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Author Topic: Is it better to turn the home air-conditioning off when I am at work?  (Read 19843 times)

lyner

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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.
 

Offline techmind

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...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.
« Last Edit: 17/08/2009 22:55:58 by techmind »
 

Offline techmind

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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)
« Last Edit: 18/08/2009 00:31:55 by techmind »
 

Offline Karsten

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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?
 

Online Bored chemist

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yes, that's right.
 

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