I own a condo (that's American for an apartment) in Tucson, Arizona. It is rented out most of the year. However, Tucson is located in the desert, so in the heat of the summer months (June-August) the condo is usually vacant. My question regards the setting of the air condition (AC) when the condo is vacant.
Currently, the AC is set at a threshold of 88 F (31 C), when the condo is vacant. This is per the recommendation of the bureau overseeing our rentals and, indeed, per the unanimous recommendation of all web pages that I have come across, e.g., AC recommendation for Tucson. The problem? We are paying $150-200 per month for cooling a condo when nobody is home.
The reason that everybody seems to agree about, is that with a higher setting (say, 100F, or, why not simply turn the thing off?) humidity could rise critically inside thus giving rise to mould problems. I get the point in a place like Florida. When air is cooled, relative humidity (RH) rises. I can see how at humid locations, this could cause problems. But in a desert, really? The AC does dehumidify the air it takes in (it has a drain for the condensed water) but warnings are prevalent that if the AC is set above 88 F, its capacity for dehumidification degrades.
Below I have plotted hourly values of weather data from a location near Tucson:
From the top you see outdoors variables: temperature (T; C) and relative humidity (RH; %); followed by derived variables: absolute humidity (AH; kg/m3), dewpoint temperature (Tdew; C) and the temperature at which the relative humidity would be 65% (T65; C).
In Northern Europe where I live, a max. indoors RH of 65% is recommended. Look at the bottom graph. Even if the AC did not dehumidify the air but just cooled it, we would need to cool it to about 32 C (90 F) to get a critical indoors RH. But let's say we cranked the AC up 100 F (38 C) or 110 F (43 C), we would never get nowhere near the critical 65% RH indoors.
But, hey, what about the outdoors humidity; it seems to get pretty close to 100% quite often. Right. Let's zoom in on such an event:
Ah, this is in the desert! The daily temperature fluctuations are extreme, and so the cool nights ramps up the RH drastically. But the AC won't be running at that time of the day; we have set it to cool when it's too hot, not to heat when it is too cold.
So, what do you think? What set point should I choose for my AC to save the most money when for several weeks, nobody is living in the condo, while not threatening the integrity of the condo? The fridge and freezer are, of course, turned off. Could I even turn off the AC too?
