BTU requirements, ceiling height adjustment ?

[The Fat Controller]

Currently refurbishing our house, have seen all the calculations and advice regarding room sizes in square metres and most assume a ceiling height of 2.7m

Any advice or links to calculators that have adjustments for increased room volume due to high ceilings would be much appreciated.

[lopburi3]

I would not worry about that - windows/walls/sun/shade/insulation/drapes/glass/ceiling/floor would all likely be much more important and they can not be more than guessed at. Hot air will rise and not need to be cooled if ceiling it higher.

[RKASA]

Most of the hand outs I have seen in the stores provide cal. using total cube of room not just floor which is an avg rule. It does matter - the air moves in a room when the A/C is on air and the air that rises moves too.

http://www.calculator.net/btu-calculator.html

you can guess at your condition of insulation - mine is good as we put bats of 4in in the over head installed proper etc. and we have 3m walls which add a lot of space to a room.

Use what you think the avg room temp would be without A/C as that is the differential your working to over come. Thailand hot hot hot.

[IMHO]

AC's are cooling down the cubic volume of the air in the room, as well as dealing with heat sources in the room (people, appliances, sunlight, radiated heat etc).

Most Thai AC calculators will assume ~3M ceiling height. If your ceiling is 4.5M high, you're asking it to make a refrigerator out of a 50% larger box. That doesn't necessarily mean 50% more BTU is required though, because your heat sources aren't also increasing by 50%. I haven't seen any calculators made for Thailand (or ones that allow you to plug in Thai climate data) that will take in all of these variables though.

We have rooms with 5.5M high ceilings and another with a large dormer with a 4.5M high peak - on those we ended up adding 30% to the normal BTU calcs.

In the case of the dormer room, we learnt the hard way by ignoring the extra ceiling height / room volume, and had to replace an inadequately spec'd AC because of it. That one now cruises along on 40c days, so +30% was maybe overshooting a little - but the 5.5M high room with +30% only just gets the job done on the hottest days.

Just how high is the ceiling?

[The Fat Controller]

Thanks for all the comments so far.

We have a one storey house which has been completely gutted and we have yet to decide the height of the new ceilings.

At the moment I am staying in a room at our apartment building and know the problems with an underpowered a/c sitting on a hot wall trying to cool me down in 42C heat !

I got the latest brochure for a/c from Power Buy, all the units in there are quoted a suitable for room sizes in SQUARE metres, no mention of volume at all.

[wpcoe]

I thought the "dead space" above the air con wall unit wasn't much of a factor in calculating cooling performance of the unit. As long as you are not occupying the space above the wall unit, what difference does it make on the cooling performance of a unit which is to cool only the area below the wall unit? Cool air sinks, warmer air rises. Unless you have something like a ceiling fan which swirls the entire cubic volume into one big homogeneous mass temperature-wise, that is.

I recall reading on a manufacturer's site some place not to place the wall unit too high on a wall in order to not exceed the cooling capacity of the unit, as the higher you put it the bigger volume of air you are trying to keep cool. The implication to me was that it was okay (and recommended) to not be concerned with the uncooled air above the wall unit.

[IMHO]

I thought the "dead space" above the air con wall unit wasn't much of a factor in calculating cooling performance of the unit. As long as you are not occupying the space above the wall unit, what difference does it make on the cooling performance of a unit which is to cool only the area below the wall unit? Cool air sinks, warmer air rises. Unless you have something like a ceiling fan which swirls the entire cubic volume into one big homogeneous mass temperature-wise, that is.

I recall reading on a manufacturer's site some place not to place the wall unit too high on a wall in order to not exceed the cooling capacity of the unit, as the higher you put it the bigger volume of air you are trying to keep cool. The implication to me was that it was okay (and recommended) to not be concerned with the uncooled air above the wall unit.

The basic principle of what you're saying is true - as anyone with a multi storey house can attest However, most split system AC's take their inlet air from the top of the indoor unit, so will still draw in hot air from above if you've just mounted it lower on a wall.

[lopburi3]

Obviously when your ceilings are low you will not have much choice but if they are high it would be much better to keep the top as far from the ceiling as you can (and still provide room coverage from fan coil). That top few cm can be very hot indeed (especially if a roof above). And when that happens you may be freezing at room level with AC set for max 30 and still not turning off because it is sensing that extremely hot ceiling air. Yes modern units pull air from top - but how far from ceiling can still make a big difference.

[The Fat Controller]

Well, the bullet has been bitten, had the sales team around today from Home Pro and have opted for two Daikin Smart Inverter units.

With the hot season sale, a couple of discount vouchers (available to anyone) and hard haggling from my wife which resulted in a "store manager discount" too, we got a great deal.

Installation is free, all we have to pay extra is for the wall brackets for the outside units and cosmetic covers for the pipework if we choose to have that.

[The Fat Controller]

Just in case anyone was wondering, list price was 69,700 and we paid 58,000.

That was for two Daikin R32 Smart Inverter, one 18,000 and one 12,000 Btu

Edited April 20, 2016 by The Fat Controller

[Issangeorge]

http://www.calculator.net/btu-calculator.html?roomwidth=3&roomwidthunit=meters&roomlength=4&roomlengthunit=meters&ceilingheight=3&ceilingheightunit=meters&insulation=poor&temperature=20&temperatureunit=c&calctype=heat&x=73&y=17