Definition
On a Korean home boiler, away mode is not a way to keep the house warm while you're out. It's a freeze-protection feature that sends just enough heat to keep the water in the pipes from freezing and bursting. The boiler stops its regular room heating and sits in a standby state that only protects the boiler and its pipes.
Away mode is a survival feature, not a heating feature
When you leave, it's easy to think that pressing the away button on the boiler controller will keep the house nicely warm. But away mode does not hold a comfortable temperature for people at all.
The details vary by manufacturer, but most away modes only fire up the boiler when the room temperature drops to roughly 8 to 10 degrees Celsius (about 46 to 50°F) or below, or when the water in the pipes cools to around 5 degrees Celsius (41°F). In other words, it isn't a mode for saving on heating bills while keeping the house livable. It's the bare-minimum defense against frozen, burst pipes.
That's why, if you leave away mode on all day in winter, you come home to a floor that has lost all its warmth and feels like ice.
Why using it for a short trip can backfire
Switching to away mode when you leave for work and turning the heat back up when you get home doesn't save as much as you'd expect. Sometimes it even burns more gas. Bringing the chilled concrete floor and the cold water in the pipes back up to 22 or 23 degrees Celsius (about 72°F) takes a huge burst of gas all at once.
That said, a cold house also loses less heat to the outside. So how much you actually lose depends on how long you're out, how well the house is insulated, and how efficient the boiler is. That's why many guides say that for short trips, it's better to turn the temperature down a little than to use away mode.
So for a short outing within a day, like commuting or a grocery run, a common recommendation is to skip away mode and set the thermostat 2 to 3 degrees lower than your usual setting.
To be more precise: the right way to leave, by boiler type
What you should do when you leave also depends on the type of boiler and how well the home is insulated. In an apartment on district heating (hot water supplied from a power plant), the floor takes a long time to warm up again once it cools. So lowering the usual temperature by just 1 to 2 degrees works better than away mode.
On the other hand, if you have your own gas boiler and you'll be gone for several days or more, such as over a holiday like Lunar New Year or on a trip, away mode is the right choice. But during a severe cold snap of minus 10 degrees Celsius (14°F) or lower, away mode alone may not be enough to stop the exposed pipes on the balcony from freezing.
For a long absence in that kind of extreme cold, instead of away mode, the trick to protecting the pipes throughout the house is to set the target temperature to about 15 to 17 degrees Celsius (59 to 63°F) or turn on the boiler's scheduled operation (for example, running 20 minutes every 3 to 4 hours).
Common misconceptions
Away mode is a power-saving mode, so using it every time you leave always saves on heating bills.
Away mode is a freeze-protection feature that only kicks in once the room temperature falls to 5 to 10 degrees Celsius or below. If you use it for just a few hours, the room gets cold, and reheating it after you get home takes a lot of gas at once. The savings are small or nonexistent, and you may even lose money.
Where you meet it
When leaving for work, you don't turn the boiler off or use away mode. You just lower the usual 22 degrees Celsius setting by 2 degrees to 20 and head out.
When you're away for 3 days over the Lunar New Year holiday, you set the room temperature to 15 degrees Celsius or turn on scheduled mode to prevent freezing in extreme cold.
In one sentence
Away mode is a freeze-protection feature, not heating. A common recommendation is to lower the temperature 2 to 3 degrees for short outings and save away mode for trips of several days or more.