Hot nights and BTUs

Edit: So that I don’t breach any copyright laws, I’ve removed the SPH article.  You can read the original article HERE.

Wonder how the auntie in the pic feels about appearing on front page news?  Thought to highlight this article for any reader who is still hesitant about installing air-conditioning during their renovation period, or deciding to omit some rooms for air-conditioning (aircon or AC).

Sure, you probably won’t be using air-conditioning most of the time/year.  But in sweltering months like June, I’m sure you’ll appreciate having cool air. Even in my place where I literally shiver under blankets in December, the unbearable heat is making us switch on the AC a whole lot more often than usual – hot nights indeed…

Which brings us to BTUs.  I only learnt about BTUs during the reno, but it’s a really important thing to bear in mind for any AC installation.  Basically AC cooling capacity is measured in British Thermal Units (BTUs) per hour.  You should try to get the right BTU blower for each room, else you risk not cooling your room effectively if the unit is too small. An oversized unit will of course waste energy.

If you see most AC ads in Singapore, the typical configuration offered for a System 3 could be 3 x 9,000 BTU paired with a 27K BTU compressor.  So is that sufficient for most homes?  Probably if you only install AC in bedrooms, assuming that each bedroom is fairly compact of course. But for the living/dining rooms, I highly doubt that a 9K BTU blower unit can cool the space properly/fast.

Taking my place as an example, let’s have a look at the master bedroom BTU requirement.

For a quick estimation, we can use the rule of thumb given by some AC specialists in Singapore:

  • Area (Length x Width) X 65 = BTU/hr   E.g. Room size 10ft X 10ft = 100ft2 X 65 = 6,500BTU/hr.

Taking my MBR that measures 4 x 4.4m, or approximately 14 x 14.5 feet, 14 x 14.4 feet x 65 = 13,104 BTU/hr.  In reality, we are using a 12K BTU unit which works fine, probably cos I could have eliminated the space taken by wardrobe [1 m = approx 3.28 ft]  I am not sure why a multiple of ’65’ too. Why not 60? Or any number between 20 to 50?  Is the number higher because of the higher humidity that we experience? (AC also remove humidity from the air, otherwise you’ll feel cold and clammy.)  Tried to Google for a while but can’t find anything.

A more accurate way to measure the BTU requirement is to also consider the ceiling height (especially if you have high ceilings), and temperature difference between the desired indoor temperature and outdoor temperature.  You may also want to boost BTU capacity if you have a east/west facing room, stay on the highest floor (with poor heat insulation from the ceiling), is using AC in the kitchen, have heat generating appliances (e.g. plasma TV), expect many people (who generate body heat!) to be in the room, etc.  There are some BTU calculators out there, but they seem to be tailored more for temperate climates and not the hot and humid tropical climate we face in Singapore.

Apart from BTU and brand of AC, other things you should check are:

  • AC compressor capacity (no point getting high BTU blowers if your compressor cannot support all the blower units)
  • R410A vs R22 refrigerant – I suggest R410A for being more environmentally friendly and future-proof.  R410A requires higher operating pressure by the way, so older pipes for R22 units may not be able to support R410A units.
  • Type of insulation used
  • Inverter vs non-inverter – again, inverter is more environmentally friendly.

For us, we chose to go to an AC specialist instead of from an electronics salesman for peace of mind about the BTU requirements. Placements of the blower units, pipping and drainage were well thought out by my IDs, contractor and AC specialist.  If you choose to DIY, you may also want to consider the aesthetics of visible vs hidden blower units (not to mention cost of carpentry for hidden units), visible trunkings and drainage pipes (so ugly!) and of course blower placement/location.

Source of news extract: The Straits Times, 8 June 2012

3 thoughts on “Hot nights and BTUs

  1. I am staying in a EM in woodlands. For effective cooling of all the 3 rooms on the 2nd floor and the study room in the 1st floor, is it advisable to go for only one compressor. What is your tick???

    • @Wei Choong: Simple math tells me no, though I suspect some unethical salesmen may say it’s ok. Just not enough power for all the rooms to be cool at the same time, but perhaps ok if there is no intent to have all the blowers switched on at the same time.

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