diff --git a/heatmiserV3/heatmiser.py b/heatmiserV3/heatmiser.py
index <HASH>..<HASH> 100644
--- a/heatmiserV3/heatmiser.py
+++ b/heatmiserV3/heatmiser.py
@@ -92,17 +92,17 @@ class HeatmiserThermostat(object):
             start_low = (start & constants.BYTEMASK)

             start_high = (start >> 8) & constants.BYTEMASK

             if function == constants.FUNC_READ:

-                payloadLength = 0

+                payload_length = 0

                 length_low = (constants.RW_LENGTH_ALL & constants.BYTEMASK)

                 length_high = (constants.RW_LENGTH_ALL >>

                                8) & constants.BYTEMASK

             else:

-                payloadLength = len(payload)

-                length_low = (payloadLength & constants.BYTEMASK)

-                length_high = (payloadLength >> 8) & constants.BYTEMASK

+                payload_length = len(payload)

+                length_low = (payload_length & constants.BYTEMASK)

+                length_high = (payload_length >> 8) & constants.BYTEMASK

             msg = [

                 destination,

-                10 + payloadLength,

+                10 + payload_length,

                 source,

                 function,

                 start_low,
Renamed payloadLength to payload_length