diff --git a/lambda-test/command-test.c b/lambda-test/command-test.c
index 74c01ca..7089aa4 100644
--- a/lambda-test/command-test.c
+++ b/lambda-test/command-test.c
@@ -27,7 +27,7 @@
 	runCommand("se");
 	assertTrue(isSimulation());
 	assertTrue(getTime() == 0);
-	assertTrue(isHeaterOn());
+	assertTrue(bit_is_set(PORTB, PB2));
 	runCommand("sd");
 	assertFalse(isSimulation());
 	assertTrue(getTime() == 0);
@@ -50,11 +50,11 @@
 	setupPorts();
 	setHeaterState(heaterStateOff);
 
-	assertFalse(isHeaterOn());
+	assertFalse(bit_is_set(PORTB, PB2));
 	runCommand("he");
-	assertTrue(isHeaterOn());
+	assertTrue(bit_is_set(PORTB, PB2));
 	runCommand("hd");
-	assertFalse(isHeaterOn());
+	assertFalse(bit_is_set(PORTB, PB2));
 
 	return true;
 }
diff --git a/lambda-test/rules-test.c b/lambda-test/rules-test.c
index b58a7aa..e35d79a 100644
--- a/lambda-test/rules-test.c
+++ b/lambda-test/rules-test.c
@@ -17,7 +17,7 @@
 /* Module rules */
 
 extern uint8_t measCount;
-extern int8_t dir;
+extern int8_t state;
 extern uint8_t airgate;
 extern Rule rules[];
 
@@ -25,26 +25,18 @@
 
 	Measurement meas = {0, 0, 0, 0};
 
-	// prevent rule warmStart from kicking in
-	setHeaterState(heaterStateOn);
-
 	resetRules(true);
-	dir = burning_down;
+	state = burning_down;
 	reason(meas);
 	assertFalse(rules[0].fired);
 
 	resetRules(true);
-	dir = none;
+	state = undefined;
 	reason(meas);
 	assertFalse(rules[0].fired);
 
 	resetRules(true);
-	dir = firing_up;
-	reason(meas);
-	assertFalse(rules[0].fired);
-
-	resetRules(true);
-	dir = burning;
+	state = firing_up;
 	reason(meas);
 	assertFalse(rules[0].fired);
 
@@ -52,30 +44,30 @@
 	meas.lambda = LAMBDA_TOO_LEAN;
 
 	resetRules(true);
-	dir = burning_down;
+	state = burning_down;
 	reason(meas);
 	assertFalse(rules[0].fired);
 
 	resetRules(true);
-	dir = none;
+	state = undefined;
 	reason(meas);
 	assertFalse(rules[0].fired);
 
 	resetRules(true);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertTrue(rules[0].fired);
 	assertTrue(50 == airgate);
 
 	// should not fire if airgate == 50
 	resetRules(false);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertFalse(rules[0].fired);
 	assertTrue(50 == airgate);
 
 	resetRules(true);
-	dir = burning;
+	// state = burning;
 	reason(meas);
 	assertTrue(rules[0].fired);
 	assertTrue(50 == airgate);
@@ -90,22 +82,17 @@
 	Measurement meas = {999, 0, 0, 0};
 
 	resetRules(true);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertFalse(rules[1].fired);
 
 	resetRules(true);
-	dir = none;
+	state = undefined;
 	reason(meas);
 	assertFalse(rules[1].fired);
 
 	resetRules(true);
-	dir = burning_down;
-	reason(meas);
-	assertFalse(rules[1].fired);
-
-	resetRules(true);
-	dir = burning;
+	state = burning_down;
 	reason(meas);
 	assertFalse(rules[1].fired);
 
@@ -113,22 +100,17 @@
 	meas.lambda = LAMBDA_TOO_LEAN;
 
 	resetRules(true);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertFalse(rules[1].fired);
 
 	resetRules(true);
-	dir = none;
+	state = undefined;
 	reason(meas);
 	assertFalse(rules[1].fired);
 
 	resetRules(true);
-	dir = burning;
-	reason(meas);
-	assertFalse(rules[1].fired);
-
-	resetRules(true);
-	dir = burning_down;
+	state = burning_down;
 	reason(meas);
 	assertTrue(rules[1].fired);
 	assertTrue(25 == airgate);
@@ -143,22 +125,17 @@
 	Measurement meas = {999, 0, 0, 0};
 
 	resetRules(true);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertFalse(rules[2].fired);
 
 	resetRules(true);
-	dir = none;
+	state = undefined;
 	reason(meas);
 	assertFalse(rules[2].fired);
 
 	resetRules(true);
-	dir = burning_down;
-	reason(meas);
-	assertFalse(rules[2].fired);
-
-	resetRules(true);
-	dir = burning;
+	state = burning_down;
 	reason(meas);
 	assertFalse(rules[2].fired);
 
@@ -166,22 +143,17 @@
 	meas.lambda = LAMBDA_MAX;
 
 	resetRules(true);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertFalse(rules[2].fired);
 
 	resetRules(true);
-	dir = none;
+	state = undefined;
 	reason(meas);
 	assertFalse(rules[2].fired);
 
 	resetRules(true);
-	dir = burning;
-	reason(meas);
-	assertFalse(rules[2].fired);
-
-	resetRules(true);
-	dir = burning_down;
+	state = burning_down;
 	reason(meas);
 	assertTrue(rules[2].fired);
 	assertTrue(0 == airgate);
@@ -194,7 +166,6 @@
 static bool testTooRich(void) {
 
 	Measurement meas = {0, 0, 0, 0};
-	dir = none;
 
 	setHeaterState(heaterStateReady);
 
@@ -240,7 +211,6 @@
 static bool testTooLean(void) {
 
 	Measurement meas = {0, 0, 0, 0};
-	dir = none;
 
 	setHeaterState(heaterStateReady);
 
@@ -282,7 +252,7 @@
 	Measurement meas = {0, 0, 0, 0};
 
 	resetRules(true);
-	dir = firing_up;
+	state = firing_up;
 
 	meas.tempI = 50;
 	measCount = 10;
@@ -328,7 +298,7 @@
 
 	resetRules(true);
 	setHeaterState(heaterStateOff);
-	dir = firing_up;
+	state = firing_up;
 	reason(meas);
 	assertTrue(100 == airgate);
 	assertTrue(heaterStateOn == getHeaterState());
@@ -370,7 +340,7 @@
 	resetRules(true);
 	resetTime();
 	Measurement meas = {0, 0, 0, 0};
-	dir = firing_up;
+	state = firing_up;
 
 	setHeaterState(heaterStateOn);
 	meas.current = 0;
@@ -387,7 +357,7 @@
 	resetRules(true);
 	resetTime();
 	Measurement meas = {0, 0, 0, 0};
-	dir = firing_up;
+	state = firing_up;
 
 	setHeaterState(heaterStateOn);
 	meas.current = 8000;
@@ -404,7 +374,7 @@
 	resetRules(true);
 	resetTime();
 	Measurement meas = {0, 0, 0, 0};
-	dir = firing_up;
+	state = firing_up;
 
 	setHeaterState(heaterStateOn);
 
diff --git a/lambda-test/sensors-test.c b/lambda-test/sensors-test.c
index 40f6e98..a534a14 100644
--- a/lambda-test/sensors-test.c
+++ b/lambda-test/sensors-test.c
@@ -185,24 +185,18 @@
 	return ! strcmp(info, MSG_RICH);
 }
 
-static bool testSetHeaterOn(void) {
+static bool testSetHeaterState(void) {
 	setHeaterState(heaterStateOn);
-	assertTrue(isHeaterOn());
+	assertTrue(bit_is_set(PORTB, PB2));
 	assertTrue(heaterStateOn == getHeaterState());
 
 	setHeaterState(heaterStateOff);
-	assertFalse(isHeaterOn());
+	assertFalse(bit_is_set(PORTB, PB2));
 	assertTrue(heaterStateOff == getHeaterState());
 
-	return true;
-}
-
-static bool testSetHeaterState(void) {
-	setHeaterState(heaterStateOff);
-	assertFalse(isHeaterOn());
-	assertTrue(heaterStateOff == getHeaterState());
-
+	setHeaterState(heaterStateOn);
 	setHeaterState(heaterStateFault);
+	assertFalse(bit_is_set(PORTB, PB2));
 	assertTrue(heaterStateFault == getHeaterState());
 
 	return true;
@@ -244,7 +238,6 @@
 static const char testToInfoOkay_P[] PROGMEM = "testToInfoOkay";
 static const char testToInfoIdeal_P[] PROGMEM = "testToInfoIdeal";
 static const char testToInfoRich_P[] PROGMEM = "testToInfoRich";
-static const char testSetHeaterOn_P[] PROGMEM = "testSetHeaterOn";
 static const char testSetHeaterState_P[] PROGMEM = "testSetHeaterState";
 static const char testGetHeaterUptime_P[] PROGMEM = "testGetHeaterUptime";
 
@@ -270,7 +263,6 @@
 		{class, testToInfoOkay_P, testToInfoOkay},
 		{class, testToInfoIdeal_P, testToInfoIdeal},
 		{class, testToInfoRich_P, testToInfoRich},
-		{class, testSetHeaterOn_P, testSetHeaterOn},
 		{class, testSetHeaterState_P, testSetHeaterState},
 		{class, testGetHeaterUptime_P, testGetHeaterUptime}
 };
diff --git a/lambda/Makefile b/lambda/Makefile
index 0630869..4dd53cb 100644
--- a/lambda/Makefile
+++ b/lambda/Makefile
@@ -3,7 +3,7 @@
 
 MCU = atmega328p
 # Currently supported are 1 MHz and 8 MHz
-F_CPU = 8000000
+F_CPU = 1000000
 # Also try BAUD = 19200 or 38400 if you're feeling lucky.
 BAUD = 9600
 
diff --git a/lambda/display.c b/lambda/display.c
index 3f032c8..c782d54 100644
--- a/lambda/display.c
+++ b/lambda/display.c
@@ -134,9 +134,9 @@
 			displayTime();
 		} else {
 			char* hint;
-			switch (getDir()) {
+			switch (getState()) {
 				case firing_up: hint = " >"; break;
-				case burning: hint = " ^"; break;
+				// case burning: hint = " ^"; break;
 				case burning_down: hint = " <"; break;
 				default: hint = " -"; break;
 			}
diff --git a/lambda/rules.c b/lambda/rules.c
index d6c8ce0..cd244b3 100644
--- a/lambda/rules.c
+++ b/lambda/rules.c
@@ -15,38 +15,47 @@
 
 #include "usart.h"
 
-FireDir dir = none;
+uint8_t measCount = MEAS_INT;
+FireState state = undefined;
 uint8_t airgate = 100;
-uint8_t measCount = 10;
 
-static int32_t tempIAvg = 0;
-static int16_t tempIMax = 0;
+static int32_t deltaAvg = 0;
+static int16_t tempIMax = TEMP_INIT;
 static int16_t tempIOldQueue[QUEUE_SIZE];
 
 /**
- * Pushes the given new value in the given fixed-size queue of old temperature
- * measurements and returns the oldest value being pushed off the queue.
+ * Pushes the given new value in the queue of old temperature measurements
+ * and returns the oldest value being pushed off the array.
  */
-static int16_t push(int16_t queue[], const size_t size, const int16_t value) {
-	int16_t last = queue[size - 1];
-	for (size_t i = size - 1; i > 0; i--) {
-		queue[i] = queue[i - 1];
+static int16_t pushQueue(const int16_t value) {
+	int16_t last = tempIOldQueue[QUEUE_SIZE - 1];
+	for (size_t i = QUEUE_SIZE - 1; i > 0; i--) {
+		tempIOldQueue[i] = tempIOldQueue[i - 1];
 	}
-	queue[0] = value;
+	tempIOldQueue[0] = value;
 
 	return last;
 }
 
 /**
+ * Initializes all elements in the queue of old temperature measurements
+ * to the given value.
+ */
+static void initQueue(const int16_t value) {
+	for (size_t i = 0; i < QUEUE_SIZE; i++) {
+		tempIOldQueue[i] = value;
+	}
+}
+
+/**
  * Reminds to set the air gate to 50% when the fire is still firing up
  * and the temperature has reached TEMP_AIRGATE_50.
  */
-static void airgate50(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if ((dir == firing_up || dir == burning) &&
-			meas.tempI >= TEMP_AIRGATE_50 && meas.lambda >= LAMBDA_TOO_LEAN &&
+static void airgate50(bool* const fired, Measurement const meas) {
+	if ((state == firing_up) &&
+			meas.tempI >= TEMP_AIRGATE_50 &&
+			meas.lambda >= LAMBDA_TOO_LEAN &&
 			airgate != 50) {
-
 		airgate = 50;
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_AIRGATE_50_0), PSTR(""), false);
 		*fired = true;
@@ -57,9 +66,9 @@
  * Reminds to set the air gate to 25% when the fire is burning down and the
  * temperature went below TEMP_AIRGATE_25.
  */
-static void airgate25(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (dir == burning_down && meas.tempI < TEMP_AIRGATE_25 &&
+static void airgate25(bool* const fired, Measurement const meas) {
+	if (state == burning_down &&
+			meas.tempI < TEMP_AIRGATE_25 &&
 			meas.lambda >= LAMBDA_TOO_LEAN && airgate > 25) {
 		airgate = 25;
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_AIRGATE_25_0), PSTR(""), false);
@@ -71,9 +80,8 @@
  * Reminds to close the air gate when the fire is burning down and the
  * temperature went below TEMP_AIRGATE_0 (no more flames).
  */
-static void airgateClose(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (dir == burning_down && meas.tempI < TEMP_AIRGATE_0 &&
+static void airgateClose(bool* const fired, Measurement const meas) {
+	if (state == burning_down && meas.tempI < TEMP_AIRGATE_0 &&
 			meas.lambda >= LAMBDA_MAX && airgate > 0) {
 		setHeaterState(heaterStateOff);
 		airgate = 0;
@@ -91,10 +99,8 @@
  * exhaust gas going into the chimney rises, more heat appears to be thrown
  * out of the chimney.
  */
-static void tooRich(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (meas.tempI > TEMP_FIRE_OUT &&
-			meas.lambda < LAMBDA_TOO_RICH &&
+static void tooRich(bool* const fired, Measurement const meas) {
+	if (meas.tempI > TEMP_FIRE_OUT && meas.lambda < LAMBDA_TOO_RICH &&
 			getHeaterState() == heaterStateReady && airgate < 50) {
 		airgate = 50;
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_AIRGATE_50_0), PSTR(""), false);
@@ -106,10 +112,8 @@
  * Notifies that the combustion is lean (again) and suggests to set the air
  * gate to 50%.
  */
-static void tooLean(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (meas.tempI > TEMP_AIRGATE_50 &&
-			meas.lambda > LAMBDA_TOO_LEAN &&
+static void tooLean(bool* const fired, Measurement const meas) {
+	if (meas.tempI > TEMP_AIRGATE_50 &&	meas.lambda > LAMBDA_TOO_LEAN &&
 			getHeaterState() == heaterStateReady && airgate > 50) {
 		airgate = 50;
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_AIRGATE_50_0), PSTR(""), false);
@@ -120,10 +124,10 @@
 /**
  * Notifies that the fire might have gone out at the beginning of firing up.
  */
-static void fireOut(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (! *fired && dir == firing_up && meas.tempI < TEMP_FIRE_OUT &&
-			tempIMax - meas.tempI > (TEMP_FIRE_OUT_RESET - TEMP_FIRE_OUT)) {
+static void fireOut(bool* const fired, Measurement const meas) {
+	if (! *fired && meas.tempI < TEMP_FIRE_OUT &&
+			tempIMax < TEMP_AIRGATE_50 &&
+			tempIMax >= TEMP_FIRE_OUT_RESET) {
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_FIRE_OUT_0), PSTR(""), false);
 		*fired = true;
 	}
@@ -136,26 +140,29 @@
  * Resets rules and some state and switches on the heating if it seems that
  * wood was added or the oven was fired up without resetting.
  * TODO make a complete reset including time?
- * TODO come up with something better than using the state of the heater
  */
-static void warmStart(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (meas.tempI > TEMP_FIRE_OUT && (dir == firing_up || dir == burning) &&
-			! isHeaterOn() && (getHeaterState() != heaterStateFault)) {
+static void warmStart(bool* const fired, Measurement const meas) {
+	if (! *fired && (state == firing_up) &&
+			meas.tempI > TEMP_FIRE_OUT && tempIMax >= TEMP_AIRGATE_50) {
 		resetRules(false);
 		airgate = 100;
 		tempIMax = meas.tempI;
-		setHeaterState(heaterStateOn);
+		if (getHeaterState() != heaterStateFault) {
+			setHeaterState(heaterStateOn);
+		}
 		*fired = true;
 	}
+	if (state == burning_down && meas.tempI < TEMP_AIRGATE_0) {
+		*fired = false;
+	}
 }
 
 /**
  * Notifies that the heater is ready and sets the corresponding state.
  */
-static void heaterReady(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (! isHeaterOn() || getHeaterState() == heaterStateReady) {
+static void heaterReady(bool* const fired, Measurement const meas) {
+	if (getHeaterState() == heaterStateOff ||
+			getHeaterState() == heaterStateReady) {
 		return;
 	}
 	if (meas.current <= milliAmpsReady && meas.current > milliAmpsDisconn) {
@@ -169,9 +176,9 @@
  * Notifies that the heater or its connection is faulty and sets the
  * corresponding state.
  */
-static void heaterFault(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (! isHeaterOn() || getHeaterState() == heaterStateFault) {
+static void heaterFault(bool* const fired, Measurement const meas) {
+	if (getHeaterState() == heaterStateOff ||
+			getHeaterState() == heaterStateFault) {
 		return;
 	}
 	if (meas.current > milliAmpsShort || meas.current < milliAmpsDisconn ||
@@ -187,9 +194,9 @@
  * Switches the heater off if it is on for 30 mins or more and there does
  * not seem to be a fire, and notifies that the fire is out.
  */
-static void heaterTimeout(bool* const fired, int8_t const dir,
-		Measurement const meas) {
-	if (! isHeaterOn() || getHeaterState() == heaterStateFault) {
+static void heaterTimeout(bool* const fired, Measurement const meas) {
+	if (getHeaterState() == heaterStateOff ||
+			getHeaterState() == heaterStateFault) {
 		return;
 	}
 	uint32_t heaterUptime = getHeaterUptime();
@@ -230,54 +237,47 @@
 		{false, tooRich},
 		{false, tooLean},
 		{false, fireOut},
-		{false, warmStart}
+		{true, warmStart}
 };
 
-int8_t getDir(void) {
-	return dir;
+int8_t getState(void) {
+	return state;
 }
 
 // called about every second
-void reason(Measurement const meas) {
+void reason(Measurement meas) {
 
-	// rules applied to each measurement
+	// rules applied at each measurement
 	size_t heaterRulesSize = sizeof(heaterRules) / sizeof(heaterRules[0]);
 	for (size_t i = 0; i < heaterRulesSize; i++) {
-		heaterRules[i].cond(&(heaterRules[i].fired), dir, meas);
+		heaterRules[i].cond(&(heaterRules[i].fired), meas);
 	}
 
-	// rules applied to every 10th measurement
-	if (measCount == 10) {
+	// evaluation of the fire state and rules applied
+	// at every MEAS_INT'th measurement
+	if (measCount == MEAS_INT) {
 		measCount = 0;
 
 		size_t rulesSize = sizeof(rules) / sizeof(rules[0]);
 		for (size_t i = 0; i < rulesSize; i++) {
-			rules[i].cond(&(rules[i].fired), dir, meas);
+			rules[i].cond(&(rules[i].fired), meas);
 		}
 
-		tempIAvg = meas.tempI + tempIAvg - ((tempIAvg - 4) >> 3);
-		int16_t tempICur = tempIAvg >> 3;
-		int16_t tempIOld = push(tempIOldQueue, QUEUE_SIZE, tempICur);
+		// difference between current and previous temperature
+		int16_t tempIOld = pushQueue(meas.tempI);
+		int16_t delta = meas.tempI - tempIOld;
 
-		// simply skip if old temperature was 0°C - in practice it never goes
-		// below 3°C anyway.
-		if (tempIOld > 0) {
-			// try to figure out if the fire is building up, burning or burning
-			// down by comparing the current temperature value with one that is
-			// 3 minutes old
-			dir = none;
-			if ((tempICur - tempIOld) >= TEMP_DELTA_UP &&
-					tempICur < TEMP_MIN && meas.lambda >= LAMBDA_BURNING) {
-				dir = firing_up;
-			}
-			if (tempICur >= TEMP_MIN || meas.lambda < LAMBDA_BURNING) {
-				dir = burning;
-			}
-			if ((tempIOld - tempICur) >= TEMP_DELTA_DOWN &&
-					tempICur < TEMP_MIN && meas.lambda >= LAMBDA_BURNING &&
-					tempIMax >= TEMP_AIRGATE_50) {
-				dir = burning_down;
-			}
+		// extra smoothing of the temperature difference
+		deltaAvg = delta + deltaAvg - (deltaAvg >> TEMP_DELTA_SMOOTH);
+		int16_t deltaSmooth = deltaAvg >> TEMP_DELTA_SMOOTH;
+
+		// try to figure out if the fire is firing up or burning down
+		state = undefined;
+		if (deltaSmooth >= TEMP_DELTA_UP) {
+			state = firing_up;
+		}
+		if (deltaSmooth <= TEMP_DELTA_DOWN && tempIMax >= TEMP_AIRGATE_50) {
+			state = burning_down;
 		}
 	}
 
@@ -285,14 +285,13 @@
 	tempIMax = MAX(tempIMax, meas.tempI);
 }
 
-void resetRules(bool const state) {
-	if (state) {
-		tempIMax = 0;
-		measCount = 10;
-		for (size_t i = 0; i < QUEUE_SIZE; i++) {
-			tempIOldQueue[i] = 0;
-		}
-		dir = none;
+void resetRules(bool const intState) {
+	if (intState) {
+		deltaAvg = 0;
+		tempIMax = TEMP_INIT;
+		initQueue(TEMP_INIT);
+		measCount = MEAS_INT;
+		state = undefined;
 		airgate = 100;
 	}
 
@@ -300,6 +299,7 @@
 	for (size_t i = 0; i < rulesSize; i++) {
 		rules[i].fired = false;
 	}
+	rules[6].fired = true;
 
 	size_t heaterRulesSize = sizeof(heaterRules) / sizeof(heaterRules[0]);
 	for (size_t i = 0; i < heaterRulesSize; i++) {
diff --git a/lambda/rules.h b/lambda/rules.h
index 10f43e3..3f97b01 100644
--- a/lambda/rules.h
+++ b/lambda/rules.h
@@ -11,33 +11,40 @@
 #include <stdbool.h>
 #include "sensors.h"
 
-#define BEEPS 20
+#define BEEPS 10
 #define LENGTH 10
 #define TONE 31
 
-/** Exhaust reaches at least this temperature when the fire is burning */
-#define TEMP_MIN 700
-/** Min. increase in temperature during AGE_TEMPI_OLD when firing up */
+/** Some rules and the fire state is evaluated every so many measurements */
+#define MEAS_INT 10
+/**
+ * Last 18 firebox temperature values updated every MEAS_INT measurements
+ * (i.e. 18 * 10 = 3 minutes @ one measurement per second)
+ */
+#define QUEUE_SIZE 18
+/** Initial temperature value */
+#define TEMP_INIT 20
+/** Smoothing of the temperature difference used to determine the fire state */
+#define TEMP_DELTA_SMOOTH 6
+/** Min. increase in temp. during QUEUE_SIZE * MEAS_INT when firing up */
 #define TEMP_DELTA_UP 10
-/** Min. decrease in temperature during AGE_TEMPI_OLD when burning down */
-#define TEMP_DELTA_DOWN 1
-/** Min. temperature at which to set the air gate to 50% when firing up  */
+/** Min. decrease in temp. during QUEUE_SIZE * MEAS_INT when burning down */
+#define TEMP_DELTA_DOWN -3
+/** Temperature at which to set the air gate to 50% when firing up  */
 #define TEMP_AIRGATE_50 500
-/** Max. temperature at which to set the air gate to 25% when burning down */
+/** Temperature at which to set the air gate to 25% when burning down */
 #define TEMP_AIRGATE_25 700
 /**
  * Max. temperature at which to set the air gate to 0% and to switch off the
  * oxygen sensor heater when burning down
  */
 #define TEMP_AIRGATE_0 450
-/** Max. temperature at which to consider the fire to have gone out */
+/** Temperature at which to consider the fire to have gone out */
 #define TEMP_FIRE_OUT 100
 /** Min. temperature at which to consider the fire to fire up again */
-#define TEMP_FIRE_OUT_RESET 125
+#define TEMP_FIRE_OUT_RESET 110
 /** Max. lambda value that can be measured */
 #define LAMBDA_MAX 2000
-/** Fire is considered fully burning if lambda is below this value */
-#define LAMBDA_BURNING 1900
 /** Combustion is considered too rich if lambda is below this value */
 #define LAMBDA_TOO_RICH 1100
 /**
@@ -46,29 +53,26 @@
  */
 #define LAMBDA_TOO_LEAN 1400
 
-/** Last 18 firebox temperature values updated every 10 seconds = 3 minutes */
-#define QUEUE_SIZE 18
-
 typedef enum {
-	none = 0,
+	undefined = 0,
 	firing_up = 1,
-	burning = 2,
+	// burning = 2,
 	burning_down = -1
-} FireDir;
+} FireState;
 
 /**
  * An attempt to create some sort of rule "object".
  */
 typedef struct {
 	bool fired;
-	void (*cond)(bool* fired, int8_t dir, Measurement meas);
+	void (*cond)(bool* fired, Measurement meas);
 } Rule;
 
 /**
- * Returns the "direction" of the fire, i.e. DIR_BURN_DOWN when it is burning
+ * Returns the current state of the fire, i.e. burning_down when it is burning
  * down.
  */
-int8_t getDir(void);
+int8_t getState(void);
 
 /**
  * Applies all rules against the given measurements.
diff --git a/lambda/sensors.c b/lambda/sensors.c
index 183c2d3..501007a 100644
--- a/lambda/sensors.c
+++ b/lambda/sensors.c
@@ -192,12 +192,7 @@
 	}
 }
 
-// TODO merge with getHeaterState()
-bool isHeaterOn(void) {
-	return bit_is_set(PORTB, PB2);
-}
-
-void setHeaterState(int8_t const state) {
+void setHeaterState(HeaterState const state) {
 	if (state == heaterStateOn) {
 		PORTB |= (1 << PB2);
 		heaterOnTime = getTime();
@@ -208,7 +203,8 @@
 	heaterState = state;
 }
 
-int8_t getHeaterState(void) {
+HeaterState getHeaterState(void) {
+	// bit_is_set(PORTB, PB2)
 	return heaterState;
 }
 
diff --git a/lambda/sensors.h b/lambda/sensors.h
index c93ce3f..39e5829 100644
--- a/lambda/sensors.h
+++ b/lambda/sensors.h
@@ -19,9 +19,9 @@
  * Oxygen sensor heater current limits at certain states in milliamps.
  */
 typedef enum {
+	milliAmpsDisconn = 100,
 	milliAmpsReady = 1350,
-	milliAmpsShort = 7500,
-	milliAmpsDisconn = 100
+	milliAmpsShort = 7500
 } HeaterMilliAmps;
 
 /**
@@ -116,12 +116,6 @@
 char* toInfo(uint16_t lambda);
 
 /**
- * Returns true if the heater of the oxygen sensor is turned on,
- * false otherwise.
- */
-bool isHeaterOn(void);
-
-/**
  * Sets the state of the heater of the oxygen sensor to the given value.
  */
 void setHeaterState(int8_t state);