My character is not taking damage.

Hey guys, this is one of my first scripts in Unity, so I am pretty weak, but any help is much appreciated. Basically I want my character to take 20 points of damage every time he moves into a fire (there is a 2.5 second delay though if he's already in the fire). My issue is that when he goes into the fire, nothing happens. His current health remains at 100. I am using Unity 5.3.1f.

This is my code so far:

 public class Fire_Damage : MonoBehaviour
 {
     public int startingHealth = 100;
     public int currentHealth;
     public Slider HealthBar;
     public float timeperBurn = 2.5f;
     public int FlameDamage = 20;
     private Player_Control player_Control;
     private bool isDead;
 
     GameObject player;
     FireHealth FCHealth;
     float timer;
     bool onFire;
     GameObject fire;
 
     void Awake()
     {
         currentHealth = startingHealth;
         player_Control = GetComponent<Player_Control>();
         player = GameObject.FindGameObjectWithTag("Player");
         FCHealth = GetComponent<FireHealth>();
         fire = GameObject.FindGameObjectWithTag("Fire");
     }
 
 
     void OnTriggerEnter (Collider other)
 
     { 
         if (other.gameObject == fire)
         {
             onFire = true;
         }
     }
 
     void OnTriggerExit (Collider other)
     {
         if (other.gameObject == fire)
         {
             onFire = false;
         }
     }
     void Death()
     {
         isDead = true;
         player_Control.enabled = false;
     }
     void Update()
         {
             if (onFire)
             {
             if (timer > Time.time)
             {
                 currentHealth -= FlameDamage;
                 timer = Time.time + timeperBurn;
                 HealthBar.value = currentHealth;
                 if (currentHealth <= 0 && !isDead)
                 {
                     Death();
                 }
             }
             }
         }
 }

In the console I only get the error that FireHealth is never used (that's my next task). So what am I missing here? Thanks.