diff --git a/README.md b/README.md
index 612c2881..7e550612 100644
--- a/README.md
+++ b/README.md
@@ -28,7 +28,7 @@ gem 'redis-cluster-client'
 | `:replica_affinity` | Symbol or String | `:random` | scale reading strategy, `:random`, `random_with_primary` or `:latency` are valid |
 | `:fixed_hostname` | String | `nil` | required if client should connect to single endpoint with SSL |
 | `:slow_command_timeout` | Integer | `-1` | timeout used for "slow" queries that fetch metdata e.g. CLUSTER NODES, COMMAND |
-| `:concurrency` | Hash | `{ model: :on_demand, size: 5}` | concurrency settings, `:on_demand`, `:pooled` and `:none` are valid models, size is a max number of workers, `:none` model is no concurrency, Please choose the one suited your environment if needed. |
+| `:concurrency` | Hash | `{ model: :none }` | concurrency settings, `:on_demand`, `:pooled` and `:none` are valid models, size is a max number of workers, `:none` model is no concurrency, Please choose the one suited your environment if needed. |
 | `:connect_with_original_config` | Boolean | `false` | `true` if client should retry the connection using the original endpoint that was passed in |
 | `:max_startup_sample` | Integer | `3` | maximum number of nodes to fetch `CLUSTER NODES` information for startup |
 
diff --git a/lib/redis_client/cluster/concurrent_worker.rb b/lib/redis_client/cluster/concurrent_worker.rb
index 998ee12b..cd70b9c6 100644
--- a/lib/redis_client/cluster/concurrent_worker.rb
+++ b/lib/redis_client/cluster/concurrent_worker.rb
@@ -71,14 +71,12 @@ def inspect
 
       module_function
 
-      def create(model: :on_demand, size: 5)
-        size = size.positive? ? size : 5
-
+      def create(model: :none, size: 5)
         case model
-        when :on_demand, nil then ::RedisClient::Cluster::ConcurrentWorker::OnDemand.new(size: size)
-        when :pooled then ::RedisClient::Cluster::ConcurrentWorker::Pooled.new(size: size)
         when :none then ::RedisClient::Cluster::ConcurrentWorker::None.new
-        else raise ArgumentError, "Unknown model: #{model}"
+        when :on_demand then ::RedisClient::Cluster::ConcurrentWorker::OnDemand.new(size: size)
+        when :pooled then ::RedisClient::Cluster::ConcurrentWorker::Pooled.new(size: size)
+        else raise ArgumentError, "unknown model: #{model}"
         end
       end
     end
diff --git a/lib/redis_client/cluster/concurrent_worker/on_demand.rb b/lib/redis_client/cluster/concurrent_worker/on_demand.rb
index 72b885b6..5beae285 100644
--- a/lib/redis_client/cluster/concurrent_worker/on_demand.rb
+++ b/lib/redis_client/cluster/concurrent_worker/on_demand.rb
@@ -5,6 +5,8 @@ class Cluster
     module ConcurrentWorker
       class OnDemand
         def initialize(size:)
+          raise ArgumentError, "size must be positive: #{size}" unless size.positive?
+
           @q = SizedQueue.new(size)
         end
 
diff --git a/lib/redis_client/cluster/concurrent_worker/pooled.rb b/lib/redis_client/cluster/concurrent_worker/pooled.rb
index 83e974b3..8677cff8 100644
--- a/lib/redis_client/cluster/concurrent_worker/pooled.rb
+++ b/lib/redis_client/cluster/concurrent_worker/pooled.rb
@@ -11,6 +11,8 @@ module ConcurrentWorker
       # So it consumes memory 1 MB multiplied a number of workers.
       class Pooled
         def initialize(size:)
+          raise ArgumentError, "size must be positive: #{size}" unless size.positive?
+
           @size = size
           setup
         end
diff --git a/lib/redis_client/cluster_config.rb b/lib/redis_client/cluster_config.rb
index 6b0504ef..7bcf7670 100644
--- a/lib/redis_client/cluster_config.rb
+++ b/lib/redis_client/cluster_config.rb
@@ -19,7 +19,7 @@ class ClusterConfig
     VALID_NODES_KEYS = %i[ssl username password host port db].freeze
     MERGE_CONFIG_KEYS = %i[ssl username password].freeze
     IGNORE_GENERIC_CONFIG_KEYS = %i[url host port path].freeze
-    MAX_WORKERS = Integer(ENV.fetch('REDIS_CLIENT_MAX_THREADS', 5))
+    MAX_WORKERS = Integer(ENV.fetch('REDIS_CLIENT_MAX_THREADS', -1)) # for backward compatibility
     # It's used with slow queries of fetching meta data like CLUSTER NODES, COMMAND and so on.
     SLOW_COMMAND_TIMEOUT = Float(ENV.fetch('REDIS_CLIENT_SLOW_COMMAND_TIMEOUT', -1))
     # It affects to strike a balance between load and stability in initialization or changed states.
@@ -110,12 +110,16 @@ def server_url
     private
 
     def merge_concurrency_option(option)
-      case option
-      when Hash
-        option = option.transform_keys(&:to_sym)
-        { size: MAX_WORKERS }.merge(option)
-      else { size: MAX_WORKERS }
+      opts = {}
+
+      if MAX_WORKERS.positive?
+        opts[:model] = :on_demand
+        opts[:size] = MAX_WORKERS
       end
+
+      opts.merge!(option.transform_keys(&:to_sym)) if option.is_a?(Hash)
+      opts[:model] = :none if opts.empty?
+      opts.freeze
     end
 
     def build_node_configs(addrs)
diff --git a/test/redis_client/test_cluster_config.rb b/test/redis_client/test_cluster_config.rb
index 8ecd23aa..a4edc483 100644
--- a/test/redis_client/test_cluster_config.rb
+++ b/test/redis_client/test_cluster_config.rb
@@ -103,6 +103,18 @@ def test_command_builder
       assert_equal(::RedisClient::CommandBuilder, ::RedisClient::ClusterConfig.new.command_builder)
     end
 
+    def test_concurrency
+      [
+        { value: nil, want: { model: :none } },
+        { value: { model: :none }, want: { model: :none } },
+        { value: { model: :on_demand, size: 3 }, want: { model: :on_demand, size: 3 } },
+        { value: { model: :pooled, size: 6 }, want: { model: :pooled, size: 6 } }
+      ].each do |c|
+        cfg = ::RedisClient::ClusterConfig.new(concurrency: c[:value])
+        assert_equal(c[:want], cfg.instance_variable_get(:@concurrency))
+      end
+    end
+
     def test_build_node_configs
       config = ::RedisClient::ClusterConfig.new
       [