diff --git a/components/pid/temp_controller_smith.py b/components/pid/temp_controller_smith.py
index 6ca8ecf..1db57be 100755
--- a/components/pid/temp_controller_smith.py
+++ b/components/pid/temp_controller_smith.py
@@ -11,10 +11,14 @@ class TempController(TempControllerBase):
 
 		# Fast model: same plant model but with zero transport delay, used
 		# to predict the current temperature without the dead time.
-		self.model = Pot(dt, {**model_params, 'Td': 0}, theta_amb)
+		self.model = Pot(dt)
+		self.model.set_plant_params({**model_params, 'Td': 0})
+		self.model.set_ambient_temperature(theta_amb)
 		# Delayed model: keeps the plant's assumed transport delay, so it
 		# can be compared like-for-like against the real (delayed) measurement.
-		self.model_delay = Pot(dt, model_params, theta_amb)
+		self.model_delay = Pot(dt)
+		self.model_delay.set_plant_params(model_params)
+		self.model_delay.set_ambient_temperature(theta_amb)
 
 		self.theta_ist_plant = 0
 		self.theta_ist_model = 0
diff --git a/components/plant/pot.py b/components/plant/pot.py
index f0ea0d8..09353ef 100644
--- a/components/plant/pot.py
+++ b/components/plant/pot.py
@@ -2,7 +2,7 @@ from components.aplant import APlant
 from components.plant import delay
 
 class Pot(APlant):
-	def __init__(self, dt, params, theta_amb=20):
+	def __init__(self, dt):
 		APlant.__init__(self)
 		self.dt = dt
 
@@ -10,31 +10,49 @@ class Pot(APlant):
 		self.x = 0
 		self.p_pot = 0
 
-		# Plant specific thermal capacity [W*s/(kg*K)]
-		self.C = params['C']
+		# Plant specific thermal capacity [W*s/(kg*K)], mass [kg], energy
+		# loss coefficient [W/(kg*K)] and transport propagation delay [s]
+		# - all None until set_plant_params() is called; process() checks
+		# for that rather than silently computing on bogus values.
+		self.C = None
+		self.M = None
+		self.L = None
+		self.Td = None
+		self.delay = None
 
-		# Plant mass [kg]
-		self.M = params['M']
+		# Plant temperature [°C] - seeded from theta_amb the first time
+		# set_ambient_temperature() is called (see there), or overridden
+		# explicitly via initial(). None until either happens;
+		# process() checks for that too.
+		self.temp = None
 
-		# Plant energy loss coefficient [W/(kg*K)]
-		# Negative input power as a function of plant mass and temperature difference T_plant and T_ambient
-		# P_loss = L * Mass * (T_plant - T_ambient)
-		self.L = params['L']
-
-		# Energy transport propagation delay
-		self.Td = params['Td']
-
-		# Plant temperature [°C]
-		self.temp = theta_amb
-
-		# Ambient temperature [°C]
-		self.theta_amb = theta_amb
+		# Ambient temperature [°C] - None until set_ambient_temperature()
+		# is called; process() checks for that as well.
+		self.theta_amb = None
 
 		# Set power [W]
 		self.p_in = 0
 
-		# Plant delay [s]
-		self.delay = delay.Delay(dt, self.Td, 0)
+	def set_plant_params(self, params):
+		self.C = params['C']
+		self.M = params['M']
+
+		# Negative input power as a function of plant mass and
+		# temperature difference T_plant and T_ambient:
+		# P_loss = L * Mass * (T_plant - T_ambient)
+		self.L = params['L']
+
+		self.Td = params['Td']
+		self.delay = delay.Delay(self.dt, self.Td, 0)
+
+	def set_ambient_temperature(self, theta_amb):
+		self.theta_amb = theta_amb
+		# Only seeds the plant's own starting temperature the first time
+		# this is called (i.e. there's no real temperature yet) - once a
+		# run is in progress, changing the ambient setting must not
+		# clobber whatever temperature has actually been simulated since.
+		if self.temp is None:
+			self.temp = theta_amb
 
 	def initial(self, temp):
 		self.temp = temp
@@ -43,6 +61,11 @@ class Pot(APlant):
 		pass
 
 	def process(self):
+		if self.delay is None:
+			raise RuntimeError("Pot.process(): plant params not set - call set_plant_params() first")
+		if self.theta_amb is None:
+			raise RuntimeError("Pot.process(): ambient temperature not set - call set_ambient_temperature() first")
+
 		self.delay.put(self.p_in)
 
 		p_loss = self.L * self.M * (self.temp - self.theta_amb)
@@ -53,9 +76,6 @@ class Pot(APlant):
 	def is_activated(self):
 		return True
 
-	def set_ambient_temperature(self, theta_amb):
-		self.theta_amb = theta_amb
-
 	def set_thermal_params(self, M, C):
 		self.M = M
 		self.C = C
diff --git a/components/sud_forecast.py b/components/sud_forecast.py
index c14c77f..fbb74bf 100644
--- a/components/sud_forecast.py
+++ b/components/sud_forecast.py
@@ -61,7 +61,9 @@ class SudForecastEstimator:
 		if not sud.load(doc) or not sud.schedule:
 			return [0.0], [start_theta], SudState.DONE, []
 
-		pot = Pot(self.dt, self.plant_params, self.theta_amb)
+		pot = Pot(self.dt)
+		pot.set_plant_params(self.plant_params)
+		pot.set_ambient_temperature(self.theta_amb)
 		pot.initial(start_theta)
 		tc = PidFactory.create(self.pid_type, self.dt, self.tempctrl_params, self.plant_params, theta_amb=self.theta_amb)
 		tc.set_enabled(True)
diff --git a/scripts/demos/pid/demo_temp_controller.py b/scripts/demos/pid/demo_temp_controller.py
index e4a80b6..7ebacfa 100644
--- a/scripts/demos/pid/demo_temp_controller.py
+++ b/scripts/demos/pid/demo_temp_controller.py
@@ -45,7 +45,9 @@ if __name__ == '__main__':
 	heatrate_soll = 1.25
 	ctrl = TempController(dt, ctrl_params)
 	ctrl.set_enabled(True)
-	plant = Pot(dt, plant_params, theta_amb)
+	plant = Pot(dt)
+	plant.set_plant_params(plant_params)
+	plant.set_ambient_temperature(theta_amb)
 	_y = np.empty(0)
 	_fb = np.empty(0)
 	_t = np.empty(0)
diff --git a/scripts/demos/pid/demo_temp_controller_smith.py b/scripts/demos/pid/demo_temp_controller_smith.py
index e9ca2ef..7534b5a 100644
--- a/scripts/demos/pid/demo_temp_controller_smith.py
+++ b/scripts/demos/pid/demo_temp_controller_smith.py
@@ -45,7 +45,9 @@ if __name__ == '__main__':
 	heatrate_soll = 1.25
 	ctrl = TempController(dt, ctrl_params, plant_params, theta_amb)
 	ctrl.set_enabled(True)
-	plant = Pot(dt, plant_params, theta_amb)
+	plant = Pot(dt)
+	plant.set_plant_params(plant_params)
+	plant.set_ambient_temperature(theta_amb)
 	_y = np.empty(0)
 	_fb = np.empty(0)
 	_t = np.empty(0)
diff --git a/scripts/demos/plant/demo_pot.py b/scripts/demos/plant/demo_pot.py
index bf06a06..45e1520 100644
--- a/scripts/demos/plant/demo_pot.py
+++ b/scripts/demos/plant/demo_pot.py
@@ -19,7 +19,9 @@ if __name__ == '__main__':
 		"gain"		: 1.0
 	}
 
-	pot = Pot(dt, pot_params, theta_amb)
+	pot = Pot(dt)
+	pot.set_plant_params(pot_params)
+	pot.set_ambient_temperature(theta_amb)
 
 	_t = np.empty(0)
 	_temp = np.empty(0)
diff --git a/scripts/demos/sud/demo_sud.py b/scripts/demos/sud/demo_sud.py
index 7685a8b..aaa1e87 100644
--- a/scripts/demos/sud/demo_sud.py
+++ b/scripts/demos/sud/demo_sud.py
@@ -49,7 +49,9 @@ if __name__ == '__main__':
 	}
 
 	ctrl = TempController(dt, ctrl_params, plant_params, theta_amb)
-	plant = Pot(dt, plant_params, theta_amb)
+	plant = Pot(dt)
+	plant.set_plant_params(plant_params)
+	plant.set_ambient_temperature(theta_amb)
 	stirrer = StirrerSim(dt)
 	heater = HeaterSim({})
 
diff --git a/server/brewpi.py b/server/brewpi.py
index 180a1a1..2ea7283 100755
--- a/server/brewpi.py
+++ b/server/brewpi.py
@@ -76,7 +76,9 @@ if __name__ == '__main__':
 	taskmgr.add(sensor_task)
 
 	# Plant
-	pot = Pot(DT, DEFAULT_PLANT_PARAMS, theta_amb)
+	pot = Pot(DT)
+	pot.set_plant_params(DEFAULT_PLANT_PARAMS)
+	pot.set_ambient_temperature(theta_amb)
 	taskmgr.add(PotTask(pot, DT_TASK, dispatcher.msgio_get("Pot")))
 
 	# Heater