How Much Air Conditioning Do You Actually Need? The Ultimate Guide to Aircon BTUs and Heat Gain in Your Home

HDB Floormaps and their heat loads

Your body is a furnace right now.

who’s a lil lightbulb

In this moment, 30 trillion of your cells are metabolising, transforming nutrients into energy. This happens even when you sleep, and keeps us alive.

As your cells metabolise, they release heat!

more activity + bigger body = more metabolism = more heat

Fun Biology Fact But a Bit Too Much Information

This heat is an inevitable byproduct of the chemical breakdown of ATP (Adenosine Triphosphate), a compound that is your body’s ‘currency of energy’. It’s needed to drive your muscle contractions, move your cells, circulate blood and even get your nerve impulses going!

In the world of aircon, one of the popular ways to measure this heat is with the BTU, The British Thermal Unit.

yum, energy

An indoor aircon unit with 18000 BTU capacity is able to remove 18000 BTU of heat (and humidity).

How much heat needs removing from your home?

Let’s start with the internal heat sources. These typically mean energy-consuming things, like appliances, lights and people.

1: Identify the heat sources inside your home

The first major heat source in your home is you!

That’s right – when you sleep, your body emanates about 18 satays of heat energy!

And as you read this, little oranges of heat are radiating from your body. (The healthy choice)

This heat leaves your body by convection and conduction from your skin to the air, which is only possible when the air is colder than your skin. This is usually the case!

Here’s a screenshot of my skin temperature ranging between 33.4 and 33.8°C , taken with this sensor I have!

My core temp at the time was 38, which was more than my battery level!

During a heat wave when the surrounding air is hotter than your skin, the body actually gains heat from conduction and convection – here it can only cool itself down by the evaporation of sweat. Because humidity limits this evaporation, heat loss becomes super difficult! This is why hot, humid climates (like Singapore) are considered much less comfortable than hot climates which are dry. (Like UAE, or certain parts of Australia!)

Other than humans, electrical and cooking equipment are a source of heat gain too. Studies from ASHRAE have been done to determine the recommended heat gain for a variety of these!

The list is long.

But it’s much simpler to assume that 100% of your appliances eventually convert electricity into heat gain.

Fun Facts about Entropy – Why All Your Electricity Becomes Heat

You might think that the light energy, the sound, and the processing power that appliances produce refutes this assumption. But light bounces around your walls until its radiation is absorbed by all surfaces in a room (some escapes out the window). Sound reverberates through the air and surfaces of your house, agitating the molecules which exert friction on each other as heat! In computers, the computation performed in CPUs are simply resistance to the circuit. They perform no energy storage and do no work – simply producing heat as electricity cascades through the billions of transistors to perform instructions.

For the laptop I’m typing this post on, that means 65W!

That means my laptop is emanating about 221.8 BTU/hour, or two plain pratas of heat during the day. Yum.

What about your entire home?

Looking at EMA’s stats on Average Monthly Household Electricity Consumption, the overall average Singapore home consumes about 470kWh of electricity a month!

That comes down to a heat generation about 470000 ÷ 30 ÷ 24 = 652.7 Watts.

Which is 2227 BTU/hour!

Plan ahead!

Because the average kWh consumption of 470kWh is quite high for smaller flats, I stratified the data like this!

Almost there!

Equally important are the sources of heat outside your walls.

2: Identify the heat sources outside your home

Even a closed, empty home heats up! Before cooling you and your family, heat enters from the hot, humid outdoor air and solar radiation outside your walls.

This happens even if you had no windows!

The amount of heat is proportional to your flat’s outer surface area, which will increase depending on your floor area:

This heat travels by conduction through your window glass and the solid concrete of your walls – when one side has a higher temperature, heat will inevitably transfer to the other side!

This heat (Q) is proportional to the wall material, the surface area of the walls (A) , their wall thickness (L) and the temperature difference (T1-T2) between the inside and outside!

A simple way to calculate the total heat gain through your walls and windows is to use the BCA’s Residence Envelope Transmittance Value (RETV).

(Sidenote: RETVs can be reduced with better insulation and materials! This can be a point of pride for residential buildings. To be eligible for a Green Mark Platinum, you need an RETV of 20 W/sqm or lower.)

Let’s take the maximum RETV – 25 W/sqm.

This leads us to the final table of internal and external loads!

I’m assuming that the floor area of a condo is about the same as a 4-roomer, gasp! As we all know, they’re going down in size.

The story doesnt end here!

The heat your house expends today may change over time, which would be wise to consider.

3. Changes over Time

Consider the changes to the number (and size) of people in your home over time.

If you want to minimize the number of times you upgrade/change air conditioning units, it’s a good idea to size for the future. Most air conditioning units last 15-20 years!

Within this time horizon, how will the loads in your house change? Should you size for the largest heat load in the next 20 years? Or the load your aircon serves 80-90% of the time? It might be tempting to just buy the biggest air conditioner you can, but over-sizing your unit can be tremendously wasteful depending on the part-load efficiency.

We’ll cover that… next semester!

I hope this post helped you understand what BTUs are and how the inside and outside of your house matters when you are looking for the right size of air conditioning!


Cool Tools Review: The Embr Wave 1, GreenTEG CORE and FOCI Wearable!

Aaaand here’s the short version!

We could end the post here, but cmon now.

So I really like strange wearables. Especially if they’re related to temperature. Or perceptions of temperature!

I want air-conditioning systems to work less for me. For this, I need to want less, which is difficult!

Tracking my data is part of a path to find the minimum conditions I need to be in comfort.

That aside, let’s get to the products!

(Fun fact: Humans have been getting cooler, one of the reasons being less chronic inflammatory conditions over the past bunch of centuries. Woohoo!)

As these products are not your usual smartphone or watch, I’ll be briefly going through their underlying technology first before going through the experience of using it. Feel free to skip those bits if you wish!

The Embr Wave

excuse the hair

With Singapore’s intense heat and furious use of aircon, the idea of bringing cooling direct to our bodies was something worth a ($300+) try to me. Exciting new powers of personalized air-conditioning would make sweatingonthewaytowork a figment of the past. I would be so cool. Ha.

So the Wave doesn’t really do that!

How it Works:

The Wave contains a peltier plate, which becomes cool on one side and hot on the other side when you pass an electric current through it.

One side of the square goes cold, the other side gets hot.

This is called thermoelectric cooling – it’s a super-cool effect which we use for regulating the temperature of scientific devices and other tiny electronics.

Scientific Devices
Source: grynx

To reject the heat on the other side, peltier plates are often combined with heat sinks (Metal blocks with a lot of ridged parts on them for more surface area) to dissipate this heat away. This explains the wiggly lines on the Embr Wave’s surface! The higher surface area is helpful to more efficiently dissipate heat when being used in Cooling mode.

From the manual

The Experience

Does it actually cool you?

Sssssorta. It’s the kind of cooling you would expect on a hot day when you dip your hands into cold water. Or suddenly touch a cold, metal doorknob.

On heating mode, it’s like when you’re in a cold room and cup your hands around a warm mug of coffee.

This pulse of cooling or heating is applied to the underside of your wrist, which has a solid amount of bloodflow and is very thermally sensitive.

bwaaaang. (Unfortunately it doesn’t turn your skin blue, that is an edit)

The pulse of cold doesn’t remain constant – it comes in waves! According to Embr, this maximizes the feeling you get from each pulse, which I can verify. Having a constant source of cold on your skin tends to be forgotten – receiving waves makes them feel ‘new’ every time.

The Embr Wave comes with a pretty great app, which is where you can set these custom modes for yourself!

Overall, the psychological effect of cold hits slowly over time – I ‘feel’ it more keenly after an extended session with the Wave (like 30-60mins). It’s not exactly the aircon-everywhere I was hoping for, but could be valuable to someone looking for that last squeeze of comfort in a space that is always slightly too hot or cold.


The design is a little blocky for laptop use, as it sticks out the bottom of your wrist! This results in occassional collisions which is not so nice. Also, the Wave 1 is charged by micro-USB (the horror!), but Embr is nice enough to include a cable in the box.

Nonetheless, Embr Labs just recently released an Embr Wave 2 in April, with a sleeker design and, according to them, stronger cooling.

I don’t have one! Sadz

The GreenTEG CORE sensor

Strap me to your chest, Jack

The CORE is super cool!

So I was walking to the mall in the BLAZING hot sun one day (normal in Singapore these days, the weather’s been nuts!) and wondered the following:

Would I sweat sooner if I walked slowly in this blazing heat, thereby spending a longer time under the skin-melting gaze of Sauron, on my way to get some KFC?

Or would I sweat sooner if I ran to my dearsweet KFC, thereby spending a shorter duration under the sun, but spending more metabolic energy to do so?

I needed some way to track my core temperature and skin temperature in real time, and graph this against my activity level. It would not be feasible to walk with a thermometer in my mouth. I didn’t have a thermometer pill.

Before investing the remainder of my life into starting a company to research and make such a method + app to track data of such method in realtime, luckily I found out about the GreenTEG CORE!

So you strap this little thing onto your chest, and it senses your skin temperature and their proprietary heat flux sensor to back-calculate your body’s core temperature to a ±0.26°C accuracy. Then it streams this real-time to an APP, which you can LOOK AT in REAL TIME. It was exactly what I wanted!

The little clips fasten it to your chest strap!

How it Works

GreenTEG uses a combination of skin temperature, heartbeat and their own research-based algorithms to generate a core temperature that hasgiven super close results to pill thermometers. The CORE is enabled with two algorithms – one incorporating heartrate (Sports Activity Algorithm) and one without (Everyday Living Algorithm). As changes in core temperature do not immediately reach the skin, the Sports Activity Algorithm takes in heartbeat data to come up with a more accurate and immediate figure for core temperature, needed by athletes who require immediate feedback for their sport!

Source: GreenTEG

So I got the heartrate sensor too!

I line em up like this. My girlfriend calls it my bra. That’s the FOCI on the right!

For those who find this uncomfortable, the CORE comes with one-day-use adhesive pads too!

As the CORE is relatively simple in function (clip+strap), it’s really all about the app!

While the CORE app doesn’t allow a direct CSV download (yet?) unless you pay a fee for a Research Mode or a Garmin device, you’ll be able to track and set alerts when your core temperature goes off the charts. Here’s me at high activity:

And another steep ramp-up:

And here’s the effect of a sudden heartrate drop:

As you can see, the body takes a while to return back to the normal core temperature of about 37°C!

While I love the functionality and will be saving the detailed analysis for another post, I do wish I could see both graphs on the same creen to observe the timings! Another gripe would be that the app sometimes resets the tracking when the bluetooth connection is lost, but GreenTEG continues to push over-the-air updates to improve this over time (this is why you get the app-enabled tech)

The FOCI Wearable

It’s a little clippy thing that goes on your waist, pushed against your diaphragm to track your breathing patterns! The FOCI was a strange little find on kickstarter, when I was looking for something nondescript to track my breathing patterns over the course of a day!

How does it work?

The FOCI tracks tiny breathing movements and translates this into a predicted cognitive state with the ‘neuro-respiratory patterns’ identified by their AI model. These states are broken down into Focused, Calm, Stressed, Fatigued, Distracted and Flow.

Each bubble is a mental state

If you click them, it displays the duration

The FOCI is 10g, super lightweight and easy to clip on without noticing, and easily hideable. However, as it can’t get a clear signal while you move around, the only accurate readings come from when you are seated.

Another micro-USB, folks!

While my personal use with the FOCI has been quite brief, so far I’ve found that it quite accurately tracks whether I am in focus or not! While the little bubble moves with your breath, making it fun to look at and track your breathing state, I’ve also found that the best way to maintain focus is to not look at it while you’re tracking yourself.

Often I go from this…
… to this…
to this.

Overall, while I am interested to see where TinyLogics takes the app and device (graph my breathing please!)

That’s all for the Embr Wave, the CORE and the FOCI! You can comment or subscribe if you like this, or let me know if you’d like to know more about these cool tools!

I’ll do another big review when I gather more interesting tech! (Reon Pocket, I’m looking at you.)


The Heart of Air-Con: The Artist’s Guide to Water-cooled Chiller Plants in Singapore

For the engineers and techs in the energy & sustainability sector, I am here for you.

This is your bread and butter!

And I hope to draw it for you with uncommon beauty.

What is a Chiller Plant?

So.. air conditioning takes up about 50% of energy consumption in our commercial buildings.

The cold is costly!

The energy which powers Air Conditioning simply moves machines that throw human heat outside of a building.


This sounds simple, but actually involves a long chain of machines, passing the heat to each other.


This magical process works mainly because of one principle:

Hot things transfer heat to cold things.

In a building, this happens a few times:

  1. Your muscles and organs (37°C) heat your skin (36°C) in your body.
  2. Your skin (36°C) heats the supply air (18°C) from your air-conditioning, making return air (28°C).
  3. The return air (28°C) heats a chilled water supply (7°C) in the aircon unit to a chilled water return (12°C) .
  4. The chilled water (12°C) heats a very cold refrigerant (5°C) in the chiller’s evaporator, to 6°C.
  5. The refrigerant is compressed (6°C -> 50°C) into a high pressure vapor in the chiller’s compressor.
  6. The hot refrigerant (50°C) heats condenser water (30°C) in the chiller’s condenser to 35°C.
  7. The condenser water (35°C) heats the ambient air (29°C) in the cooling tower…
  8. … which is where all our heat goes! The ambient. The sky. The outside.

Is this bad for the environment?

Technically, without air conditioning, we would walk around heating the air anyway.

What these machines do is essentially increasing the gradient between hot and cold.

That’s no problem, right?

The thing is, not all machines are efficient – they produce their own heat while working.

Due to inefficiency!

And the energy they consume from power plants.. requires more heat to generate too! Ideally, 100% of the electricity we supply to machines is converted into the useful work we use the machines for. However, some of this heat is inevitably converted to heat and sound produced by friction between mechanical parts and electrical resistance.

The more machinery we run to operate an air-conditioning system, the more heat is generated in the world. Sounds kinda funny, but we don’t really feel the effects of this when we’re chilling indoors!

On to the machines themselves!

1. The Air Cycle (Air Handling Units)

  • Humans do stuff. Their muscles and organs get warm, and this heat is transferred to their skin, which is transferred to the air around their skin.
  • This hot membrane of air transfers heat all over the air! Hotter air particles have more kinetic energy, and jiggle faster than their cold neighbouring particles. These hotter particles rise above their inactive neighbours who remain relatively still (AKA Meritocracy). At the ceiling of a room, a fan sucks in these hotties, blowing them past a very cold chilled water coil inside an Air Handling Unit (AHU). This coil’s water is at about 7°C. That’s cold!
  • The hot jiggly particles collide against this coil, losing their jiggliness to the coil surface. The cold water inside the coil takes gets heated from 7°C to 12°C. Meanwhile, the air is cooled from ~28°C to about 18°C. The resulting average room temperature is somewhere around 24°C, which is the temperature your thermostat tries to maintain.
  • The cycle repeats. Cold air hits human, human heats air, hot air heats water, hot air becomes cold. Ugga bugga.

2. The Water Loop

  • Chilled water just took some heat, and needs to drop it quickly. It flows through the Chilled Water Return pipes to the chiller plant, where it will be cooled back to 7°C again, by the chiller.
  • Chillers have a simple function – Make chilled water cold again.
  • They do this by passing the 12°C water through super-cold refrigerant fluid, which is about 5°C in the evaporator.
  • After the 12°C water transfers its heat to the refrigerant, it leaves the chiller at 7°C, on its merry way back to the AHUs.

3. The Refrigerant Loop

  • Air passed heat to water. Water passed heat to refrigerant. Refrigerants are fluids with ideal properties for containing and releasing energy rapidly, but I’ll be skipping those details for now (low boiling temperature! Rapid phase-change material!),but in summary is a fluid that can be very cold at room temperature. This enables it to be the ‘colder thing’ which the already-very-cold 12°C water can give its heat to.
  • Once the refrigerant takes this heat, it is compressed into a hot, high-pressure vapor, which enables it to now be the ‘hot thing’ to transfer heat to another water loop, the condensor water loop.

The whole operation of a chiller looks like this:

Which I hope makes sense of this!

4. The Condenser Water Loop – Throwing it all out

  • In the chiller’s condenser, hot refrigerant now passes its heat to water (Yes, a separate circuit). This water goes to a cooling tower, those huge grilled-boxes you see on top of many buildings in Singapore.
cooling tower vs. chiller – cooling towers on roof
  • Cooling towers do exactly what you do to cool soup – blow air over it. The hot water is trickled down the serrated walls of metal, and air is sucked through these walls by a huge fan in the centre of the box’s ceiling.
  • The water that collects at the bottom is at about outdoor temperature, where it is pumped back to the chiller to be heated again.
  • This stage is called Heat Rejection, the final stage of any air conditioning cycle. It is where the heat of hundreds of warm humans is amalgamated and tossed back out into the air, where it is more easily forgotten.

Is this all necessary?

We’ve all suffered the heat here – air-con definitely makes it feel better. While I don’t have air-conditioning at home, I seem to survive quite well with fans. Even a cool breeze outdoors on a sunny day makes me feel comfortable enough to forget the humidity.

I’m not so sure what the best way is – what do you think?

Is it too much for some comfort?