diff --git a/Puffer/module.php b/Puffer/module.php
index 699fd6f..eb4aaf1 100644
--- a/Puffer/module.php
+++ b/Puffer/module.php
@@ -61,7 +61,7 @@ class Puffer extends IPSModule
         $maxAT = $this->ReadPropertyInteger("MaxAT_Temp");  // z.B. 0
         $minAT = $this->ReadPropertyInteger("MinAT_Temp");  // z.B. 20
 
-        $m = ($maxVT - $minVT) / ($minAT - $maxAT);
+        $m = ($minVT - $maxVT) / ($maxAT - $minAT);
         $this->SetValue("Steigung", $m);