diff --git a/manual/factory_firmware.md b/manual/factory_firmware.md
index 354bbeb9..a4025eea 100644
--- a/manual/factory_firmware.md
+++ b/manual/factory_firmware.md
@@ -10,7 +10,7 @@ First install on the requested platform the standard firmware and configure ever
 
 First create a directory `data` inside the following project path: `src/`.
 
-As the export removes all your password you need to add them again to the `json` file. Open the `json` file with a text editor and search for all the `"pwd": ""`. Between the second bunch of quotation marks you have to place the password.
+As the export removes all your passwords you need to add them again to the `json` file. Open the `json` file with a text editor and search for all the `"pwd":""` sections. Between the second bunch of quotation marks you have to place the password.
 
 *Note: It's recommended to keep all information in one line to save space on the ESP littlefs partition*
 
@@ -26,35 +26,21 @@ ahoy
         ...
 ```
 
-## modify platform.ini to build factory binary
-
-Open the file `src/platformio.ini` and uncomment the following line `#post:../scripts/add_littlefs_binary.py` (remove the `#`)
-
 ## build firmware
 
 Choose your prefered environment and build firmware as usual. Once the process is finished you should find along with the standard `firmware.bin` an additional file called `firmware.factory.bin`. Both files are located here: `src/.pio/build/[ENVIRONMENT]/`
 
 ## Upload to device
 
-Navigate to the firmware output directory `src/.pio/build/[ENVIRONMENT]/` and open a terminal.
-
-### ESP32
-
-Python:
-`esptool.py -b 921600 write_flash --flash_mode dio --flash_size detect 0x1000 bootloader.bin 0x8000 partitions.bin 0x10000 firmware.factory.bin`
-
-Windows:
-`esptool.exe -b 921600 write_flash --flash_mode dio --flash_size detect 0x1000 bootloader.bin 0x8000 partitions.bin 0x10000 firmware.factory.bin`
-
-### ESP32-S3 (OpenDTU Fusion Board)
+Navigate to the firmware output directory `src/.pio/build/[ENVIRONMENT]/` and open a terminal or vice versa.
 
 Python:
-`esptool.py -b 921600 write_flash --flash_mode dio --flash_size detect 0x10000 firmware.factory.bin`
+`esptool.py -b 921600 write_flash --flash_mode dio --flash_size detect 0x0 firmware.factory.bin`
 
 Windows:
-`esptool.exe -b 921600 write_flash --flash_mode dio --flash_size detect 0x10000 firmware.factory.bin`
+`esptool.exe -b 921600 write_flash --flash_mode dio --flash_size detect 0x0 firmware.factory.bin`
 
-For a 4MB flash size the upload should be finished within 22 seconds.
+The upload should be finished within one minute.
 
 ## Testing
 
@@ -62,10 +48,9 @@ Reboot your ESP an check if all your settings are present.
 
 ## Keep updated with 'Mainline'
 
-From time to time a new version of AhoyDTU will be published. To get this changes into your alread prepared factory binary generation environment you have to do only a few steps:
+From time to time a new version of AhoyDTU will be published. To get the changes into your already prepared factory binary generation environment you have to do only a few steps:
 
-1. revert the changes of `platformio.ini` by executing from repository root: `git checkout src/platformio.ini`
-2. pull new changes from remote: `git pull`
-3. modify the `platformio.ini` again as you can read above (remove comment)
-4. build and upload
-5. enjoy
+1. pull new changes from remote: `git pull`
+2. check if the `data` folder is still there and contains the `settings.json`
+3. build and upload
+4. enjoy
diff --git a/scripts/add_littlefs_binary.py b/scripts/add_littlefs_binary.py
index db766f45..8f6556f9 100644
--- a/scripts/add_littlefs_binary.py
+++ b/scripts/add_littlefs_binary.py
@@ -1,10 +1,13 @@
 import os
 import subprocess
 import shutil
+from SCons.Script import DefaultEnvironment
 Import("env")
 
 
 def build_littlefs():
+    if os.path.isfile('data/settings.json') == False:
+        return # nothing to do
     result = subprocess.run(["pio", "run", "--target", "buildfs", "--environment", env['PIOENV']])
     if result.returncode != 0:
         print("Error building LittleFS:")
@@ -13,6 +16,13 @@ def build_littlefs():
         print("LittleFS build successful")
 
 def merge_bins():
+    if os.path.isfile('data/settings.json') == False:
+        return # nothing to do
+
+    BOOTLOADER_OFFSET = 0x0000
+    PARTITIONS_OFFSET = 0x8000
+    FIRMWARE_OFFSET   = 0x10000
+
     flash_size = int(env.BoardConfig().get("upload.maximum_size", "4194304"))
     app0_offset = 0x10000
     if env['PIOENV'][:7] == "esp8266":
@@ -31,6 +41,12 @@ def merge_bins():
     start = os.getcwd()
     os.chdir('.pio/build/' + env['PIOENV'] + '/')
 
+    with open("bootloader.bin", "rb") as bootloader_file:
+        bootloader_data = bootloader_file.read()
+
+    with open("partitions.bin", "rb") as partitions_file:
+        partitions_data = partitions_file.read()
+
     with open("firmware.bin", "rb") as firmware_file:
         firmware_data = firmware_file.read()
 
@@ -38,11 +54,16 @@ def merge_bins():
         littlefs_data = littlefs_file.read()
 
     with open("firmware.factory.bin", "wb") as merged_file:
-        # fill gap with 0xff
+        merged_file.write(b'\xFF' * BOOTLOADER_OFFSET)
+        merged_file.write(bootloader_data)
+
+        merged_file.write(b'\xFF' * (PARTITIONS_OFFSET - (BOOTLOADER_OFFSET + len(bootloader_data))))
+        merged_file.write(partitions_data)
+
+        merged_file.write(b'\xFF' * (FIRMWARE_OFFSET - (PARTITIONS_OFFSET + len(partitions_data))))
         merged_file.write(firmware_data)
-        if len(firmware_data) < (littlefs_offset - app0_offset):
-            merged_file.write(b'\xFF' * ((littlefs_offset - app0_offset) - len(firmware_data)))
 
+        merged_file.write(b'\xFF' * (littlefs_offset - (FIRMWARE_OFFSET + len(firmware_data))))
         merged_file.write(littlefs_data)
 
     os.chdir(start)
@@ -51,6 +72,5 @@ def main(target, source, env):
     build_littlefs()
     merge_bins()
 
-
 # ensure that script is called once firmeware was compiled
 env.AddPostAction("$BUILD_DIR/${PROGNAME}.bin", main)
diff --git a/src/CHANGES.md b/src/CHANGES.md
index 34ced30b..93e0d834 100644
--- a/src/CHANGES.md
+++ b/src/CHANGES.md
@@ -3,6 +3,7 @@
 ## 0.8.120 - 2024-05-18
 * fix crash if invalid serial number was set -> inverter will be disabled automatically
 * improved and fixed factory image generation
+* fix HMT-1800-4T number of inputs #1628
 
 ## 0.8.119 - 2024-05-17
 * fix reset values at midnight if WiFi isn't available #1620
diff --git a/src/hm/hmSystem.h b/src/hm/hmSystem.h
index cb4103bd..81507105 100644
--- a/src/hm/hmSystem.h
+++ b/src/hm/hmSystem.h
@@ -69,9 +69,13 @@ class HmSystem {
                     iv->ivRadioType = INV_RADIO_TYPE_NRF;
                 }
             } else if(iv->config->serial.b[5] == 0x13) {
-                    iv->ivGen = IV_HMT;
+                iv->ivGen = IV_HMT;
+                if(iv->config->serial.b[4] == 0x61)
+                    iv->type = INV_TYPE_4CH;
+                else
                     iv->type = INV_TYPE_6CH;
-                    iv->ivRadioType = INV_RADIO_TYPE_CMT;
+
+                iv->ivRadioType = INV_RADIO_TYPE_CMT;
             } else if(iv->config->serial.u64 != 0ULL) {
                 DPRINTLN(DBG_ERROR, F("inverter type can't be detected!"));
                 iv->config->enabled = false;