Air Conditioner Buying Guide: Inverter, Size and Stars

Our air conditioner buying guide starts with one rule: pick the size for your room first, then choose inverter or fixed speed, then compare energy stars. Most buyers do it in the reverse order and end up with a unit that is too big or too costly to run.

1. Match the size to the room

Air conditioners in Vietnam are sold by horsepower, or HP. One HP is commonly sold as about 9,000 BTU per hour. BTU per hour is the cooling the unit delivers. A bigger number means more cooling.

A common seller rule of thumb is about 600 BTU per hour for each square metre of floor. By that rule, a 15 square metre bedroom needs about 1 HP, and a 25 square metre living room needs about 1.5 HP. Treat these as a starting point only.

Add capacity if the room faces the afternoon sun, sits under a roof, has large windows, or holds many people or a kitchen. Ask the seller to show the sum they used for your room.

Do not buy far too big. An oversized unit cools the room fast and then switches off, which can leave the air damp and the room uneven. A unit that is too small runs all day and never reaches the set temperature.

Sources: Wikipedia, "Air conditioning" (https://en.wikipedia.org/wiki/Air_conditioning), on the ton of cooling and BTU per hour. The 9,000 BTU per HP figure and the 600 BTU per square metre rule are common seller rules of thumb, not an official standard.

2. Inverter or fixed speed

A fixed speed unit has a compressor that is either fully on or fully off. It cools hard, stops, then starts again when the room warms up.

An inverter unit changes the compressor speed. It runs fast at first to cool the room, then slows down and keeps the temperature steady. Because it does not keep stopping and starting, it usually uses less power and makes less noise.

Inverter models cost more at the shop. The extra price pays back faster the more hours you run the unit. If you cool a bedroom all night, an inverter is a sound choice. If you use a spare room for a few hours a week, a fixed speed unit may be enough.

Sources: Wikipedia, "Air conditioner inverter" (https://en.wikipedia.org/wiki/Air_conditioner_inverter), on how an inverter controls compressor speed compared with a compressor that is either on or off.

3. Read the energy stars

Vietnam uses an energy label with one to five stars, set under the Ministry of Industry and Trade. More stars means the unit uses less electricity for the same cooling. A five star unit costs more to buy but less to run.

Compare stars only between units of similar size. A small five star unit still uses less power in total than a large three star unit, so size comes first. Also look at the power draw in watts on the label, and not only the star count.

A simple check: take the watts on the label, multiply by the hours you will run the unit each month, divide by 1,000, and multiply by your tariff per kWh. This gives a rough monthly cost for each model you are comparing.

Sources: the one to five star energy label is set in Circular 07/2012/TT-BCT of the Ministry of Industry and Trade, dated 4 April 2012 (https://english.luatvietnam.vn/circular-no-07-2012-tt-bct-dated-april-04-2012-of-the-ministry-of-industry-and-trade-defining-the-energy-labeling-for-means-and-equipment-using-ener-69613-doc1.html). The efficiency idea (cooling output divided by power input) is explained at Wikipedia, "Seasonal energy efficiency ratio" (https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio).

4. Check the features that matter

Look at the copper coil, the warranty on the compressor, and the noise level of the indoor unit in decibels. A longer compressor warranty is a good sign. Check that the remote has a sleep or timer mode, since it cuts night time running.

Ask where the outdoor unit will go and how long the pipe run will be. Long pipes and tight spots lower performance and raise the install bill.

5. Plan the install and the upkeep

A good unit fitted badly will cool badly. Ask who installs it, what is included in the price, and whether pipe, cable and the wall bracket are extra. Keep the receipt and the warranty card together.

Clean the indoor filter every two to four weeks when you use the unit daily. A clogged filter blocks air and makes the unit work harder. Have the full system serviced once a year.

Quick checklist

• Measure the room in square metres and note sun, roof and people.
• Pick the HP, then decide on inverter or fixed speed by how many hours you will run it.
• Compare energy stars and watts between units of the same size.
• Get the install, pipe and warranty terms in writing.

Sources: Wikipedia, "Air conditioning" (https://en.wikipedia.org/wiki/Air_conditioning); Wikipedia, "Air conditioner inverter" (https://en.wikipedia.org/wiki/Air_conditioner_inverter); Wikipedia, "Seasonal energy efficiency ratio" (https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio).