A/C Sizing help needed.... Dazed and confused

[Baht Man]

Help me out here.
I'm currently building and called 2 local a/c folks out here.

Nimitz Air quoted a 36,000 btu cheap China model for about 50,000 Baht.
Standard Air quoted 2, 44,700 Mitubishi units at about 176,000 Baht, more than double to capacity of Nimitz.

I quized both of them and Nimitz said due to ceiling height and insulated walls and ceiling, a single unit was enough.
They then requoted using Carrier, Mit etc.

Standard said maybe I could use one large unit at about 42,000 and a smaller one at 35,000.

These are cassette, ceiling mounted units.

Why such a differece ????

The room is 75 sq.m (810 sf.) and most walls are double block walls with insulation between.I will insulate the ceiling as well.

A lot of the ceiling height is at 5.5m and I calculated total room cu.ft. at 19,400 and cu.meter at 550.

Any help would be appreciated.

[bluejets]

As a rough guide , we used to calculate at 40 BTU per sq ft floor area.

As you might imagine this is just a starting point.

Heat loads such as amount of glass window area, especially that facing West, type of structure and insulation properties of same, number of people normally using and coming and going. Larger than normal ceiling height say over 2.4m will have some bearing on the heat load as hot air is usually pocketed above door height in those areas. Also what is the temp difference at which you will run your system..?? Those trying to conserve energy will run at 25 degrees C in summer so it is clear that if outside temp is say 40 degrees, then heaps more energy required than a day at say 30 degrees.

Biggest problem one will have if you under-estimate requirement and the system will run continuously and do nothing for you.

There are many places on the web where you can look for insulating properties of different building materials and also BTU calculations.

Just as an example..... http://www.wikihow.com/Calculate-BTU-Per-Square-Foot

http://www.goodhousekeeping.com/home/ai ... calculator

http://www.airconexperts.co.za/btu_calc.html (this one seems to support my reasoning)

For building insulation............ http://en.wikipedia.org/wiki/Building_insulation

cheers..Jorgo

[Baht Man]

Thanks BJ.
Yea, I've been to a lot of sites to determine a/c size.
810 sf. x 40 =32,400.
Seems a bit low to me.
Again just a starting point, as you note.

Front of house faces east, has a lot of glass (green tint, single pane with low e and will get morning sun only +10%.
At the 5.5 meter height over the windows I was planning on using switched exhaust fans (think bathroom) to suck the heated air out for a short time.

Being an interior insulated room helps and even using most modifiers, size maxes out at around 54,000 btu's.

There are a number of houses beihg built near here that use 2 units in the large rooms. The owners I've talked to use them both to get the temps down then revert to using only 1 to maintain it.

Nimitz (50,000 btu quote) told me, well, if it's not big enough, you can always add another. That's not possible in my build.

Your thoughts on double units?

Thanks for the input.

[tutone]

Try cooloff.org on the internet and then click on their cooling calculator. It factors in a lot of different things such as ceiling height, windows, construction material, shading, etc. It's an American site so when it asks you to pick a geographical area in the U.S. just pick a hot place like Miami or one of the Southwestern cities. I used this when I bought my 3 a/c units and it has worked well for me. All of the shops where we looked at a/c units wanted to sell me bigger units, but I stuck with the information I had and they have been more than adequate. I also checked a couple of other sites just to see if there were any large discrepancies between recommendations.

[glalt]

I don't think any formula will accurately hit the mark. Keep in mind that if you have too much capacity you will have a damp clammy cold because the units will not run enough to effectively dehumidify the area. I'd put in one decent size unit, maybe 35,000 BTU and see how it does. You can always add another unit if one won't keep up to suit you. I wouldn't worry about your high ceiling because the cool air will stay down low.

[Baht Man]

It would be tough to add a second unit, after the build, as I'd prefer ceiling mounted cassette type and access would be almost impossible, maybe.
I'm aware of oversizing and humidity issues.
Thanks for the input.