Your best bet is to start by getting ahold of the controls diagram if it even exits. If not trace and make your own diagram, Then analyze. Write down all components part numbers and start googling or go to supply house for data sheets. Hot deck cold deck multizones many times used hi lo signal selectors so the hot deck temp is set by the coldest zone (all t/stat branch lines go to selector inputs and unused ports are jumpered. Branch line of hi lo selector to cpa port of receiver controller ( remote set point adjustment). Receiver controller has hot deck sensor and branch line goes to N.O. valve. Pressure at valve indicates hot deck temp is in throttling range. Zero pressure valve is being told to open and raise hot deck temp. There are variations depending on who's controls but principle is the same.
I didn't write the book I just read it!