Will be gone for 3 wks... leave A/C on?

[ewankho]

Maybe you could try those automatic timers that will turn on and shut off your appliances on certain times of the day. I saw some in Japan Homes but haven't tried them.

[chowfun]

http://irc.nrc-cnrc.gc.ca/pubs/rr/rr191/rr191.pdf

[chowfun]

That's complete bollocks, as anyone with a rudimentary education in physics could tell you.

You have me convinced...Not!

[timklip]

cue the light, airy elevator music while PDLM frantically scours the net for information...ANY information... to refute chow fun's claim....

Who was it that said

"better to remain silent and be thought a fool, than to open your mouth and remove all doubt"?

ohhhh yeahhh.... it was PDLM's high school Rudimentary Physics 101 teacher...

[PDLM]

I can't really see how the study quoted by chowfun is at all applicable to Hong Kong.

It is a study on a "basement + 2 floors" house built with insulation designed to withstand a Canadian winter and with a combined heating and cooling system. The "summer" study with the "single temperature" setting is in an environment where the maximum daytime temperature on any of the 14 days was 28.7 degrees degrees and the night time temperature was as low as 8.9 degrees. So the system was both cooling and heating at different points in the 24-hour cycles (in fact it was heating for about twice as long as it was cooling on most "summer" days as far as I can tell from the data).

The whole thing is just so utterly unlike the HK environment that it is not worth any further discussion.

[DavidHK]

wading into this issue unprepared I recall my technical lectures from uni on thermal mass. Low thermal mass has a faster response time for temp change than a high thermal mass building. Take a lightweight timber frame building vs a stone castle. The lightweight structure will take less time to heat when its cold than the castle as the castles thermal mass will 'store' the cold and radiate cool air.

Now obviously this needs a better description but I hope you have got the concept.

Google the BRE HQ in the UK and look at how the exposed concrete ceiling works (how it should work as from memory the measured results are not as good as the simulations suggest). The windows are opened at night to allow the cold air to cool the concrete sofits. The concrete then radiates cold air during the day (that only works when there is difference between day and night temps).

However I doubt very much that an uninsulated thin concrete walled HK apartment has enough mass to have a noticeable effect (the soft furnishings will have no effect at all). The knock out blow is that the A/C will use lot more energy staying on during the day than any minor radiant gains (or losses if I take the way chowfuns post is worded). There is also a lot of energy loss in the storage process so using power to cool air to then cool concrete really sucks as an idea. So while PDLM's answer was a little short he's right that its bollocks. Put the A/C on when you want it. If your A/C has a dehumidify function then by all means use that while you are away (if you can get to/see the condenser / chiller unit in your apartment you can see it/hear it turning off and on. On dehumidify mode it doesn’t need to run very often compared to a cooling mode so the energy use is a lot lower). My own not very scientific experiment showed that a stand-alone dehumidifier was cheaper to run than an A/C split unit in dehumidify mode but it was a close run thing. I should add that A/C units differ a lot in terms of efficiency so my experiment is only really relevant to the models of units and my apartment size.

Apologies for the over long post and glancing relevance.

Edit: This was written while PDLM wrote his response. Once again I'll concur with the assessment - these things only work in temparate climates. You cannot take environmental design practices from Europe or The US and expect them to work here. They just don't.

[chowfun]

I can't really see how the study quoted by chowfun is at all applicable to Hong Kong.

It is a study on a "basement + 2 floors" house built with insulation designed to withstand a Canadian winter and with a combined heating and cooling system. The "summer" study with the "single temperature" setting is in an environment where the maximum daytime temperature on any of the 14 days was 28.7 degrees degrees and the night time temperature was as low as 8.9 degrees. So the system was both cooling and heating at different points in the 24-hour cycles (in fact it was heating for about twice as long as it was cooling on most "summer" days as far as I can tell from the data).

The whole thing is just so utterly unlike the HK environment that it is not worth any further discussion.

The link I provided was a study on heating and cooling and it touched how surprisingly little money or energy you save by turning the AC off. It definitively shows that maintaining a cool temperature uses up less energy than turning off and on the AC and wait for it to cool things from the hot natural temperature.

It sounds like you've made up your mind already. I provided scientific data. You provided your personal opinion. Any conclusion anyone reaches from these posts are strictly up to them.

[MovingIn07]

Chowfun - PDLM was rude but correct. DavidHK was polite and also correct. One might think that a strategic withdrawal would be in order about now.

[hktraveller]

Chowfun - PDLM was rude but correct. DavidHK was polite and also correct. One might think that a strategic withdrawal would be in order about now.

The highest savings occur when you keep the temperature as high as possible and get down the humidity.Then as opposed to temperature, low humidity spreads allover the flat more easily. I run one single aircon (2HP) at 27degrees plus one high capacity dehumidifier
in a 1800 sqft. flat with all doors open and it is comfortable in all the rooms. 27 degrees is not much different to the outside temperature, so that bad insulation is no issue. At 50% humidity I even sometime feel cold if temperature gets below 27 degrees. MY electricity bill never peaks over 900 per month, and I have a computer also running 24/7. Average about 600-700.

[PDLM]

It sounds like you've made up your mind already. I provided scientific data. You provided your personal opinion.

You provided scientific data applicable to a large house in Canada, built to full Canadian winter insulation standards using a centralised heating/cooling system with a single engine. As a simple example of how inappropriate to the HK situation this is, one of the major findings was that if you switched the cooling system off for 7 hours during the day then it took 7 hours for the room temperature to come back down after it was switched on again. Even when I lived in a rooftop apartment in the middle of a HK summer I have never had a situation in HK where the air-con didn't get the temperature down within 30 minutes.

As DavidHK stated, the thermal mass of the building is a key issue. The thermal mass of a large winter-insulated test house in Canada is hugely greater than that of a thinwalled HK apartment. Moreover the heating/cooling system in Canada is completely different. And the savings stated related to periods where for two thirds of the time the system was heating not cooling.

I'm not disputing that the paper you referenced is accurate scientific data. I'm simply saying that it is not at all relevant to the question of cooling and dehumidifying HK apartments.