Can you hook two central air con compressors to one thermostat if you have two thermostats side by side one for upstairs and the downstairs bedrooms and one for the living dining rooms and the den?

Answer 1

No. You must keep a separate thermostat for each air conditioning unit even if the two thermostats have been physically installed side-by-side.

The reason is very simple to understand: a thermostat is, very roughly, a bit like the cruise control in a car. When the speed drops from the set speed the cruise control gives the engine some more gas until it senses that the car's speed is fast enough and then it reduces the gas supply to the engine to maintain the set speed. No car engine can share the same cruise control with another engine. It is exactly the same for two air conditioning units - they each need to have their own thermostat.

Now this particular question itself raises yet another question: Why has the installer placed both thermostats alongside each other? That is not good practice at all!

The installer was either "cutting corners" - to save having to do some extra wiring for each separate thermostat - or was plain incompetent.

To ensure that a particular air conditoner is controlled correctly its thermostat must be installed somewhere within the "zone of cooling" for that air conditioner.

You should have been asked which areas you want to be controlled the closest. If you chose say the main bedroom for Zone A and the living room for Zone B, that is where the thermostats for each air conditioner should have been placed. But, as they have been installed side-by-side, if they are both set to the same temperature then the zone furthest away from the thermostats may never be cooled enough or may be over-cooled, whilst the zone closest to the thermostats may be controlled ok.

To understand the reason why that would be true, let's consider what would result if each thermostat were set to a different temperature: it is obvious that the thermostat set to the cooler temperature will keep its ac unit running long after the thermostat set to the higher temperature has turned off its own ac unit.

The resulting overall effect depends on which of the two zones is set to be cooler: * If the "set-to-be-cooler" zone is the zone closest to the position of the two thermostats then that zone will be controlled ok but the more distant "set-to-be-warmer" zone may never get any air conditioning!

That more distant zone could just heat up to the temperature of the outside air because its "own" thermostat is telling it that the air around it (meaning the thermostat, not the ac unit!) is cool enough already...

* If the "set-to-be-cooler" zone is the zone furthestfrom the position of the two thermostats then that zone will, again, never be controlled properly because the ac unit in the nearer "set-to-be-warmer" zone will always turn off its ac unit to maintain a temperature that is warmer than the other, more distant zone requires.

So, again, that more distant zone will always get too much cooling because its "own" thermostat is telling it that the air around it (meaning the thermostat, not the ac unit!) is not warm enough yet!