HeatPump | Arduino library to control Mitsubishi Heat Pumps

There are two problems in your implementation.

The first one is that the QThread actually runs the function in the separate thread just the first time (when start() is called), while the QTimer will always execute it in the main thread, where it was created. run() should never be explicitly called.

Then, and this is the main point of your issue, the signal should always be emitted only when the value changes, not every time it is requested.

Finally, since requests are potentially blocking, the command sent from the UI should also be threaded. We already have a thread running, so we could use that along with a python Queue.

There are also other issues:

• as the documentation explicitly says, terminate() is discouraged, as using it could cause problems and stability issues; using a simple flag is way more simple and safer; in any case, with your original code it wouldn't do nothing anyway, unless called as soon as the thread is started: as stated above, each further execution was done in the wrong thread, so terminate() will do nothing useful;
• the second try/except block is pointless, as there is no possibility for that type of exception in there; the only exception that would make sense at that point is the KeyError for the dictionary lookup;
• the stateChanged signal is for tri-state checkboxes, for standard dual state the proper signal is toggled;

A proper implementation must keep the thread running in a while loop, and eventually quit when necessary. Using the queue timeout argument we also get the 6 second interval, so there is no need for timers.