Index: firmware/App/Services/PIControllers.c
===================================================================
diff -u -rd3671cad1447db7ad496ad6282324ef7570c5625 -rd00ad426d849083922332f832e88e7137a1cad60
--- firmware/App/Services/PIControllers.c	(.../PIControllers.c)	(revision d3671cad1447db7ad496ad6282324ef7570c5625)
+++ firmware/App/Services/PIControllers.c	(.../PIControllers.c)	(revision d00ad426d849083922332f832e88e7137a1cad60)
@@ -25,22 +25,31 @@
  *  @{
  */
 
-// ********** private definitions **********
+// ********** private definitions **********
 
+/// Enumeration of PI controller direction.
+typedef enum controller_Directions
+{
+    CONTROLLER_BIDIRECTIONAL = 0,          ///< Controller runs bidirectional so it only covers positive and negative values
+    CONTROLLER_UNIDIRECTIONAL,             ///< Controller run unidirectional so it only covers positive values
+    NUM_OF_CONTROLLELR_DIRECTIONS          ///< Number of PI controllers directions
+} PI_CONTROLLER_DIRECTIONS_T;
+
 /// Record for PI controller.
 typedef struct {
     // -- PI's parameters --
-    F32 Kp;                         ///< Proportional Value.
-    F32 Ki;                         ///< Integral Value.
-    F32 uMax;                       ///< Maximum control signal.
-    F32 uMin;                       ///< Minimum control signal.
+    F32 Kp;                                 ///< Proportional Value.
+    F32 Ki;                                 ///< Integral Value.
+    F32 uMax;                               ///< Maximum control signal.
+    F32 uMin;                               ///< Minimum control signal.
     // -- PI's signals --
-    F32 referenceSignal;            ///< reference signal.
-    F32 measuredSignal;             ///< measured signal.
-    F32 errorSignal;                ///< reference - measured signal.
-    F32 errorSumBeforeWindUp;       ///< error signal before windup correction.
-    F32 errorSum;                   ///< error integral after windup correction.
-    F32 controlSignal;              ///< actual control signal.
+    F32 referenceSignal;                    ///< reference signal.
+    F32 measuredSignal;                     ///< measured signal.
+    F32 errorSignal;                        ///< reference - measured signal.
+    F32 errorSumBeforeWindUp;               ///< error signal before windup correction.
+    F32 errorSum;                           ///< error integral after windup correction.
+    F32 controlSignal;                      ///< actual control signal.
+    PI_CONTROLLER_DIRECTIONS_T direction;   ///< PI controller control direction.
 } PI_CONTROLLER_T;
 
 #define MIN_KI                      NEARLY_ZERO                         ///< minimum integral coefficient - cannot be zero.
@@ -51,11 +60,11 @@
 /// PI Controllers - initial configurations.
 static PI_CONTROLLER_T piControllers[ NUM_OF_PI_CONTROLLERS_IDS ] =
 {   // Kp   Ki  uMax  uMin  ref meas  err  esw esum ctrl
-    { 0.0, 0.0, 1.0,  0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 },  // PI_CONTROLLER_ID_RO_PUMP
-    { 0.0, 0.0, 0.99, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 },  // I_CONTROLLER_ID_RO_PUMP_RAMP_UP
-    { 0.0, 0.0, 3000, 300, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 },  // PI_CONTROLLER_ID_DRAIN_PUMP
-    { 0.0, 0.0, 1.39, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 },  // PI_CONTROLLER_ID_PRIMARY_HEATER
-    { 0.0, 0.0, 0.50, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0 }   // PI_CONTROLLER_ID_TRIMMER_HEATER
+    { 0.0, 0.0, 1.0,  0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, CONTROLLER_UNIDIRECTIONAL },  // PI_CONTROLLER_ID_RO_PUMP
+    { 0.0, 0.0, 0.99, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, CONTROLLER_UNIDIRECTIONAL },  // I_CONTROLLER_ID_RO_PUMP_RAMP_UP
+    { 0.0, 0.0, 3000, 300, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, CONTROLLER_BIDIRECTIONAL },   // PI_CONTROLLER_ID_DRAIN_PUMP
+    { 0.0, 0.0, 1.39, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, CONTROLLER_UNIDIRECTIONAL },  // PI_CONTROLLER_ID_PRIMARY_HEATER
+    { 0.0, 0.0, 0.50, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, CONTROLLER_UNIDIRECTIONAL }   // PI_CONTROLLER_ID_TRIMMER_HEATER
 };
 
 /*********************************************************************//**
@@ -153,9 +162,19 @@
         SET_CONTROLLER( controller, controllerID );
 
         controller->referenceSignal = referenceSignal;
-        controller->measuredSignal = measuredSignal;
-        // calculate error signal
-        controller->errorSignal = referenceSignal - measuredSignal;
+        controller->measuredSignal = measuredSignal;
+
+        // Calculate error signal
+        if ( controller->direction == CONTROLLER_UNIDIRECTIONAL )
+        {
+            // To make sure that the outcome will be positive
+            controller->errorSignal = fabs( referenceSignal ) - ( referenceSignal < 0.0 ? ( measuredSignal * -1.0 ) : measuredSignal );
+        }
+        else
+        {
+            controller->errorSignal = referenceSignal - measuredSignal;
+        }
+
         controller->errorSum += controller->errorSignal;
         // anti-windup
         controller->errorSumBeforeWindUp = controller->errorSum;