Heat demand calculation

Heat demand arises from up to four components that are recorded separately and then added hour by hour: building space heating, hot water, process heat and – where present – an externally imported load profile. Network losses can be added on top. Only what is activated as a building block in the project is recorded; building, external heat demand and process heat have to be ticked there individually.

This page describes the working sequence in the program. The calculation method itself is described by the fundamentals page Heat demand calculation, the origin of the weather data by the page Climate data. Heat demand is the starting quantity of every further design step: generator output, storage volume and, in the end, economic viability depend directly on it.

Contents of this page

Building space heating from the catalogue

For the building space heating you select a building from the catalogue. It is filtered by residential or non-residential use, building type and construction period, whose classes range from “before 1919” through to BEG 55 and BEG 40; a full-text search with wildcards helps with large catalogues. The assigned building becomes a project copy in which you adjust the reference quantity – usually the floor area in square metres. Changes therefore only take effect in the project and leave the catalogue untouched.

Demand from consumption data

If consumption data are available instead of areas, you enter them directly: oil consumption in litres per year, gas consumption in cubic metres or megawatt hours, or general fuel consumption in megawatt hours.

Taking the annual utilisation ratio of the previous plant into account, EPOS-Plan converts the consumption into useful heat and scales the building model so that it reproduces exactly this demand – the annual profile is thereby tied to the actual bill. An existing building can be described realistically in this way from the heating bill alone, without component areas and U-values having to be known in detail.

Characteristics of the building type

The physical characteristics of a building type comprise floor area and area per user, room height, window areas separated by south, north and east/west, the window transmittance, the areas of external wall, roof, ground floor and other components with the associated U-values, thermal bridge supplements with their connection lengths, the air change rate and the thermal mass of the construction, which is classified as light, heavy or very heavy.

A second page holds the usage conditions: set room temperature during the day, night setback, weekend and holiday setback, maximum room temperature and four freely definable holiday periods for winter, Easter, summer and autumn. In non-residential buildings in particular these entries have a noticeable influence on the annual demand, because setback times and holidays account for a considerable part of the year.

Calculating the annual profile

From these inputs the program calculates, for each of the 365 days, the solar gains through the window areas, the specific transmission and ventilation losses and from them – taking the thermal mass of the building and the set-back periods into account – the daily heat load. Each daily value is then distributed to hourly values by a type-dependent 24-hour distribution curve. Eight curves by season and day of the week are available for residential buildings, five curves by season and cloud cover for non-residential buildings.

The result is a continuous annual profile of the heating output, from which the maximum heat load and the load duration curve are derived. Both quantities are the basis of the subsequent generator design.

Hot water and process heat

Hot water and process heat are recorded on the same pattern: you select a type from the catalogue, enter the annual consumption, and the program distributes it over the year using the stored monthly shares and a weekly profile with 168 hourly values. The weekly profiles can be edited day by day in a dedicated editor, copied and applied to all days; a chart shows the result immediately.

For every temperature level and every process you create a separate entry. It thus remains traceable which share of the demand occurs at which temperature level – a distinction that becomes important at the latest when the question arises which generator can cover the respective share at all.

External load profile and network losses

Alternatively, a heat load profile that already exists can be imported as an hourly series and added directly to the calculated demand. That is the route for objects for which a measurement is available, and for connected consumers that are not to be modelled through a building model.

Network losses are entered either as a percentage or as an absolute annual figure; they are distributed evenly over the year. They concern the distribution in the heating network – without a network, that is for a single building, the figure stays at zero.

Notes for practice

Check the demand against known consumption data before you add technology: a demand that is twenty per cent wrong devalues every subsequent design. Where a bill is available, the route through consumption is to be preferred to the route through floor area – the building model is then scaled so that annual total and actual consumption match.

A hot water demand set too low makes summer operation look too favourable; where a circulation system is present, its losses have to be included. Summer is the period in which solar thermal, heat pump and CHP are decided – if the base load is set too small there, the result of the entire simulation shifts.

After the calculation, check the maximum heat load and the load duration curve. The peak load determines the peak-load output, the flat part of the duration curve the range in which a base-load generator achieves many operating hours. Both quantities say more about the design than the annual total alone.

Keep in mind that climate region and building data act together: changing the region alters solar gains and heat load and makes a new calculation necessary.

How things continue after the heat demand is shown by the Program workflow overview; the next step is the Electricity demand calculation, before it lies Project management. The engineering background is collected under Fundamentals › Heat demand calculation, worked cases under Examples.