Low　temperature　heating　and　high　temperature　cooling　systems　such　as　thermally　activated　building　systems　(TABS)　offer　the　chance　to　use　low　exergy　sources,　which　can　be　very　beneficial　financially　as　well　as　ecologically　when　using　renewable　energy　sources.　The　above　has　led　to　a　considerable　increase　of　water　based　radiant　systems　in　modern　buildings　and　a　need　for　reliable　simulation　tools　to　predict　the　indoor　environment　and　energy　performance.　This　paper　describes　the　comparison　of　the　building　simulation　tools　IDA　ICE,　IES　,　EnergyPlus　and　TRNSYS.　The　simulation　tools　are　compared　to　each　other　using　the　same　room　and　boundary　conditions.　The　results　show　significant　differences　in　predicted　room　temperatures,　heating　and　cooling　degree　hours　as　well　as　thermal　comfort　in　winter　and　summer.