feasible to leave 2nd room open and let the living room unit to clean the air?
Of course, just that for us we have 2 kids and my MIL would come over during weekend to help, so we'd need 1 x living room, 1 x computer room and 1 x MBR. 2 x Toilet doors closed, 1 bedroom closed and 1 x kitchen closed.
You can leave 1 more room opened. That room is likely to have an aircon FCU, so that's even better. Just that it'd cost a fair bit of $$$ in electricity bill
2 x 9000 btu/hr FCU = approx. 750W for a 2 tick compressor. 1.5kW fixed consumption. It's about 40 cents per hour (including GST) operation.
Make sure the aircon FCUs and external compressor fins and fan blades are decently cleaned so that they are operating at max efficiency, that's why learning to DIY clean is so good. You don't have to wait for the aircon man by fixing an appt and wait for them (Sometimes they rubber band 4hrs one). Sibeh jialet if you do once every 3 months as advised.
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BTW, one more interesting thing that you'd notice is that while the living room is at 29 deg C or so, maybe hitting 28 deg C after a longer period, it'd be pretty dry. RH can dip to 40% range, maybe 30% if switched on longer. Esp if you are in the airflow of a fan, you will feel comfy and not perspire. My kids and wife perspire easily, they did not perspire yesterday at all in the living room.
The reason is coz the 100% duty cycle for the aircons would continue to dehumidify the air (aircon drainage pipe would continue to flow one). However, the cold dry air from the rooms flows out to the living room and gets heated up by the warmer walls/floors....maybe plasma TV heats it up. The rooms would be cold an dry, but the living room would be warmer and just as dry. So that aids the living room comfort level.
While the room's floors/walls/electrical appliances heat up the air, the RH % stays the same (assuming that the unit is pretty air tight), only humidifying factor is the number of human occupants in the space. So yes, 2 x 9000 btu/hr units trying to cool 60 sqm of space can lead to pretty low humidity levels over a longer period of time but temperature won't get freezing cold.
That is how the refrigerative type of dehumidifier works. There is a cooling part (like your aircon FCU) and before the air is exhausted out, there is a heating element that re-heats the air which has been dried.
I don't forsee the external compressor/electrical fan motor breaking down if it's been maintained/installed properly in a windy open place and it's a good reliable brand (free of dust clog-ups) and you break up the 100% duty cycle by not operating for more than 6hrs at a time (rest for say 20 mins).
But then again, I do see some pretty screwed up installations coz the aircon ledge designs are screwed up in the first place. You have 2 compressors and there is some "short circuit" of the exhaust air back to the intake coz the aircon ledge is small + partially blocked and one external unit needs to be angled towards the second unit thus heating the second unit up as it ingests some of the hot air.
For this example, if the general wind direction is such that during certain months (NE or SW monsoon period) it causes the output of the hot air of that compressor to blow towards the intake fins, then really it may reduce the efficiency and life of the fan/compressor. Issue of both the angle of the compressor + one compressor is placed behind the other, it can make a difference. The intake would draw in the hot air, esp if aided by the wind.
