diff --git a/src/heisskleber/console/sender.py b/src/heisskleber/console/sender.py
index da685bd..90f4796 100644
--- a/src/heisskleber/console/sender.py
+++ b/src/heisskleber/console/sender.py
@@ -1,6 +1,7 @@
+import json
 from typing import Any, TypeVar
 
-from heisskleber.core import Packer, Sender, json_packer
+from heisskleber.core import Packer, Sender
 
 from .config import ConsoleConf
 
@@ -13,7 +14,7 @@ class ConsoleSender(Sender[T]):
     def __init__(
         self,
         config: ConsoleConf,
-        packer: Packer[T] = json_packer,  # type: ignore[assignment]
+        packer: Packer[T] = lambda x: json.dumps(x),  # type: ignore[assignment]
     ) -> None:
         self.verbose = config.verbose
         self.pretty = config.pretty
@@ -22,7 +23,8 @@ class ConsoleSender(Sender[T]):
     async def send(self, data: T, topic: str | None = None, **kwargs: dict[str, Any]) -> None:
         """Serialize data and write to console output."""
         serialized = self.packer(data)
-        output = f"{topic}:\t{serialized.decode()}" if topic else serialized.decode()
+        serialized = serialized.decode() if isinstance(serialized, bytes | bytearray) else serialized
+        output = f"{topic}:\t{serialized}" if topic else serialized
         print(output)  # noqa: T201
 
     def __repr__(self) -> str:
diff --git a/src/heisskleber/core/__init__.py b/src/heisskleber/core/__init__.py
index c678aec..6a3f1b5 100644
--- a/src/heisskleber/core/__init__.py
+++ b/src/heisskleber/core/__init__.py
@@ -3,7 +3,7 @@
 from typing import Any
 
 from .config import BaseConf, ConfigType
-from .packer import JSONPacker, Packer, PackerError
+from .packer import JSONPacker, Packer, PackerError, Payload
 from .receiver import Receiver
 from .sender import Sender
 from .unpacker import JSONUnpacker, Unpacker, UnpackerError
@@ -29,6 +29,7 @@ __all__ = [
     "ConfigType",
     "Packer",
     "PackerError",
+    "Payload",
     "Receiver",
     "Sender",
     "Unpacker",
diff --git a/src/heisskleber/core/packer.py b/src/heisskleber/core/packer.py
index 3ea010e..9baaca4 100644
--- a/src/heisskleber/core/packer.py
+++ b/src/heisskleber/core/packer.py
@@ -2,10 +2,12 @@
 
 import json
 from abc import abstractmethod
-from typing import Any, Protocol, TypeVar
+from typing import Any, Protocol, TypeAlias, TypeVar
 
 T_contra = TypeVar("T_contra", contravariant=True)
 
+Payload: TypeAlias = str | bytes | bytearray
+
 
 class PackerError(Exception):
     """Raised when unpacking operations fail.
@@ -38,7 +40,7 @@ class Packer(Protocol[T_contra]):
     """
 
     @abstractmethod
-    def __call__(self, data: T_contra) -> bytes:
+    def __call__(self, data: T_contra) -> Payload:
         """Packs the data dictionary into a bytes payload.
 
         Arguments:
diff --git a/src/heisskleber/core/unpacker.py b/src/heisskleber/core/unpacker.py
index 5fd305a..df94803 100644
--- a/src/heisskleber/core/unpacker.py
+++ b/src/heisskleber/core/unpacker.py
@@ -6,6 +6,8 @@ from typing import Any, Protocol, TypeVar
 
 T_co = TypeVar("T_co", covariant=True)
 
+Payload = str | bytes | bytearray
+
 
 class UnpackerError(Exception):
     """Raised when unpacking operations fail.
@@ -20,10 +22,11 @@ class UnpackerError(Exception):
 
     PREVIEW_LENGTH = 100
 
-    def __init__(self, payload: bytes) -> None:
+    def __init__(self, payload: Payload) -> None:
         """Initialize the error with the failed payload and cause."""
         self.payload = payload
-        preview = payload[: self.PREVIEW_LENGTH] + b"..." if len(payload) > self.PREVIEW_LENGTH else payload
+        dots = b"..." if isinstance(payload, bytes | bytearray) else "..."
+        preview = payload[: self.PREVIEW_LENGTH] + dots if len(payload) > self.PREVIEW_LENGTH else payload
         message = f"Failed to unpack payload: {preview!r}. "
         super().__init__(message)
 
@@ -37,7 +40,7 @@ class Unpacker(Protocol[T_co]):
     """
 
     @abstractmethod
-    def __call__(self, payload: bytes) -> tuple[T_co, dict[str, Any]]:
+    def __call__(self, payload: Payload) -> tuple[T_co, dict[str, Any]]:
         """Unpacks the payload into a data object and optional meta-data dictionary.
 
         Args:
@@ -76,7 +79,7 @@ class JSONUnpacker(Unpacker[dict[str, Any]]):
 
     """
 
-    def __call__(self, payload: bytes) -> tuple[dict[str, Any], dict[str, Any]]:
+    def __call__(self, payload: Payload) -> tuple[dict[str, Any], dict[str, Any]]:
         """Unpack the payload."""
         try:
             return json.loads(payload), {}
diff --git a/src/heisskleber/core/utils.py b/src/heisskleber/core/utils.py
index c42418b..61b0c08 100644
--- a/src/heisskleber/core/utils.py
+++ b/src/heisskleber/core/utils.py
@@ -1,7 +1,7 @@
 import asyncio
-from collections.abc import Coroutine
+from collections.abc import Callable, Coroutine
 from functools import wraps
-from typing import Any, Callable, ParamSpec, TypeVar
+from typing import Any, ParamSpec, TypeVar
 
 P = ParamSpec("P")
 T = TypeVar("T")
diff --git a/src/heisskleber/file/sender.py b/src/heisskleber/file/sender.py
index 6b6e728..29224be 100644
--- a/src/heisskleber/file/sender.py
+++ b/src/heisskleber/file/sender.py
@@ -1,13 +1,15 @@
 import asyncio
 import contextlib
+import json
 import logging
+from collections.abc import Callable
 from concurrent.futures import ThreadPoolExecutor
 from datetime import datetime
-from io import BufferedWriter
+from io import TextIOWrapper
 from pathlib import Path
 from typing import Any, TypeVar
 
-from heisskleber.core import Packer, Sender, json_packer
+from heisskleber.core import Packer, PackerError, Sender
 from heisskleber.file.config import FileConf
 
 T = TypeVar("T")
@@ -15,6 +17,19 @@ T = TypeVar("T")
 logger = logging.getLogger("heisskleber.file")
 
 
+def json_packer(data: dict[str, Any]) -> str:
+    """Pack to json string."""
+    try:
+        return json.dumps(data)
+    except json.JSONDecodeError:
+        raise PackerError(data) from None
+
+
+def csv_packer(data: dict[str, Any]) -> str:
+    """Create csv string from data."""
+    return ",".join(map(str, data.values()))
+
+
 class FileWriter(Sender[T]):
     """Asynchronous file writer implementation of the Sender interface.
 
@@ -22,34 +37,48 @@ class FileWriter(Sender[T]):
     Files are named according to the configured datetime format.
     """
 
-    def __init__(self, config: FileConf, packer: Packer[T] = json_packer) -> None:  # type: ignore[assignment]
+    def __init__(
+        self,
+        config: FileConf,
+        packer: Packer[T] = json_packer,  # type: ignore[assignment]
+        header_func: Callable[[T], list[str]] | None = None,
+    ) -> None:
         """Initialize the file writer.
 
         Args:
-            base_path: Directory path where files will be written
             config: Configuration for file rollover and naming
+            header_func: Function to extract header from T
             packer: Optional packer for serializing data
         """
         self.base_path = Path(config.directory)
         self.config = config
+        self.header_func = header_func
         self.packer = packer
 
         self._executor = ThreadPoolExecutor(max_workers=1)
         self._loop = asyncio.get_running_loop()
+        self._header: list[str] | None = None
 
-        self._current_file: BufferedWriter | None = None
-        self._rollover_task: asyncio.Task | None = None
+        self._current_file: TextIOWrapper | None = None
+        self._rollover_task: asyncio.Task[None] | None = None
         self._last_rollover: float = 0
         self.filename: Path = Path()
 
-    async def _open_file(self, filename: Path) -> BufferedWriter:
+    async def _open_file(self, filename: Path) -> TextIOWrapper:
         """Open file asynchronously."""
-        return await self._loop.run_in_executor(self._executor, lambda: filename.open(mode="ab"))
+        return await self._loop.run_in_executor(self._executor, lambda: filename.open(mode="a"))
 
     async def _close_file(self) -> None:
         if self._current_file is not None:
             await self._loop.run_in_executor(self._executor, self._current_file.close)
 
+    async def _write_header(self) -> None:
+        if not self._header or not self._current_file:
+            return
+        for line in self._header:
+            await self._loop.run_in_executor(self._executor, self._current_file.write, line)
+            await self._loop.run_in_executor(self._executor, self._current_file.write, "\n")
+
     async def _rollover(self) -> None:
         """Close current file and open a new one."""
         if self._current_file is not None:
@@ -60,6 +89,7 @@ class FileWriter(Sender[T]):
         self._current_file = await self._open_file(self.filename)
         self._last_rollover = self._loop.time()
         logger.info("Rolled over to new file: %s", self.filename)
+        await self._write_header()
 
     async def _rollover_loop(self) -> None:
         """Background task that handles periodic file rollover."""
@@ -83,10 +113,15 @@ class FileWriter(Sender[T]):
             await self.start()
         if not self._current_file:
             raise RuntimeError("FileWriter not started")
+        if not self._header and self.header_func is not None:
+            self._header = self.header_func(data)
+            await self._write_header()
 
         payload = self.packer(data)
+        if isinstance(payload, bytes | bytearray):
+            payload = payload.decode()
         await self._loop.run_in_executor(self._executor, self._current_file.write, payload)
-        await self._loop.run_in_executor(self._executor, self._current_file.write, b"\n")
+        await self._loop.run_in_executor(self._executor, self._current_file.write, "\n")
 
     async def start(self) -> None:
         """Start the file writer and rollover background task."""
diff --git a/src/heisskleber/mqtt/sender.py b/src/heisskleber/mqtt/sender.py
index 743d419..c627691 100644
--- a/src/heisskleber/mqtt/sender.py
+++ b/src/heisskleber/mqtt/sender.py
@@ -8,7 +8,7 @@ from typing import Any, TypeVar
 
 import aiomqtt
 
-from heisskleber.core import Packer, Sender, json_packer
+from heisskleber.core import Packer, Payload, Sender, json_packer
 from heisskleber.core.utils import retry
 
 from .config import MqttConf
@@ -34,7 +34,7 @@ class MqttSender(Sender[T]):
     def __init__(self, config: MqttConf, packer: Packer[T] = json_packer) -> None:  # type: ignore[assignment]
         self.config = config
         self.packer = packer
-        self._send_queue: asyncio.Queue[tuple[bytes, str]] = asyncio.Queue()
+        self._send_queue: asyncio.Queue[tuple[Payload, str]] = asyncio.Queue()
         self._sender_task: asyncio.Task[None] | None = None
 
     async def send(self, data: T, topic: str = "mqtt", qos: int = 0, retain: bool = False, **kwargs: Any) -> None:
diff --git a/src/heisskleber/serial/sender.py b/src/heisskleber/serial/sender.py
index f433bdd..01e0a9a 100644
--- a/src/heisskleber/serial/sender.py
+++ b/src/heisskleber/serial/sender.py
@@ -54,6 +54,7 @@ class SerialSender(Sender[T]):
             await self.start()
 
         payload = self.packer(data)
+        payload = payload.encode() if isinstance(payload, str) else payload
         try:
             await asyncio.get_running_loop().run_in_executor(self._executor, self._ser.write, payload)
             await asyncio.get_running_loop().run_in_executor(self._executor, self._ser.flush)
diff --git a/src/heisskleber/udp/sender.py b/src/heisskleber/udp/sender.py
index 8693db4..cfc1952 100644
--- a/src/heisskleber/udp/sender.py
+++ b/src/heisskleber/udp/sender.py
@@ -81,6 +81,7 @@ class UdpSender(Sender[T]):
         """
         await self._ensure_connection()  # we know that self._transport is intialized
         payload = self.pack(data)
+        payload = payload.encode() if isinstance(payload, str) else payload
         self._transport.sendto(payload)  # type: ignore [union-attr]
 
     def __repr__(self) -> str:
diff --git a/src/heisskleber/zmq/receiver.py b/src/heisskleber/zmq/receiver.py
index 4a80dfd..880d23d 100644
--- a/src/heisskleber/zmq/receiver.py
+++ b/src/heisskleber/zmq/receiver.py
@@ -71,7 +71,7 @@ class ZmqReceiver(Receiver[T]):
             topic: The topic or list of topics to subscribe to.
 
         """
-        if isinstance(topic, (list, tuple)):
+        if isinstance(topic, list | tuple):
             for t in topic:
                 self._subscribe_single_topic(t)
         else:
diff --git a/src/heisskleber/zmq/sender.py b/src/heisskleber/zmq/sender.py
index 955c393..99265be 100644
--- a/src/heisskleber/zmq/sender.py
+++ b/src/heisskleber/zmq/sender.py
@@ -35,6 +35,7 @@ class ZmqSender(Sender[T]):
         if not self.is_connected:
             await self.start()
         payload = self.packer(data)
+        payload = payload.encode() if isinstance(payload, str) else payload
         logger.debug("sending payload %(payload)b to topic %(topic)s", {"payload": payload, "topic": topic})
         await self.socket.send_multipart([topic.encode(), payload])