[Don_1]

Do you have external shutters on your windows? These will help prevent the increase in temperature within your home during the day. As others have said, good insulation is the best option.

You can get a programmable timer which can be set for different on/off times on Mon - Fri and Sat/Sun with manual override and 1 hour boost.

[Karsten]

Now I am wondering though, if you use AC all the time, would it make sense to let the house warm up while you are not there and then have the AC work harder to dool it down when you arrive? Or is it better to keep the house at a cool level with the AC (not as cool as you want it when you are there) and turn it on higher when you arrive? I ws once told the opposite for heating systems: Don't turn the heat off while absent since the heating system has to work harder to heat up the house that has cooled down so much. It is better to leave the heat on a little bit even when you are not there. Does the same apply to AC? Or to neither?

[lyner]

Have you read the thread, this far, Karsten?
Heat loss / gain is proportional to the temperature difference. Once at ambient temperature, the house won't lose or gain any heat. There is a limit to how much the A/C or heater will have to do to get back to a temperature you want, this way. If you run the system constantly, you are using maximum energy  all the time. No comparison!

[Karsten]

Do you have external shutters on your windows? These will help prevent the increase in temperature within your home during the day. As others have said, good insulation is the best option.

You can get a programmable timer which can be set for different on/off times on Mon - Fri and Sat/Sun with manual override and 1 hour boost.

I own a mobile home that was built in 2000. Not huge amount on insulation but not bad either. The place does not increase in temperature dramatically until the sun creeps around to the side where the windows are. Then it makes a noticeable difference to keep the sun out. I am in the process of rebuilding the whole thing and all the mirrors I found are leaning against the windows on that side to help with keeping summer heat out. White curtains too. All windows are closed during the day. At night I open a few windows on the North side of the house and put a window fan in a window on the south side to push out warm air while sucking in cold night time air through the other open windows. I even seal the area around the window fan a bit to not suck in air through that window since it is still warm there. Works really well. The house is slightly warmer than the outside only in the late afternoon. With outdoor shutters it would be even better. Hard to find those though. I may have to build them.

I spend a few weeks in a simple house on a Greek island once. Two weeks of no cloud weather. No AC - always really comfortable. Thick stone walls, tiny windows, white paint, good roof overhang. No humid weather though.

Many architects and builders have come to rely on AC rather than basic construction principles to deal with heat or cold. Even termites "know" about passive solar principles and how to cool your home with the help of the sun. Too many of us just build whatever pleases the eye or satisfies status symbol needs and battle the temperature fluctuations with machines.

[Karsten]

Have you read the thread, this far, Karsten?
Heat loss / gain is proportional to the temperature difference. Once at ambient temperature, the house won't lose or gain any heat. There is a limit to how much the A/C or heater will have to do to get back to a temperature you want, this way. If you run the system constantly, you are using maximum energy  all the time. No comparison!

I have seen video tapes that had melted while being left in the car in the summer. I have read of readings of 100C or higher of dashboards. The ambient temperatures did not cause this.

In the summer a house can reach much higher temperatures than the ambient temperatures. Especially those that are not built smart. A house will warm up all day as long as the sun shines on it, or even worse in it. I would not be surprised to find that some homes can reach inside temps of 60C on 40C days. Does it still make sense to let it get this hot if you want the inside to be 25C during the evening/night time? 

[lyner]

You seem to have got the general principle, K. So what's the difference between over a day and over a week?
You even added a little spice concerning high daytime temperatures, which makes it an even stronger case in favour of not running the AC when you're not home at midday.

But, for houses with little or no insulation, they're pouring money down the drain, Summer or Winter.

[techmind]

...I would not be surprised to find that some homes can reach inside temps of 60C on 40C days. Does it still make sense to let it get this hot if you want the inside to be 25C during the evening/night time? 

While it'd be better for any houses which got that hot to take other measures (like shades / blinds / lighter coloured paints...) the principle still holds. That is, if the aircon is on all day, you are maintaining a greater temperature differential, so you will maximise the rate at which heat flows into the building ... all of which you're going to get rid of with the aircon. Without aircon, eventually the house will approach an equilibrium where it ceases to get any hotter, at this point the net energy (heat) flow into the building becomes zero. With the air-con on, you never reach that equilibrium. Since the job of the aircon is to pump out the heat which flows in, it makes sense not to do anything to increase the rate at which heat flows in - keeping the temperature low when it doesn't need to be is counterproductive.

[techmind]

It's probably worth clarifying some definitions - specifically the difference between 'heat' and temperature.

In physics, 'heat' is thermal energy. It can be measured in Joules, just like electrical or any other type of energy. Similarly the flow of heat can be measured in Joules per second (aka Watts) just like other forms of energy.

When you add 'heat' (thermal energy) to something it's temperature gets hotter. When you remove 'heat' (thermal energy) the something gets colder.


The amount by which the temperature changes when you add or remove heat depends on the thermal mass (strictly heat capacity) of the item you're heating/cooling. For example, the amount of heat energy to make a one penny (or one cent) coin red-hot would probably only change the temperature of a bucket of water (much greater heat capacity) by one or two degrees Celcius.

'Heat' (thermal energy) always flows from a hotter body or environment to a cooler one if they are in thermal contact. Heat flow ceases when the bodies are at the same temperature - ie thermal equilibrium is established.

A perfectly insulated body will retain the same temperature (be it hot or cold) forever. An imperfectly insulated body will gain or lose heat (thermal energy) from the surroundings at a rate proportional to the difference between the bodies' temperature and that of the surroundings.

If you forcibly keep adding heat to a perfectly-insulated body, its temperature will keep rising as long as you keep adding heat. Perhaps picture an electric fire inside a polystyrene box (do not try this at home!!!).


Aircon or a refridgerator is a machine which uses external energy to pump heat from one place to another (usually against its natural flow from hot to cold).


In a dynamic equilibrium where you're adding (or removing) heat from something which is poorly insulated, you will reach an equilibrium i.e. steady temperature when the rate of heat loss (or gain) through the poor insulation is equal to the rate at which you're adding (or removing) heat with your heater (or aircon).


In common experience especially where houses are concerned, the insulation is far from perfect, and we know that if you put your central heating on maximum (and overrode the thermostat) your house would not get hotter forever, but would eventually reach an equilibrium (maybe at 25C on a freezing cold winter's day, or 50C on a baking hot summer's day). The sustained difference in temperature between the inside of your house and the outside of the walls for a given heat input (boiler) or extraction (aircon) can be measured in degrees C per Watt - and this figure effectively measures the overall thermal insulation of the house.
In a small terraced house, a 10kW boiler might heat the house to 30 degrees above ambient when run flat out - this would indicate a thermal conductivity equivalent to 30 degrees per 10000Watts, ie 0.003 C/W. Turned upside down, as 333 W/C this tells you you require 333 watts of heating (cooling) to maintain the temperature ONE degree above (below) ambient. Which goes to explain why a one or 2 kW electric heater is utterly pathetic when the central heating has failed in a house in winter (it'll only raise the temperature by 3-6C)!

(Yes it's a bit more complicated if the temperature isn't uniform within the house, or the external temperature isn't uniform eg sun shining on one side - but this is the basic principle)

[Karsten]

I think I get it now. A higher temp difference results in more heat flow than just letting it react to the temps outside and cool it down once.

This means though that what I learned about heating a house (keeping the heat on a little bit to not let it cool down too much) is nonsense. Unless there is a danger that pipes will freeze it is more efficient to control the heat with a thermostat, let it cool down all day, and bring it back up to comfortable levels when/shortly before you get home.

Right?

[Bored chemist]

yes, that's right.