This is my design for the HVAC "control system," which is really just a 4"x5" board with these seven relays on it. Rather than buy a fully integrated (and proprietary) roof air conditioning system involving separate thermostats and controls, we got the simpler (and cheaper) self-contained units with integrated controls, which also lets us use whatever brand units we want, and even to mix and match. The problem with these is that the fans run constantly (the internal thermostats only switch off the compressors), and we did not want that. This board lets us control the three roof airs with two conventional residential-type thermostats (we bought the digital programmable ones). Since we were already putting the 'stats in place for the roof airs, we decided to use the same 'stats to control both the Webasto hydronic heat system and the supplemental electric heaters (when shore or generator power is available), and this board also manages that function.