diff --git a/server/mergin/sync/private_api_controller.py b/server/mergin/sync/private_api_controller.py
index aaa77216..13f059b9 100644
--- a/server/mergin/sync/private_api_controller.py
+++ b/server/mergin/sync/private_api_controller.py
@@ -4,7 +4,6 @@
 import os
 from datetime import datetime, timedelta, timezone
 from urllib.parse import quote
-from blinker import signal
 from connexion import NoContent
 from flask import (
     render_template,
@@ -353,17 +352,17 @@ def download_project(id: str, version=None):  # noqa: E501 # pylint: disable=W06
             f"attachment; filename*=UTF-8''{file_name}"
         )
         return resp
-
-    temp_zip_path = project_version.zip_path + ".partial"
-    # to be safe we are not in vicious circle remove inactive partial zip
-    if os.path.exists(temp_zip_path) and datetime.fromtimestamp(
-        os.path.getmtime(temp_zip_path), tz=timezone.utc
-    ) < datetime.now(timezone.utc) - timedelta(
-        seconds=current_app.config["PARTIAL_ZIP_EXPIRATION"]
-    ):
-        move_to_tmp(temp_zip_path)
-
-    if not os.path.exists(temp_zip_path):
-        create_project_version_zip.delay(project_version.id)
+    # GET request triggers background job if no partial zip or expired one
+    if request.method == "GET":
+        temp_zip_path = project_version.zip_path + ".partial"
+        # create zip if it does not exist yet or has expired
+        partial_exists = os.path.exists(temp_zip_path)
+        is_expired = partial_exists and datetime.fromtimestamp(
+            os.path.getmtime(temp_zip_path), tz=timezone.utc
+        ) < datetime.now(timezone.utc) - timedelta(
+            seconds=current_app.config["PARTIAL_ZIP_EXPIRATION"]
+        )
+        if not partial_exists or is_expired:
+            create_project_version_zip.delay(project_version.id)
 
     return "Project zip being prepared, please try again later", 202
diff --git a/server/mergin/sync/tasks.py b/server/mergin/sync/tasks.py
index f56fb273..ce92171e 100644
--- a/server/mergin/sync/tasks.py
+++ b/server/mergin/sync/tasks.py
@@ -114,9 +114,17 @@ def create_project_version_zip(version_id: int):
         return
 
     zip_path = project_version.zip_path + ".partial"
-    # already running job
     if os.path.exists(zip_path):
-        return
+        mtime = datetime.fromtimestamp(os.path.getmtime(zip_path), tz=timezone.utc)
+        expiration_time = datetime.now(timezone.utc) - timedelta(
+            seconds=current_app.config["PARTIAL_ZIP_EXPIRATION"]
+        )
+        if mtime > expiration_time:
+            # partial zip is recent -> another job is likely running
+            return
+        else:
+            # partial zip is too old -> remove and creating new one
+            os.remove(zip_path)
 
     os.makedirs(os.path.dirname(zip_path), exist_ok=True)
     try:
diff --git a/server/mergin/tests/test_celery.py b/server/mergin/tests/test_celery.py
index a5d07f47..50420cf0 100644
--- a/server/mergin/tests/test_celery.py
+++ b/server/mergin/tests/test_celery.py
@@ -4,6 +4,8 @@
 
 import os
 from datetime import datetime, timedelta
+from pathlib import Path
+
 from flask import current_app
 from flask_mail import Mail
 from unittest.mock import patch
@@ -144,16 +146,44 @@ def test_remove_deleted_project_backups(client):
 
 
 def test_create_project_version_zip(diff_project):
+    """Test celery tasks for creating project zip and cleaning them up"""
     latest_version = diff_project.get_latest_version()
-    assert not os.path.exists(latest_version.zip_path)
+    zip_path = Path(latest_version.zip_path)
+    partial_zip_path = Path(latest_version.zip_path + ".partial")
+    assert not zip_path.exists()
+    create_project_version_zip(diff_project.latest_version)
+    assert zip_path.exists()
+    assert not partial_zip_path.exists()
+    before_mtime = zip_path.stat().st_mtime
+    # mock expired partial zip -> celery removes it and creates new zip
+    partial_zip_path.parent.mkdir(parents=True, exist_ok=True)
+    partial_zip_path.touch()
+    new_time = datetime.now() - timedelta(
+        seconds=current_app.config["PARTIAL_ZIP_EXPIRATION"] + 1
+    )
+    modify_file_times(partial_zip_path, new_time)
+    create_project_version_zip(diff_project.latest_version)
+    assert (
+        not partial_zip_path.exists()
+    )  # after creating zip archive, celery remove partial zip
+    after_mtime = zip_path.stat().st_mtime
+    assert before_mtime < after_mtime
+    # mock valid partial zip -> celery skips creating new zip and returns
+    before_mtime = zip_path.stat().st_mtime
+    partial_zip_path.parent.mkdir(parents=True, exist_ok=True)
+    partial_zip_path.touch()
     create_project_version_zip(diff_project.latest_version)
-    assert os.path.exists(latest_version.zip_path)
-    assert not os.path.exists(latest_version.zip_path + ".partial")
-    remove_projects_archives()
-    assert os.path.exists(latest_version.zip_path)
+    assert (
+        partial_zip_path.exists()
+    )  # celery does not create zip archive and does not clean partial zip
+    after_mtime = zip_path.stat().st_mtime
+    assert before_mtime == after_mtime
+    os.remove(partial_zip_path)
+    remove_projects_archives()  # zip is valid -> keep
+    assert zip_path.exists()
     new_time = datetime.now() - timedelta(
         days=current_app.config["PROJECTS_ARCHIVES_EXPIRATION"] + 1
     )
     modify_file_times(latest_version.zip_path, new_time)
-    remove_projects_archives()
+    remove_projects_archives()  # zip has expired -> remove
     assert not os.path.exists(latest_version.zip_path)
diff --git a/server/mergin/tests/test_private_project_api.py b/server/mergin/tests/test_private_project_api.py
index 27021ecb..54575114 100644
--- a/server/mergin/tests/test_private_project_api.py
+++ b/server/mergin/tests/test_private_project_api.py
@@ -2,15 +2,16 @@
 #
 # SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-MerginMaps-Commercial
 
-import datetime
 import json
 import os
-from unittest.mock import patch
-
+import shutil
 import pytest
+from unittest.mock import patch
+from pathlib import Path
+from datetime import datetime, timedelta
 from flask import url_for
 
-from ..app import db
+from ..app import db, current_app
 from ..sync.models import AccessRequest, Project, ProjectRole, RequestStatus
 from ..auth.models import User
 from ..config import Configuration
@@ -20,6 +21,7 @@
     login,
     create_project,
     create_workspace,
+    modify_file_times,
 )
 
 
@@ -174,7 +176,7 @@ def test_project_access_request(client):
 
     # try with inactive project
     rp = create_project("removed_project", test_workspace, user)
-    rp.removed_at = datetime.datetime.utcnow()
+    rp.removed_at = datetime.utcnow()
     db.session.commit()
 
     resp = client.post(
@@ -193,7 +195,7 @@ def test_project_access_request(client):
     assert resp.status_code == 200
     resp = client.get("/app/project/access-requests?page=1&per_page=10")
     assert resp.json.get("count") == 1
-    rp.removed_at = datetime.datetime.utcnow()
+    rp.removed_at = datetime.utcnow()
     db.session.commit()
     access_request = AccessRequest.query.filter(
         AccessRequest.project_id == rp.id
@@ -375,7 +377,7 @@ def test_admin_project_list(client):
     assert resp.json.get("count") == 1
     # mark as inactive
     p = Project.query.get(resp.json["items"][0]["id"])
-    p.removed_at = datetime.datetime.utcnow()
+    p.removed_at = datetime.utcnow()
     p.removed_by = user.id
     db.session.commit()
 
@@ -423,19 +425,55 @@ def test_admin_project_list(client):
 
 
 test_download_proj_data = [
-    (None, 202),
-    ("v1", 202),
-    ("v100", 404),
-    (None, 200),
+    # zips do not exist, version not specified -> call celery task to create zip with latest version
+    (0, 0, 0, None, 202, 1),
+    # expired partial zip exists -> call celery task
+    (0, 1, 1, None, 202, 1),
+    # valid partial zip exists -> return, do not call celery
+    (0, 1, 0, None, 202, 0),
+    # zips do not exist, version specified -> call celery task with specified version
+    (0, 0, 0, "v1", 202, 1),
+    # specified version does not exist -> 404
+    (0, 0, 0, "v100", 404, 0),
+    # zip is ready to download
+    (1, 0, 0, None, 200, 0),
 ]
 
 
-@pytest.mark.parametrize("version,expected", test_download_proj_data)
+@pytest.mark.parametrize(
+    "zipfile,partial,expired,version,exp_resp,exp_call", test_download_proj_data
+)
 @patch("mergin.sync.tasks.create_project_version_zip.delay")
-def test_download_project(mock_create_zip, client, version, expected, diff_project):
-    if expected == 200:
-        project_version = diff_project.get_latest_version()
-        os.mknod(project_version.zip_path)
+def test_download_project(
+    mock_create_zip,
+    client,
+    zipfile,
+    partial,
+    expired,
+    version,
+    exp_resp,
+    exp_call,
+    diff_project,
+):
+    """Test download endpoint responses and celery task calling"""
+    # prepare initial state according to testcase
+    project_version = diff_project.get_latest_version()
+    if zipfile:
+        zip_path = Path(project_version.zip_path)
+        if zip_path.parent.exists():
+            shutil.rmtree(zip_path.parent, ignore_errors=True)
+        zip_path.parent.mkdir(parents=True, exist_ok=True)
+        zip_path.touch()
+    if partial:
+        temp_zip_path = Path(project_version.zip_path + ".partial")
+        shutil.rmtree(temp_zip_path.parent, ignore_errors=True)
+        temp_zip_path.parent.mkdir(parents=True, exist_ok=True)
+        temp_zip_path.touch()
+        if expired:
+            new_time = datetime.now() - timedelta(
+                seconds=current_app.config["PARTIAL_ZIP_EXPIRATION"] + 1
+            )
+            modify_file_times(temp_zip_path, new_time)
     resp = client.get(
         url_for(
             "/app.mergin_sync_private_api_controller_download_project",
@@ -443,17 +481,16 @@ def test_download_project(mock_create_zip, client, version, expected, diff_proje
             version=version if version else "",
         )
     )
-    if expected == 200:
-        os.remove(project_version.zip_path)
-    assert resp.status_code == expected
-    assert mock_create_zip.called if expected == 202 else not mock_create_zip.called
-    if not version and expected != 200:
+    assert resp.status_code == exp_resp
+    assert mock_create_zip.called == exp_call
+    if not version and exp_call:
         call_args, _ = mock_create_zip.call_args
         args = call_args[0]
         assert args == diff_project.latest_version
 
 
 def test_large_project_download_fail(client, diff_project):
+    """Test downloading too large project is refused"""
     resp = client.get(
         url_for(
             "/app.mergin_sync_private_api_controller_download_project",
@@ -494,4 +531,24 @@ def test_remove_abandoned_zip(mock_prepare_zip, client, diff_project):
     )
     assert mock_prepare_zip.called
     assert resp.status_code == 202
-    assert not os.path.exists(partial_zip_path)
+
+
+@patch("mergin.sync.tasks.create_project_version_zip.delay")
+def test_download_project_request_method(mock_prepare_zip, client, diff_project):
+    """Test head request does not create a celery job"""
+    resp = client.head(
+        url_for(
+            "/app.mergin_sync_private_api_controller_download_project",
+            id=diff_project.id,
+        )
+    )
+    assert not mock_prepare_zip.called
+    assert resp.status_code == 202
+    resp = client.get(
+        url_for(
+            "/app.mergin_sync_private_api_controller_download_project",
+            id=diff_project.id,
+        )
+    )
+    assert mock_prepare_zip.called
+    assert resp.status_code == 202