diff --git a/lambda-test/command-test.c b/lambda-test/command-test.c
index c447ccc..74c01ca 100644
--- a/lambda-test/command-test.c
+++ b/lambda-test/command-test.c
@@ -21,7 +21,7 @@
 
 static bool testIsSimulation(void) {
 	setupPorts();
-	setHeaterOn(false);
+	setHeaterState(heaterStateOff);
 
 	assertFalse(isSimulation());
 	runCommand("se");
@@ -48,7 +48,7 @@
 
 static bool testHeater(void) {
 	setupPorts();
-	setHeaterOn(false);
+	setHeaterState(heaterStateOff);
 
 	assertFalse(isHeaterOn());
 	runCommand("he");
diff --git a/lambda-test/rules-test.c b/lambda-test/rules-test.c
index 1ef15d3..b58a7aa 100644
--- a/lambda-test/rules-test.c
+++ b/lambda-test/rules-test.c
@@ -16,7 +16,7 @@
 
 /* Module rules */
 
-extern uint16_t age;
+extern uint8_t measCount;
 extern int8_t dir;
 extern uint8_t airgate;
 extern Rule rules[];
@@ -25,6 +25,9 @@
 
 	Measurement meas = {0, 0, 0, 0};
 
+	// prevent rule warmStart from kicking in
+	setHeaterState(heaterStateOn);
+
 	resetRules(true);
 	dir = burning_down;
 	reason(meas);
@@ -276,32 +279,33 @@
 
 static bool testFireOut(void) {
 
-	resetRules(true);
 	Measurement meas = {0, 0, 0, 0};
+
+	resetRules(true);
 	dir = firing_up;
 
 	meas.tempI = 50;
-	age = 0;
+	measCount = 10;
 	reason(meas);
 	assertFalse(rules[5].fired);
 
 	meas.tempI = TEMP_FIRE_OUT;
-	age = 0;
+	measCount = 10;
 	reason(meas);
 	assertFalse(rules[5].fired);
 
 	meas.tempI = TEMP_FIRE_OUT_RESET;
-	age = 0;
+	measCount = 10;
 	reason(meas);
 	assertFalse(rules[5].fired);
 
 	meas.tempI = TEMP_FIRE_OUT - 1;
-	age = 0;
+	measCount = 10;
 	reason(meas);
 	assertTrue(rules[5].fired);
 
 	meas.tempI = TEMP_FIRE_OUT_RESET;
-	age = 0;
+	measCount = 10;
 	reason(meas);
 	assertFalse(rules[5].fired);
 
@@ -312,25 +316,22 @@
 
 static bool testWarmStart(void) {
 
+	Measurement meas = {101, 0, 0, 0};
+
 	resetRules(true);
-	Measurement meas = {0, 0, 0, 0};
-
-	age = 0;
+	setHeaterState(heaterStateOff);
 	airgate = 50;
-
-	setHeaterOn(false);
 	reason(meas);
 	assertTrue(50 == airgate);
 	assertTrue(heaterStateOff == getHeaterState());
 	assertFalse(rules[6].fired);
 
-	age = 0;
+	resetRules(true);
+	setHeaterState(heaterStateOff);
 	dir = firing_up;
-
-	setHeaterOn(false);
 	reason(meas);
 	assertTrue(100 == airgate);
-	assertTrue(heaterStateUp == getHeaterState());
+	assertTrue(heaterStateOn == getHeaterState());
 	assertTrue(rules[6].fired);
 
 	cancelAlert();
@@ -344,17 +345,17 @@
 	resetTime();
 	Measurement meas = {0, 0, 0, 0};
 
-	setHeaterOn(false);
+	setHeaterState(heaterStateOff);
 	meas.current = milliAmpsReady;
 	reason(meas);
 	assertTrue(heaterStateOff == getHeaterState());
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 	meas.current = 5000;
 	reason(meas);
-	assertTrue(heaterStateUp == getHeaterState());
+	assertTrue(heaterStateOn == getHeaterState());
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 	meas.current = milliAmpsReady;
 	reason(meas);
 	assertTrue(heaterStateReady == getHeaterState());
@@ -371,7 +372,7 @@
 	Measurement meas = {0, 0, 0, 0};
 	dir = firing_up;
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 	meas.current = 0;
 	reason(meas);
 	assertTrue(heaterStateFault == getHeaterState());
@@ -388,7 +389,7 @@
 	Measurement meas = {0, 0, 0, 0};
 	dir = firing_up;
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 	meas.current = 8000;
 	reason(meas);
 	assertTrue(heaterStateFault == getHeaterState());
@@ -405,7 +406,7 @@
 	Measurement meas = {0, 0, 0, 0};
 	dir = firing_up;
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 
 	// more than 5 mins
 	addTime(301);
@@ -425,7 +426,7 @@
 	resetTime();
 	Measurement meas = {0, 0, 0, 0};
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 
 	// equal or more than 30 mins below TEMP_FIRE_OUT
 	addTime(1800);
@@ -447,7 +448,7 @@
 	resetTime();
 	Measurement meas = {0, 0, 0, 0};
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 
 	// equal or more than 3 hours below TEMP_AIRGATE_0
 	addTime(10800UL);
@@ -463,100 +464,6 @@
 	return true;
 }
 
-static bool testReasonDirBurnUp(void) {
-
-	resetRules(true);
-	Measurement meas = {0, 0, 2000, 0};
-
-	age = 0;
-	reason(meas);
-	assertTrue(dir == none);
-
-	meas.tempI = 9;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == none);
-
-	meas.tempI = TEMP_FIRE_OUT;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == firing_up);
-
-	meas.tempI = TEMP_AIRGATE_25 - 1;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == firing_up);
-
-	meas.tempI = TEMP_AIRGATE_25 - 1;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == none);
-
-	meas.tempI = TEMP_AIRGATE_25;
-	meas.lambda = LAMBDA_BURNING - 1;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == burning);
-
-	meas.tempI = TEMP_AIRGATE_25 + 1;
-	meas.lambda = LAMBDA_BURNING;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == burning);
-
-	cancelAlert();
-
-	return true;
-}
-
-static bool testReasonDirBurnDown(void) {
-
-	resetRules(true);
-	Measurement meas = {999, 0, 1999, 0};
-
-	age = 0;
-	reason(meas);
-	assertTrue(dir == none);
-
-	meas.tempI = 800;
-	meas.lambda = LAMBDA_BURNING - 1;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == burning);
-
-	meas.tempI = TEMP_AIRGATE_25;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == burning);
-
-	meas.tempI = TEMP_AIRGATE_25 - 1;
-	meas.lambda = LAMBDA_BURNING - 1;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == burning);
-
-	meas.tempI = TEMP_AIRGATE_25 - 1;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == none);
-
-	meas.tempI = TEMP_FIRE_OUT;
-	meas.lambda = LAMBDA_MAX;
-	age = 180;
-	reason(meas);
-	assertTrue(dir == burning_down);
-
-	cancelAlert();
-
-	return true;
-}
-
 /* Test "class" */
 static const char class[] PROGMEM = "rules";
 
@@ -574,26 +481,22 @@
 static const char testHeaterFaultNoheat_P[] PROGMEM = "testHeaterFaultNoheat";
 static const char testHeaterTimeout0_P[] PROGMEM = "testHeaterTimeout0";
 static const char testHeaterTimeout1_P[] PROGMEM = "testHeaterTimeout1";
-static const char testReasonDirBurnUp_P[] PROGMEM = "testReasonDirBurnUp";
-static const char testReasonDirBurnDown_P[] PROGMEM = "testReasonDirBurnDown";
 
 /* Tests */
 static TestCase const tests[] = {
-		{class, testAirgate50_P, testAirgate50},
+ 		{class, testAirgate50_P, testAirgate50},
 		{class, testAirgate25_P, testAirgate25},
 		{class, testAirgateClose_P, testAirgateClose},
 		{class, testTooRich_P, testTooRich},
 		{class, testTooLean_P, testTooLean},
-		{class, testFireOut_P, testFireOut},
+ 		{class, testFireOut_P, testFireOut},
 		{class, testWarmStart_P, testWarmStart},
 		{class, testHeaterReady_P, testHeaterReady},
 		{class, testHeaterFaultNoconn_P, testHeaterFaultNoconn},
 		{class, testHeaterFaultShort_P, testHeaterFaultShort},
 		{class, testHeaterFaultNoheat_P, testHeaterFaultNoheat},
 		{class, testHeaterTimeout0_P, testHeaterTimeout0},
-		{class, testHeaterTimeout1_P, testHeaterTimeout1},
-		{class, testReasonDirBurnUp_P, testReasonDirBurnUp},
-		{class, testReasonDirBurnDown_P, testReasonDirBurnDown}
+		{class, testHeaterTimeout1_P, testHeaterTimeout1}
 };
 
 TestClass rulesClass = {tests, sizeof(tests) / sizeof(tests[0])};
diff --git a/lambda-test/sensors-test.c b/lambda-test/sensors-test.c
index f324958..40f6e98 100644
--- a/lambda-test/sensors-test.c
+++ b/lambda-test/sensors-test.c
@@ -186,11 +186,11 @@
 }
 
 static bool testSetHeaterOn(void) {
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 	assertTrue(isHeaterOn());
-	assertTrue(heaterStateUp == getHeaterState());
+	assertTrue(heaterStateOn == getHeaterState());
 
-	setHeaterOn(false);
+	setHeaterState(heaterStateOff);
 	assertFalse(isHeaterOn());
 	assertTrue(heaterStateOff == getHeaterState());
 
@@ -198,7 +198,7 @@
 }
 
 static bool testSetHeaterState(void) {
-	setHeaterOn(false);
+	setHeaterState(heaterStateOff);
 	assertFalse(isHeaterOn());
 	assertTrue(heaterStateOff == getHeaterState());
 
@@ -211,7 +211,7 @@
 static bool testGetHeaterUptime(void) {
 	addTime(10);
 
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 	assertTrue(0 == getHeaterUptime());
 
 	addTime(10);
diff --git a/lambda/Makefile b/lambda/Makefile
index 4dd53cb..0630869 100644
--- a/lambda/Makefile
+++ b/lambda/Makefile
@@ -3,7 +3,7 @@
 
 MCU = atmega328p
 # Currently supported are 1 MHz and 8 MHz
-F_CPU = 1000000
+F_CPU = 8000000
 # Also try BAUD = 19200 or 38400 if you're feeling lucky.
 BAUD = 9600
 
diff --git a/lambda/alert.c b/lambda/alert.c
index 59371c1..bc83fc2 100644
--- a/lambda/alert.c
+++ b/lambda/alert.c
@@ -84,3 +84,7 @@
 bool isAlertActive(void) {
 	return alertActive;
 }
+
+bool isBeeping(void) {
+	return beepCount > 0;
+}
diff --git a/lambda/alert.h b/lambda/alert.h
index 7111dea..cc91e73 100644
--- a/lambda/alert.h
+++ b/lambda/alert.h
@@ -49,4 +49,9 @@
  */
 bool isAlertActive(void);
 
+/**
+ * Returns true if (an alert) is currently beeping, false otherwise.
+ */
+bool isBeeping(void);
+
 #endif /* ALERT_H_ */
diff --git a/lambda/command.c b/lambda/command.c
index 5392ec2..59e7b9e 100644
--- a/lambda/command.c
+++ b/lambda/command.c
@@ -43,7 +43,7 @@
 		resetTime();
 		resetDisplay();
 		resetRules(true);
-		setHeaterOn(true);
+		setHeaterState(heaterStateOn);
 		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("sd")) == 0) {
@@ -66,12 +66,12 @@
 	}
 	else if (strcmp_P(fields[0], PSTR("he")) == 0) {
 		// oxygen sensor heater enable
-		setHeaterOn(true);
+		setHeaterState(heaterStateOn);
 		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("hd")) == 0) {
 		// oxygen sensor heater disable
-		setHeaterOn(false);
+		setHeaterState(heaterStateOff);
 		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("cm")) == 0) {
diff --git a/lambda/display.c b/lambda/display.c
index 20a2893..3f032c8 100644
--- a/lambda/display.c
+++ b/lambda/display.c
@@ -88,6 +88,7 @@
 }
 
 void cycleDisplay(void) {
+	bool beeping = isBeeping();
 	if (isAlertActive()) {
 		// button pressed during alert
 		cancelAlert();
@@ -97,8 +98,10 @@
 			position = displayPosCurrent;
 		}
 	}
+	if (! beeping) {
+		beep(1, 1, 31);
+	}
 	updatePending = true;
-	beep(1, 1, 31);
 }
 
 void updateMeas(Measurement const meas) {
diff --git a/lambda/integers.c b/lambda/integers.c
index af2d6cd..b5fb14b 100644
--- a/lambda/integers.c
+++ b/lambda/integers.c
@@ -22,4 +22,3 @@
 			((num - ((den < 0) ? den + 1 : den - 1)) / den) :
 			((num + ((den < 0) ? den + 1 : den - 1)) / den);
 }
-
diff --git a/lambda/lambda.c b/lambda/lambda.c
index 873537c..c5daf8d 100644
--- a/lambda/lambda.c
+++ b/lambda/lambda.c
@@ -57,7 +57,7 @@
 	alert_P(1, 1, 31, PSTR(MSG_WELCOME), PSTR(""), false);
 	// spend some time on being polite
 	while (getTime() < 3) {}
-	setHeaterOn(true);
+	setHeaterState(heaterStateOn);
 
 	uint32_t time = 0;
 	Measurement meas;
diff --git a/lambda/rules.c b/lambda/rules.c
index 7a81821..d6c8ce0 100644
--- a/lambda/rules.c
+++ b/lambda/rules.c
@@ -15,13 +15,27 @@
 
 #include "usart.h"
 
-uint8_t age = 0;
 FireDir dir = none;
 uint8_t airgate = 100;
+uint8_t measCount = 10;
 
-static Measurement rulesMeasMax = {0, 0, 2000, 0};
-static Measurement rulesMeasPrev = {0, 0, 2000, 0};
-static bool prevInit = false;
+static int32_t tempIAvg = 0;
+static int16_t tempIMax = 0;
+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.
+ */
+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];
+	}
+	queue[0] = value;
+
+	return last;
+}
 
 /**
  * Reminds to set the air gate to 50% when the fire is still firing up
@@ -32,6 +46,7 @@
 	if ((dir == firing_up || dir == burning) &&
 			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;
@@ -60,7 +75,7 @@
 		Measurement const meas) {
 	if (dir == burning_down && meas.tempI < TEMP_AIRGATE_0 &&
 			meas.lambda >= LAMBDA_MAX && airgate > 0) {
-		setHeaterOn(false);
+		setHeaterState(heaterStateOff);
 		airgate = 0;
 		alert_P(BEEPS, LENGTH, TONE,
 				PSTR(MSG_AIRGATE_CLOSE_0), PSTR(""), false);
@@ -108,7 +123,7 @@
 static void fireOut(bool* const fired, int8_t const dir,
 		Measurement const meas) {
 	if (! *fired && dir == firing_up && meas.tempI < TEMP_FIRE_OUT &&
-			rulesMeasMax.tempI - meas.tempI > (TEMP_FIRE_OUT_RESET - TEMP_FIRE_OUT)) {
+			tempIMax - meas.tempI > (TEMP_FIRE_OUT_RESET - TEMP_FIRE_OUT)) {
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_FIRE_OUT_0), PSTR(""), false);
 		*fired = true;
 	}
@@ -120,19 +135,17 @@
 /**
  * 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 ((dir == firing_up || dir == burning) && ! isHeaterOn() &&
-			getHeaterState() != heaterStateFault) {
-		// it seems wood has been added or - probably more likely - oven
-		// was fired up without resetting. Should probably work that way
-		// anyway, making manual reset unnecessary?
-		// TODO make a complete reset including time?
+	if (meas.tempI > TEMP_FIRE_OUT && (dir == firing_up || dir == burning) &&
+			! isHeaterOn() && (getHeaterState() != heaterStateFault)) {
 		resetRules(false);
 		airgate = 100;
-		rulesMeasMax.tempI = meas.tempI;
-		setHeaterOn(true);
+		tempIMax = meas.tempI;
+		setHeaterState(heaterStateOn);
 		*fired = true;
 	}
 }
@@ -164,7 +177,6 @@
 	if (meas.current > milliAmpsShort || meas.current < milliAmpsDisconn ||
 			(getHeaterUptime() >= 300 && getHeaterState() != heaterStateReady)) {
 		// short circuit or disconnected or did not warm up within 5 minutes
-		setHeaterOn(false);
 		setHeaterState(heaterStateFault);
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_HEATER_FAULT_0),
 				PSTR(MSG_HEATER_FAULT_1), true);
@@ -183,12 +195,12 @@
 	uint32_t heaterUptime = getHeaterUptime();
 	if (heaterUptime >= 1800 && meas.tempI < TEMP_FIRE_OUT &&
 			meas.lambda >= LAMBDA_MAX) {
-		setHeaterOn(false);
+		setHeaterState(heaterStateOff);
 		alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_FIRE_OUT_0), PSTR(""), false);
 	}
 	if (heaterUptime >= 10800 && meas.tempI < TEMP_AIRGATE_0 &&
 			meas.lambda >= LAMBDA_MAX) {
-		setHeaterOn(false);
+		setHeaterState(heaterStateOff);
 		if (airgate > 0) {
 			alert_P(BEEPS, LENGTH, TONE, PSTR(MSG_AIRGATE_CLOSE_0), PSTR(""),
 					false);
@@ -235,51 +247,51 @@
 	}
 
 	// rules applied to every 10th measurement
-	if (age % 10 == 0) {
+	if (measCount == 10) {
+		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);
 		}
+
+		tempIAvg = meas.tempI + tempIAvg - ((tempIAvg - 4) >> 3);
+		int16_t tempICur = tempIAvg >> 3;
+		int16_t tempIOld = push(tempIOldQueue, QUEUE_SIZE, tempICur);
+
+		// 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;
+			}
+		}
 	}
 
-	age++;
-
-	// init previous measurements with current measurements
-	if (! prevInit) {
-		rulesMeasPrev = meas;
-		prevInit = true;
-	}
-
-	// try to figure out if the fire is building up, burning or burning down
-	// by comparing current measurements with ones that are 3 minutes old.
-	if (age >= AGE_MEAS_PREV) {
-		dir = none;
-		if ((meas.tempI - rulesMeasPrev.tempI) >= TEMP_DELTA_UP &&
-				meas.tempI < TEMP_MIN && meas.lambda >= LAMBDA_BURNING) {
-			dir = firing_up;
-		}
-		if (meas.tempI >= TEMP_MIN || meas.lambda < LAMBDA_BURNING) {
-			dir = burning;
-		}
-		if ((rulesMeasPrev.tempI - meas.tempI) >= TEMP_DELTA_DOWN &&
-				meas.tempI < TEMP_MIN && meas.lambda >= LAMBDA_BURNING &&
-				rulesMeasMax.tempI >= TEMP_AIRGATE_50) {
-			dir = burning_down;
-		}
-
-		rulesMeasPrev = meas;
-		age = 0;
-	}
-
-	rulesMeasMax.tempI = MAX(rulesMeasMax.tempI, meas.tempI);
+	measCount++;
+	tempIMax = MAX(tempIMax, meas.tempI);
 }
 
 void resetRules(bool const state) {
 	if (state) {
-		prevInit = false;
-		rulesMeasMax.tempI = 0;
-
-		age = 0;
+		tempIMax = 0;
+		measCount = 10;
+		for (size_t i = 0; i < QUEUE_SIZE; i++) {
+			tempIOldQueue[i] = 0;
+		}
 		dir = none;
 		airgate = 100;
 	}
diff --git a/lambda/rules.h b/lambda/rules.h
index 6285921..10f43e3 100644
--- a/lambda/rules.h
+++ b/lambda/rules.h
@@ -15,13 +15,11 @@
 #define LENGTH 10
 #define TONE 31
 
-/** Age of previous measurements to compare against */
-#define AGE_MEAS_PREV 180
 /** Exhaust reaches at least this temperature when the fire is burning */
 #define TEMP_MIN 700
-/** Min. increase in temperature during AGE_MEAS_PREV when firing up */
+/** Min. increase in temperature during AGE_TEMPI_OLD when firing up */
 #define TEMP_DELTA_UP 10
-/** Min. decrease in temperature during AGE_MEAS_PREV when burning down */
+/** 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  */
 #define TEMP_AIRGATE_50 500
@@ -48,6 +46,9 @@
  */
 #define LAMBDA_TOO_LEAN 1400
 
+/** Last 18 firebox temperature values updated every 10 seconds = 3 minutes */
+#define QUEUE_SIZE 18
+
 typedef enum {
 	none = 0,
 	firing_up = 1,
diff --git a/lambda/sensors.c b/lambda/sensors.c
index 61ae08a..183c2d3 100644
--- a/lambda/sensors.c
+++ b/lambda/sensors.c
@@ -192,23 +192,19 @@
 	}
 }
 
-void setHeaterOn(bool const on) {
-	if (on) {
-		PORTB |= (1 << PB2);
-		heaterState = heaterStateUp;
-		heaterOnTime = getTime();
-		alert_P(1, 1, 31, PSTR(MSG_HEATER_UP_0), PSTR(MSG_HEATER_UP_1), true);
-	} else {
-		PORTB &= ~(1 << PB2);
-		heaterState = heaterStateOff;
-	}
-}
-
+// TODO merge with getHeaterState()
 bool isHeaterOn(void) {
 	return bit_is_set(PORTB, PB2);
 }
 
 void setHeaterState(int8_t const state) {
+	if (state == heaterStateOn) {
+		PORTB |= (1 << PB2);
+		heaterOnTime = getTime();
+		alert_P(1, 1, 31, PSTR(MSG_HEATER_UP_0), PSTR(MSG_HEATER_UP_1), true);
+	} else if (state == heaterStateOff || state == heaterStateFault) {
+		PORTB &= ~(1 << PB2);
+	}
 	heaterState = state;
 }
 
diff --git a/lambda/sensors.h b/lambda/sensors.h
index 5019ecc..c93ce3f 100644
--- a/lambda/sensors.h
+++ b/lambda/sensors.h
@@ -30,7 +30,7 @@
 typedef enum {
 	heaterStateFault = -1,
 	heaterStateOff = 0,
-	heaterStateUp = 1,
+	heaterStateOn = 1,
 	heaterStateReady = 2
 } HeaterState;
 
@@ -116,11 +116,6 @@
 char* toInfo(uint16_t lambda);
 
 /**
- * Turns the heater of the oxygen sensor on or off.
- */
-void setHeaterOn(bool on);
-
-/**
  * Returns true if the heater of the oxygen sensor is turned on,
  * false otherwise.
  */