Tempered air secondary system

[Jonnymac]

Hello,

I am modelling a building for SBEM in apache that has a primary heating system of underfloor heating. The is also mechanical supply and extract ventilation. The Supply is via an AHU with LTHW connection to temper the air and a dx coil to cool the supply air in summer. Both the tempered and cooled air are sized to condition the space. The extract is only in operation when dumping air during overheating.

What is the best way to model this, do I select a CV air system and assign it as a secondary system? If I do this will it not automatically assume this system is a system to heat and cool the space to comfortable temperatures. May be I should just assign Mech vent at zone level.

Advice would be appreciated.