Keys for assessing heat demand for a renewable heating project
Assessment of the heat demands of a building is key to the design of a replacement heating system and the business case for it. For a full business case assessment, a heating system will need to be designed and costed by heating surveyors or engineers. However, it is possible to get an idea of the size of system based on figures available from historical energy bills for the building or other sources of information.
Fundamental quantities
The two key features that drive the size, cost and technical feasibility of a greenspace heat scheme are the peak heat demand and annual heat demand. These quantities apply to a single building, but also to a number of buildings collectively in the context of a heat network. Most heat pump schemes supply all of (or the vast majority of) connected buildings’ heat demand – so the capacity of the system that is installed is directly matched to the buildings’ peak and annual heat demand.1
The cost of a greenspace heat scheme will be highly influenced by the peak heat demand value. This quantity is the maximum amount of heat that will need to be supplied at any one moment. For space heating applications, this corresponds to the amount of heat that a building needs to keep inside conditions comfortable when the weather is especially cold. The higher the peak heat demand, the larger the heat pump, the larger the ground or water source heat collector and the larger the heat distribution system (e.g. pipework) that is required. The peak heat demand is measured in kilowatts (kW).
The scheme’s financial and non-financial benefits (including carbon emissions and air quality benefits) are influenced by another value, the annual heat demand of the connected buildings. Some subsidies may also be based on this value. Along with the peak heat demand, the annual heat demand also has a bearing on the size of the ground or water source heat collector. Annual heat demand is a measure of the total amount of energy delivered in the form of heating in the course of a year, and is measured in kilowatt-hours (kWh).
Both peak and annual demand values are needed to establish the amount of heat that can be generated from a given area of land (or vice versa, how much land is required to generate a given amount of heat), although assumptions can be made where information is not available. Similarly, estimates or known values for both quantities are required to assess financial viability. Known values or sensible estimates can be obtained from various sources as shown in Table 1 – the best sources for generating reliable assessments are listed first.
| Source | Value discovered (peak or annual) | Comment | |
|---|---|---|---|
| Lower-confidence assessments Higher-confidence assessments | Heat loss survey | Can be both | Requires dedicated visit by a specialist surveyor, which will incur costs. |
| Design information from the time of build/refit | Can be both | A starting point from which the impact of changes in heating requirements since the design was made – if any - can be taken into account in order to project future heat demand. For example, changes in occupancy, building use or energy efficiency measures. | |
| Utility bills or meter readings | Annual heat demand | A starting point from which the impact of changes in heating requirements since the bills were issued – if any - can be taken into account in order to project future heat demand. For example, changes in occupancy, building use or energy efficiency measures. | |
| Energy Performance Certificate (EPC) or Display Energy Certificate (DEC) | Annual heat demand | EPCs or DECs are not available for all properties. The heat demand figures shown are calculated according to a standard methodology that does not account for occupancy or user behaviour. May require conversion of fuel consumption figures into heat demand. | |
| Current heating system capacity | Peak heat demand | Normally represents an ‘upper bound’ for the peak heat demand, since the current system may have been designed to include some spare capacity. Future design work for the greenspace heat scheme could bring down the capacity requirements and therefore the cost. | |
| Partial inference using an assumed capacity factor (see dedicated section below) | The unknown quantity is inferred from the known quantity | Low confidence in the capacity factor assumption will result in low confidence in the inferred quantity. | |
| Benchmark values for different types of building | Annual heat demand | Energy consumption can vary widely between buildings of the same use type, even when the floor area and age are similar. Benchmark values can be used as starting points for opportunity searches, but building-specific data will be needed to improve confidence when assessing the prospects for an identified opportunity. |
New-build heat demand
For new buildings, the only sources from Table 1 that could be available are design information and benchmark values. The heat demand of a similar existing building could also be used in place of benchmark values.
Adjusting for energy efficiency improvements
As part of a heating system upgrade to an existing building, it is worth considering carrying out upgrades to the building fabric and services at the same time to improve energy efficiency. Increased insulation and controlled ventilation will result in a reduced heating capacity and energy requirement. There are measures which may reduce the impact on system sizing due to hot water requirements, such as increasing hot water storage or reducing hot water flow rates with water-efficient taps.
The respective impacts of various possible energy efficiency measures are listed on a building’s Energy Performance Certificate (EPC) or Display Energy Certificate (DEC).2 If energy efficiency measures are to be installed at the same time as the greenspace heat scheme implementation, these projected impacts may be used to modify the baseline values for a building’s annual heat demand.
Capacity factor
The relationship between the values of peak heat demand and annual heat demand is described by the capacity factor. This is the ratio between the average heat output and the peak heat output. It is also equivalent to the annual heat demand divided by the peak heat demand multiplied by 8,760 hours (the number of hours in a year):
Capacity factor= average heat output /(kW)peak heat output (kW)= annual heat demand (kWh)/peak heat demand (kW) ×8760 hours
Another term used to describe the same relationship is the full-load-equivalent hours (FLEQ) figure,3 which is the annual heat demand divided by the peak heat demand, or the capacity factor multiplied by 8,760 hours:
FLEQ hours= capacity factor ×8,760 hours= annual heat demand (kWh)peak heat demand (kW)
When either the peak heat demand or the annual heat demand is known – but not both - an assumption can be made for the capacity factor which will allow an estimate of the unknown quantity to be inferred. If there are estimates for both values, the capacity factor conversion can be applied to the more reliable value in order to sense-check the other.
Typical capacity factors
The capacity factor for space heating loads depends on many factors, including occupancy patterns, climatic conditions and the fabric of the building itself. Typical values for space heating loads range between 10% and 30%. For other types of heating load the range of capacity factor can be much wider: for situations where consistent amounts of heat are applied to a process that runs constantly throughout the year, the capacity factor can be close to 100%.
When a capacity factor assumption must be made to assess the capacity of schemes providing space heating, a suitable value would be 17% for an intermittently occupied building (e.g. an office) and 24% for a continuously occupied building (e.g. a care home).4
When using the capacity factor as an input to an estimation of the ground heat resource (see 5.2 - Estimating the heat resource), it is prudent to use slightly higher figures of 20% for intermittently occupied buildings and 27% for continuously occupied buildings.5
Annual heat demand
Sources of information or estimates for annual heat demand include (in decreasing order of preference):
Heat loss survey - best
Building design information - best
Utility bills or meter readings
EPC or DEC figures
Inferring from peak heat demand by assuming a capacity factor
Benchmark values
If the required value is not available from building design information or a heat survey, the annual heat demand can be estimated from utility bills or meter readings, from EPC or DEC figures or using benchmark values.
If reliable information is available about the peak heat demand but not the annual heat demand, the latter can be calculated via the use of a capacity factor figure or assumption:
Annual heat demand (kWh) = peak heat demand (kW) x capacity factor x 8,760 (hours)
Where a capacity factor assumption must be made, a suitable value would be 17% for an intermittently occupied building and 24% for a continuously occupied building.
To estimate annual heat demand using energy bills or meter readings, firstly, determine what kind of existing heating there is.
Using utility bills or meter readings for buildings currently using gas, oil or LPG heating
Where all heating is from fuels like gas, oil or LPG, the total amount of fuel used per annum is the starting point for the annual heat demand calculation. Unless there is a significant usage of fuel for purposes other than heating - e.g. a kitchen with relatively high utilisation of gas hobs and ovens - then assume all fuel is used for space heating and hot water generation.
Gas bills should report gas usage in kWh, the unit required for further calculations. However, where meter readings are used, the meter will be measuring a volume of gas. The amount of gas used between two readings is calculated by subtracting the first meter reading from the second one. This must then be converted to kWh. Many online converters and guides will help with this.
An outline of the calculations is:
Work out if the meter is imperial or metric. Imperial meters measure cubic feet (ft3) where one unit is 100ft3. Metric meters measure cubic metres (m3).
Calculate how many units or m3 have been used between two dates by subtracting the reading from the earlier date from the reading at the later date.
Convert the 100s of cubic feet to cubic metres, if necessary.6
Convert the volume of gas (now in cubic metres) to energy. This is done by multiplying by a value which is approximately 40 (reference7).
Bills for oil, LPG and similar fuels may only state the consumption in litres or kilograms rather than kWh. In this case, a conversion must be made in a similar manner to step 4 above; however, the multiplier is different for each fuel.
The resulting value is the amount of fuel energy that has been measured, in kWh. This is not the same as the building’s heat demand: not all energy in the fuel is delivered as useful heat due to the inefficiency of converting chemical energy to useable heat energy (boiler efficiencies are always less than 100%8). A rough assumption would be that newer condensing boiler systems operate at 90% efficiency with older boiler systems 80% efficient.
To convert the fuel usage in kWh into a heat demand in kWh, multiply by the efficiency. If bill or meter information does not cover a whole year, values can be extrapolated to match the shape of typical month-by-month profiles for heat demand from other buildings.
Using utility bills or meter readings for buildings currently using electric heating
Where all heating is electric, heating load is mixed in with other loads. The ratio of heating to other loads will vary greatly depending on the building use.
If enough detail is available, an estimate of heat demand can be obtained by subtracting summer energy use from winter energy use. The unit of measurement in electricity meters is already kilowatt-hours and does not require efficiency to be taken into account (efficiency is close to 100%). As with gas, the energy used between two dates is the earlier meter reading subtracted from the later one.
If this is not possible, then the total electricity use serves as an “upper bound” for the annual heat demand only.
If bill or meter information does not cover a whole year, values can be extrapolated to match the shape of typical month-by-month profiles for heat demand from other buildings.
Estimation for buildings currently using a mix of existing electric heating and gas/oil/LPG boilers
Where heating is provided by a mixture of electricity and fuel combustion, and billing or meter information is available that allows the contribution that each makes to the heating supply to be quantified, these contributions can be summed to obtain a total annual heat demand.
Peak heat demand
Sources of information or estimates for peak heat demand include (in decreasing order of preference):
Heat loss survey - best
Building design information - best
Current heating system capacity
Inferring from annual heat demand by assuming a capacity factor
If the required value is not available from building design information or a heat survey, the peak heat demand for existing buildings may be inferred from the capacity of the existing heating system – see next section.
If reliable information is available about the annual heat demand but not the peak heat demand, the latter can be calculated via the use of a capacity factor figure or assumption:
Peak heat demand kW=Annual heat demand (kWh)capacity factor ×8,760 hours
Where a capacity factor assumption must be made, a suitable value would be 17% for an intermittently occupied building and 24% for a continuously occupied building.
Making sense of current heating system capacity
In many instances, the capacity of the existing source(s) of heat – normally the boiler or boilers – can be used as a guide to the building’s peak heat demand. It must be remembered that the current heat generation equipment might have been deliberately oversized, or may incorporate redundancy – where this is the case, the capacity indicated can be considered to be an ‘upper bound’ on the building’s peak heat load.
Where the heating system also provides hot water on demand (e.g. a gas combi boiler), the maximum instantaneous output of the heat generation equipment will normally exceed the building’s peak heat load as it would be seen by the designer of a heat pump system.9 In this instance, the boiler’s capacity cannot be used to estimate the building’s peak heat demand, so an alternative source must be used for that information.
The capacity of the heat generation equipment should be stated on boilerplates, in manuals or in installation documentation.10 If none of these are available, it is worth looking up the make and model online. Correct identification of the exact model in a range may require the model or serial number.
Building benchmarks
Table 2 suggests some benchmarks for different types of non-domestic building that could be used to estimate annual heat demand when no other information is available.
| Building type | Annual heat demand benchmark estimate (kWh per m2 per annum) |
|---|---|
| Offices and public buildings | 100 |
| Schools | 130 |
| Buildings for cultural activities | 170 |
| Sports and leisure facilities | 280 – 1,000 (swimming pools have highest demand) |
Footnotes:
It is possible to design a heat pump system that meets only part of a building’s heat demand, with the remainder being supplied from an alternative source – an arrangement called a ‘bivalent system’. However, this guide assumes that the greenspace heat scheme is to supply all of the connected buildings’ heat demand.
Not all buildings have EPCs or DECs.
FLEQ is the quantity used in guidelines from the Microgeneration Certification Scheme (MCS). MCS system sizing tables for ground source heat pumps cover the range from 1,200 FLEQ hours (a 14% capacity factor) to 3,600 FLEQ hours (a 41% capacity factor).
FLEQ equivalent values would be 1,500 hours and 2,100 hours respectively.
FLEQ equivalent values would be 1,800 hours and 2,400 hours respectively.
1 unit = 100ft3. Multiply the number of units by 2.83 (or number of cubic feet by 0.0283).
This value comes from an industry figure for atmospheric pressure multiplied by a published calorific value that depends on the composition of the gas.
Sometimes boiler efficiencies are quoted on a basis that ignores a component of the input energy; in this case, the quoted efficiency of condensing boilers can exceed 100%. However, the total useful energy out is always less than the total energy input.
Heat pump systems that provide hot water as well as space heating always include hot water storage, i.e. a hot water tank, that allows hot water to be generated more slowly and over longer time periods.
Old or non-European boilers may require conversion of the capacity to metric units (kW).