CIBSE Gain calculation - Importance of accurate input data

	Product(s):	Hevacomp
	Version(s):	V8i
	Environment:	N/A
	Area:	Design Database
	Subarea:	Results

Overview

Unusual results are likely to occur if Building model data is inadequate.

Method

Comprehensive and consistent data input is essential to the accuracy of calculation results. This can be seen in the CIBSE Gain calculation in particular.

The CIBSE Gain calculation methodology considers every surface within the selected room(s), so each is important to the final result. This can be seen in the equations shown in the CIBSE Gain manual:

For example, results will be inaccurate if the Room Data method is used and room volume is entered but not every room surface. Take the following room:

In the above, all room surfaces have been entered except a roof. If CIBSE Gain is run with Fresh Air Loads enabled, then Sensible gains will be excessive:

Upon entering the Roof surface and completing the room, Sensible gains will fall. It should be noted, however, that Latent gains will increase:

These changes are due to the additional heat gain through the Roof surface area, but also the new surface impacting on Fresh air load infiltration, through correction factors shown above.