diff --git a/components/pid/temp_controller.py b/components/pid/temp_controller.py
index 986fa3d..a23e95a 100644
--- a/components/pid/temp_controller.py
+++ b/components/pid/temp_controller.py
@@ -5,7 +5,15 @@ class TempController(TempControllerBase):
 	def __init__(self, dt):
 		TempControllerBase.__init__(self, dt)
 		self.dt = dt
-		self.last_theta_ist = 20
+		# None until the first process() tick - rather than hardcoding a
+		# starting point (e.g. 20), which would make that very first
+		# tick's heatrate come out as a bogus spike whenever the real
+		# starting temperature isn't actually 20 (see
+		# components/sud_forecast.py's SudForecastEstimator, which builds
+		# a fresh TempController on every call - unlike the real one,
+		# which only ever goes through this exactly once, it has no
+		# earlier ticks to have already settled past that spike by).
+		self.last_theta_ist = None
 		self.heatrate_ist = 0
 
 	def process(self):
@@ -13,7 +21,7 @@ class TempController(TempControllerBase):
 
 		# Process Kalman
 		self.theta_ist = self.theta_ist_set
-		heatrate = (self.theta_ist - self.last_theta_ist)/self.dt*60
+		heatrate = 0 if self.last_theta_ist is None else (self.theta_ist - self.last_theta_ist)/self.dt*60
 
 		alpha = 0.1
 		self.heatrate_ist = (1-alpha) * self.heatrate_ist + alpha*heatrate
diff --git a/components/pid/temp_controller_smith.py b/components/pid/temp_controller_smith.py
index a806c2c..18bb534 100755
--- a/components/pid/temp_controller_smith.py
+++ b/components/pid/temp_controller_smith.py
@@ -6,7 +6,15 @@ class TempController(TempControllerBase):
 	def __init__(self, dt):
 		TempControllerBase.__init__(self, dt)
 		self.dt = dt
-		self.last_theta_ist = 20
+		# None until the first process() tick - rather than hardcoding a
+		# starting point (e.g. 20), which would make that very first
+		# tick's heatrate come out as a bogus spike whenever the real
+		# starting temperature isn't actually 20 (see
+		# components/sud_forecast.py's SudForecastEstimator, which builds
+		# a fresh TempController on every call - unlike the real one,
+		# which only ever goes through this exactly once, it has no
+		# earlier ticks to have already settled past that spike by).
+		self.last_theta_ist = None
 		self.heatrate_ist = 0
 
 		# Fast model: same plant model but with zero transport delay, used
@@ -60,7 +68,7 @@ class TempController(TempControllerBase):
 		# so the dead time drops out of the feedback path.
 		self.theta_ist = self.theta_ist_model + (self.theta_ist_plant - self.theta_ist_model_delay)
 
-		heatrate = (self.theta_ist - self.last_theta_ist)/self.dt*60
+		heatrate = 0 if self.last_theta_ist is None else (self.theta_ist - self.last_theta_ist)/self.dt*60
 
 		alpha = 0.1
 		self.heatrate_ist = (1-alpha) * self.heatrate_ist + alpha*heatrate