In accordance with the SAP 2005 manual (revision 1): "A secondary system is always specified when the main system is electric storage heaters or off-peak electric underfloor heating. Note that building regulations or other regulations may make additional specifications in relation to secondary heaters, which should be followed in the case of compliance calculations." This is to allow for heating at on peak times where the main "Off Peak" system is not available. Where no heating system is present to fulfil this demand the default system applicable is Portable Electric Heaters. The Lifespan SAP Team