def search_lhn_all_edges(city):
    """
    Returns all edge tuples, which are of type heating or heating_and_deg.
    If no heating network exists, returns empty list

    Parameters
    ----------
    city : object
        City object of pyCity_calc

    Returns
    -------
    list_lhn_edges : list (of tuples)
        Returns list of LHN tuples (holding start and stop node ids)
    """

    list_lhn_edges = []

    for edge in city.edges(data=True):
        if 'network_type' in edge[2]:
            if (edge[2]['network_type'] == 'heating' or
                        edge[2]['network_type'] == 'heating_and_deg'):
                list_lhn_edges.append((edge[0], edge[1]))

    return list_lhn_edges