_base - T_extreme) * k₁

5. Divide the fixed demand constant k₀ by 7 (if calculated on a weekly basis) or 30.5 (if monthly) to compute the daily non-weather-related demand, and add this to the quantity derived from the step 4.

Note: This analysis is valid for a building heated 7 days per week. Additional capacity must be allowed if you consider that there might be a need to boost the building from cold on the peak day.

Worked example

A building is found to have fixed demand of 2,100 kWh per week and weather-related demand of 100 kWh per degree day (base temperature: 15.5C). What is its peak daily demand? Assume a worst-case outside air temperature of -1C.

1. The weather-dependent demand on the coldest day is (15.5-(-1))*100 = 1,650 kWh
2. Fixed demand is 2,100/7 = 300 kWh
3. Hence total demand on the coldest day is 1,650 + 300 = 1,950 kWh