Introduction
The program CIBSE Heat Loss & Heat Gain (APcalc) performs heat loss and heat gain calculations according to the procedures laid down in CIBSE Guide A (1995, 1999, 2001, 20006).
Heat Loss Methodology
The Heat Loss program applies the ‘Simple Model’ described in Sections and Appendix 5A.1 of the CIBSE Guide (1995 edition, reprinted 1999). The main features of this method are as follows:
- The method calculates heat loss by conduction, infiltration and mechanical ventilation.
- Steady state conditions are assumed in the basic calculation.
- An allowance for intermittent operation may be made using the factor derived in Appendix 5.A5 of the Guide.
- No allowance is made for casual or solar gains.
- Heat gains from adjacent rooms may be accounted for at the user’s option, using the Modified U-value method described in Section 5.5. of the Guide.
- Long-wave radiation exchange is modelled using a single radiant temperature for each room.
- Outside environmental temperature is assumed to equal outside air temperature.
- Convective/radiant surface resistances may be modified by the user.
- The radiant fraction of the heating plant and the radiant fraction sensed by the thermostat are under the user’s control.
The program Vista provides facilities for viewing the results of the Heat Loss analysis.
Heat Gain Methodology
The Heat Gains program is based on the ‘Simple Model’ described in Section and Appendix 5A.2 of the CIBSE Guide [ 0 ]. The method has been extended within the guidelines set out in the Guide (see Reference and Basic models – Section and Appendix 5.A7) to provide a more general and accurate treatment of solar gains through glazing. The main features of the ‘Simple Model’ are as follows:
- The method calculates heat gains and losses by conduction, infiltration and mechanical ventilation.
- Conduction calculations are based on the CIBSE Admittance method.
- The calculations are performed for the 24 hours of each design day, on the hour.
- The program makes allowance for casual and solar gains.
- Heat gains from adjacent rooms may be accounted for at the user’s option, using the Modified U-value method described in Section of the Guide.
- Long-wave radiation exchange is modelled using a single radiant temperature for each room.
- Outside environmental temperature includes an allowance for solar gain on opaque surfaces
- Convective/radiant surface resistances may be modified by the user.
- The radiant fraction sensed by the thermostat is under the user’s control.
- The cooling plant is assumed to be purely convective.
- Solar gain through glazing is accounted for through the use of solar gain factors.
Extensions to the ‘Simple Model’ are applied as follows:
External shading may be taken into account through the use of shading files generated by SunCast and calculations of local shading by window recesses and overhangs.
The solar gain factor treatment applied in the ‘Simple Model’ is replaced by an approach that allows the software to deal with a greater variety of glazing systems and to treat solar gain entering rooms more accurately. Properties of glazing units are calculated from first principles using the transmission, absorption and reflection characteristics of each layer of the glazing. These properties define the performance of the glazing over a range of incidence angles and distinguish between beam and diffuse solar radiation. In place of solar gain factors, which involve approximations unnecessary in a computer implementation, the software uses a direct solution method based on the principles set out in the Guide to calculate how the solar gain entering a room is absorbed by and conducted through its surfaces.
Details of the solar algorithms are provided under Solar Radiation below.
The program calculates solar fluxes incident on the exterior of the building rather than using tables of cooling load and sol-air temperature.
The user may specify a proportion of the solar gain entering a room that is estimated to be lost be re-transmission out of the windows as short-wave radiation.
Control for heating, as well as cooling, may be specified.
The user has control over the weather data used to drive the calculation.
Solar Radiation
Differences between Solar Gain Treatment in Heat Gain and ApacheSim
The treatment of solar gains in Heat Gain has much in common with the treatment in Apache Simulation. The following differences should, however, be noted. In Heat Gains:
- Meteorological solar variables are calculated from parameters entered in APlocate.
- Incident solar fluxes are calculated on the assumption of isotropic sky radiation.
- Shading calculations do not include internal solar tracking.
- No solar radiation is transferred between rooms.
- A fixed ‘Solar reflected fraction’ is applied (as set in Space Data).
- The internal distribution of solar radiation follows CIBSE admittance procedure principles.
Meteorological Solar Variables
The program generates design values of direct solar irradiance for clear sky conditions using the following parameters set in APlocate:
- Latitude & longitude.
- Standard meridian (time zone) and local time correction.
- Height above sea level.
- Haze factor.
- Precipitable water content.
A detailed description of the method is provided in the APlocate User Guide.
Values of diffuse solar radiation are evaluated by the program as a function of solar altitude only, and correspond to clear sky basic diffuse irradiances in Table A2.25 of the CIBSE Guide A2 .
Incident Solar Fluxes
The calculation of solar fluxes incident on external building surfaces follows the procedure used in Apache Simulation, with sky radiation treated as isotropic. See Apache Simulation Calculation Methods.
Treatment of Solar Radiation in Heat Gain
In Heat Gain, external shading factors read from a SunCast or SunCast Lite shading file are applied to the incident solar radiation beam. However there is no solar tracking in this program and no transference of diffuse radiation between rooms.
Shading
Shading of the beam component of solar radiation may be modelled in three ways in Heat Gain:
- Shading and solar tracking calculations performed by SunCast.
- Shading calculations performed by APsim for construction-based shading devices.
SunCast shading applies to both glazed and opaque surfaces. Construction-based shading only applies to glazing.
SunCast Shading Files
Shading data generated by SunCast for the 15th day of selected months is stored on a shading file with extension *.shd. The data for a given month comprises hourly data describing the exposure of exterior building surfaces to beam solar radiation. In the case of the SunCast file, internal solar tracking information is also stored on the file, but this data is ignored by Heat Gain. If the shading file is specified at run time, APsim reads the data and uses it to modify the beam component of solar radiation for shaded external surfaces.
Construction-based Shading Devices
Shading devices attached to constructions created in APcdb are modelled as described in Apache Simulation Calculation Methods.
‘Solar Reflected Fraction’ Parameter
This parameter sets the proportion of solar gain entering a room that is assumed to be lost by re-transmission through external glazing.
Distribution of Solar Radiation
Solar radiation entering a room is distributed internally according to principles set out in the CIBSE Admittance Procedure.
Special Element Adjacencies
Windows in elements assigned the adjacencies ‘Outside air with offset temp.’ and ‘Temp. from profile’, and those in partition elements linking to inactive spaces, receive no external solar radiation. Any radiation transmitted out of the building through such windows in the course of the solar distribution is lost from the model.
Solar Transmission, Absorption & Reflection by Glazing
The solar properties of glazing elements are derived as described in Apache Simulation Calculation Methods.
Solar Absorption & Reflection by Opaque Surfaces
External opaque building surfaces absorb and reflect solar radiation according to their solar absorptance as assigned in APcdb. SunCast and SunCast Lite shading data is applied to external opaque surfaces.
External Long-wave Radiation
Long-wave radiation loss from external surfaces in Heat Gain is assumed to take the following values:
Walls: 21 W/m2
Roofs: 91 W/m2
References
- CIBSE Guide A Environmental Design. The Chartered Institution of Building Services Engineers, London, 1999.
- Atmospheric Effects on Solar Radiation for Computer Analysis of Cooling Loads for Buildings at Various Location Heights , Journal of the Institution of Heating and Ventilating Engineers Volume 39 (Feb. 1972)