diff --git a/src/addon_io.c b/src/addon_io.c
new file mode 100644
index 0000000..39befcc
--- /dev/null
+++ b/src/addon_io.c
@@ -0,0 +1,35 @@
+#include <libopencm3/stm32/gpio.h>
+
+#include "pinout.h"
+
+#include "addon_io.h"
+
+static const uint32_t OUTPUT_LIST[2] = {ADDON_ISO_IO_OUT1, ADDON_ISO_IO_OUT2};
+
+void addon_io_init(void)
+{
+	// Isolated outputs configuration: output, initially low = off
+	gpio_clear(ADDON_ISO_IO_PORT, ADDON_ISO_IO_OUT1 | ADDON_ISO_IO_OUT2);
+	gpio_mode_setup(ADDON_ISO_IO_PORT, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE, ADDON_ISO_IO_OUT1 | ADDON_ISO_IO_OUT2);
+
+	// Isolated inputs configuration: input, no pull resistors
+	gpio_mode_setup(ADDON_ISO_IO_PORT, GPIO_MODE_INPUT, GPIO_PUPD_NONE, ADDON_ISO_IO_IN);
+}
+
+
+void addon_io_iso_out_on(uint8_t idx)
+{
+	gpio_set(ADDON_ISO_IO_PORT, OUTPUT_LIST[idx]);
+}
+
+
+void addon_io_iso_out_off(uint8_t idx)
+{
+	gpio_clear(ADDON_ISO_IO_PORT, OUTPUT_LIST[idx]);
+}
+
+
+bool addon_io_read_iso_in(void)
+{
+	return gpio_get(ADDON_ISO_IO_PORT, ADDON_ISO_IO_IN) != 0;
+}
diff --git a/src/addon_io.h b/src/addon_io.h
new file mode 100644
index 0000000..511fcc5
--- /dev/null
+++ b/src/addon_io.h
@@ -0,0 +1,14 @@
+#ifndef ADDON_IO_H
+#define ADDON_IO_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+void addon_io_init(void);
+
+void addon_io_iso_out_on(uint8_t idx);
+void addon_io_iso_out_off(uint8_t idx);
+
+bool addon_io_read_iso_in(void);
+
+#endif // ADDON_IO_H
diff --git a/src/charge_control.c b/src/charge_control.c
index e1898e8..117bfd5 100644
--- a/src/charge_control.c
+++ b/src/charge_control.c
@@ -8,6 +8,7 @@
 #include "charge_pump.h"
 #include "rs485.h"
 #include "flash_config.h"
+#include "addon_io.h"
 
 #include "charge_control.h"
 
@@ -243,12 +244,18 @@ static void solar_fsm_update(uint64_t uptime_ms, struct MeasurementResult *meas)
 		case CHARGE_LOW_EXTERNAL_TEMPERATURE:
 			if(charge_state_entered) {
 				power_switch_solar_off();
+
+				// switch on the heater via the isolated I/O addon board
+				addon_io_iso_out_on(0);
 			}
 
 			// this state can only be entered if the BMP280 measurement is valid, so
 			// no need to check it again here.
 			if(bmp280_get_temperature() > external_temperature_recovery) {
 				charge_state = CHARGE_WAIT_CHARGEPUMP;
+
+				// switch the heater off again when this state is left
+				addon_io_iso_out_off(0);
 				break;
 			}
 			break;
diff --git a/src/pinout.h b/src/pinout.h
index 84a24eb..d1893e7 100644
--- a/src/pinout.h
+++ b/src/pinout.h
@@ -43,6 +43,8 @@
 #define RS485_TX_PIN      GPIO6
 #define RS485_RX_PIN      GPIO7
 
+/*** Expansion connector signals ***/
+
 /* BMP280 I²C */
 
 #define BMP280_I2C_PORT   GPIOA
@@ -50,4 +52,12 @@
 #define BMP280_I2C_SCL    GPIO9
 #define BMP280_I2C_SDA    GPIO10
 
+/* Isolated inputs and outputs on I/O addon board */
+
+#define ADDON_ISO_IO_PORT       GPIOA
+
+#define ADDON_ISO_IO_OUT1       GPIO5
+#define ADDON_ISO_IO_OUT2       GPIO6
+#define ADDON_ISO_IO_IN         GPIO7
+
 #endif // PINOUT_H