diff --git a/octoprint_enclosure/__init__.py b/octoprint_enclosure/__init__.py
index d446981..55eb548 100644
--- a/octoprint_enclosure/__init__.py
+++ b/octoprint_enclosure/__init__.py
@@ -695,6 +695,7 @@ class EnclosurePlugin(octoprint.plugin.StartupPlugin, octoprint.plugin.TemplateP
 
                     try:
                         calculated_duty = ((current_temp - temp_a) * (duty_b - duty_a) / (temp_b - temp_a)) + duty_a
+                        calculated_duty = self.constrain(calculated_duty, duty_a,100)
                     except:
                         calculated_duty = 0