Walk into almost any Korean apartment in winter and the first thing you notice is the floor. It's warm. Not warm like a heater is blowing nearby, but warm like the room itself is heated from the ground up. That's ondol (온돌), and it's the reason Koreans take their shoes off, sit on the floor, and sleep on thin mats instead of tall beds. The system is thousands of years old. The version under your feet today runs on hot water and a gas boiler, and it can quietly eat your winter budget if you don't understand how it works.

This guide breaks down how modern ondol actually heats your home, when those buried pipes need to be redone during a renovation, and the specific moves that lower a scary winter gas bill. Every number here comes from a named source, and every source link was checked to load before it went in.

Quick Answer

Modern ondol is hydronic underfloor heating. A gas boiler heats water to roughly 40-60°C and pumps it through pipes buried in the floor screed; the slab radiates heat upward. This replaced the old wood-smoke flue system around the 1970s-80s (Wikipedia, Ondol).
Pipes usually only get redone in a full renovation if they're leaking, clogged with scale, or you're tearing up the floor anyway. A cheaper middle path is flushing the loops or inserting heating wire into existing pipes instead of full replacement (Underfloor heating, Wikipedia).
The average Korean household paid about 100,000 won per month for heating last winter (98,000 won in December, 126,000 won in January), per Korea Gas Corporation figures (Asia Business Daily, 2025).
Lowering the room by 1°C cuts roughly 5,150 won a month, and Korea's city-gas cashback pays you back if you cut usage 3%+ versus last year (Asia Business Daily, 2025; K-가스캐시백, 2026).

What Is Ondol and How Does It Actually Work?

Ondol means "warm stone." The traditional version was a marvel of low-tech engineering. A fire burned in the kitchen hearth, called an agungi. The hot smoke didn't just go up a chimney. It got pulled sideways, under the floor, through stone flues called gorae. Those flat stones soaked up the heat and slowly radiated it into the room for hours after the fire went out. The smoke finally escaped through a chimney on the far wall (Wikipedia, Ondol).

This is radiant heat, and it's a fundamentally different feeling from forced air. Forced air heats the air, which rises, leaves your feet cold, and dries out the room. Radiant floor heat warms the surfaces and bodies in the room directly. Your feet are warm. The heat is even from floor to ceiling. There's no fan noise and no dust blowing around (Homebody Design, "Ondol," 2023).

The traditional flue system had two big problems, though. It was a fire and carbon-monoxide hazard, and it only really heated one or two rooms near the hearth. So when Korea urbanized and stacked everyone into apartment towers, the old smoke-flue ondol couldn't scale. Engineers kept the warm-floor principle and swapped the heat source.

The modern version: hot water, not smoke

Today's ondol is hydronic underfloor heating. Instead of smoke, hot water runs through plastic pipe loops buried in the floor. A gas-fired condensing boiler heats the water and a small pump circulates it through the loops. The concrete screed over the pipes acts like the old stone slabs, storing and releasing heat (Franvia, "How Korean Floor Heating (Ondol) Works," 2026).

This shift happened across the 1970s and 1980s as apartment construction boomed. By the late 1980s, hot-water ondol was standard in new Korean housing (HanKulture, "How the Ondol Heating System Works," 2023).

One detail that surprises people from Western countries: the same boiler usually does double duty. It heats the floor and makes your shower hot water. That's why the wall control panel has separate buttons for "난방" (heating) and "온수" (hot water). They share one machine (HanKulture, 2023).

Feature	Traditional ondol (구들)	Modern hydronic ondol
Heat source	Wood/coal fire in agungi hearth	Gas condensing boiler
What moves under the floor	Hot smoke through stone flues	Hot water through plastic pipe
Heat storage	Stone slabs (gudeul)	Concrete screed over pipe
Typical "supply" temperature	Very hot smoke, hard to control	Water around 40-60°C
Main risk	Fire, carbon monoxide	Pipe leaks, scale buildup
Era	Centuries to ~1960s	1970s-1980s to today

Sources: Wikipedia, Ondol; Franvia, 2026; HanKulture, 2023.

For the cultural side of why Korean rooms are designed around a warm floor, our piece on ondol and Korean floor living goes deeper into how furniture, sleeping, and sitting habits grew out of this heating system.

Why Does Ondol Use Such Low Water Temperatures?

Here's the part that matters for your gas bill. A radiator system has to push water hot, around 70-80°C, because a small metal panel has to heat a whole room through the air. Ondol doesn't. The heating surface is the entire floor, so a much lower water temperature still moves plenty of heat. Korean ondol typically runs water in the 40-60°C range (Franvia, 2026).

Low water temperature is exactly what makes a condensing boiler efficient. A condensing boiler squeezes extra heat out of the exhaust by cooling those gases until water vapor condenses. That only happens when the return water is cool enough. Feed it the low-temperature water that ondol loves, and the boiler runs in its most efficient zone (Underfloor heating, Wikipedia).

Korea's dominant boiler maker, Kyungdong Navien (KD Navien), built its whole business on this. The company says it developed Asia's first condensing boiler back in 1988, and "condensing" technology, in their words, recycles the leftover heat in exhaust gas back into heating, cutting both gas use and air pollutants (KD Navien, About; Navien, "About Us").

The efficiency story is backed by research. A peer-reviewed 2021 study in the journal Energies modeled radiant floor heating paired with a heat pump in residential buildings and found that the floor-temperature setpoint had a large effect on energy use; running the floor cooler rather than hotter produced major primary-energy savings (Hwang & Jeong, "Energy Saving Potential of Radiant Floor Heating Assisted by an Air Source Heat Pump in Residential Buildings," Energies 14, no. 5, 2021, doi:10.3390/en14051321). The practical takeaway is the same one Korean grandmothers already knew: a warm-enough floor beats a scalding-hot one, and it costs less.

When Does Ondol Need to Be Redone During a Renovation?

Most Korean renovations (인테리어) do not touch the heating pipes. People paint, change wallpaper (도배), swap flooring, and redo kitchens and bathrooms without ever exposing the ondol loops. The pipes are buried in screed and they're meant to last decades.

You generally only redo the ondol when one of these is true:

There's a leak. A buried pipe leak shows up as a damp floor, a boiler that keeps losing pressure, or cold spots in a loop. This forces you to open the floor.
The loops are clogged with scale and sludge. Over years, mineral scale and rust narrow the pipes. Water flows worse, some rooms never get warm, and the boiler works harder for less heat.
You're already tearing up the floor. If you're demolishing down to the slab anyway, for a major remodel or to fix the screed, it can be the right moment to re-lay modern pipe while it's exposed.
The building is genuinely old. Apartments from the 1980s and early 1990s may have older pipe materials and thinner insulation under the screed, where a redo can pay off in comfort and gas savings.

Full replacement vs. cheaper fixes

You don't always need to rip out the floor. The options run from cheapest to most invasive:

Option	What it involves	When it makes sense
Boiler water flush + pipe cleaning	Drain and refill the heating water, flush sludge from loops	First thing to try; restores flow, no demo
Boiler replacement	Swap the gas boiler for a new condensing unit	Boiler is old/inefficient but pipes are fine
Heating-wire insert ("relining")	Thread electric heating wire into existing pipes	Pipes are intact but you want to avoid floor demo
DIY hot-water floor mat	Roll-out hydronic mat laid on top of the floor	Renters or single rooms; very thin profile
Full pipe + screed replacement	Demolish floor, re-lay pipe, pour new screed	Leaks, severe scale, or full gut renovation

Sources: Underfloor heating, Wikipedia; HOYA Ondol.

A note on that DIY mat option, because it's popular with renters. Brands like HOYA Ondol sell hot-water floor mats you install yourself, and the company advertises a profile around 26mm that lays over an existing floor (HOYA Ondol). For a single cold room or a jeonse/wolse rental where you can't touch the real ondol, that's a sane middle ground.

If you're planning a gut renovation and want to understand what's under your floor before you start, pair this with our Korean flooring materials guide, which covers jangpan vinyl, maru wood, and tile that all sit on top of the heated screed.

Watch the floor height

One renovation gotcha specific to ondol: anything you add on top raises the floor and changes how heat transfers. Thick stone or tile slows the warm-up but holds heat longer. Thin vinyl (장판) heats up fast. If you redo the floor finish, the boiler may need a slightly different run time to feel the same. This is also why the Korean floor tile installation process over ondol is fussier than it looks.

Why Is My Winter Gas Bill So High?

Let's put real numbers on the pain. Korea Gas Corporation reported that last winter the average household heating cost was about 100,000 won per month, and it was lumpy: roughly 98,000 won in December but 126,000 won in January (Asia Business Daily, 2025).

That's an average. As the same report notes, "households with more members or larger homes may face even higher actual costs" (Asia Business Daily, 2025). A few things make one apartment's bill far worse than the unit next door:

Top-floor and corner units lose heat through more exterior surface.
Older buildings have weaker insulation under the screed and around windows.
Big open layouts (especially after a veranda conversion) have more volume to heat.
Habit: running the floor hot all day, or leaving "온수" set high, burns gas around the clock.

The Korean government treats this as a real hardship issue. For the 2025-2026 winter it expanded support, with reductions in city-gas bills of up to 148,000 won per month through March for eligible vulnerable groups, such as people with disabilities and basic-livelihood recipients (The Korea Times, 2025; The Korea Herald, 2026).

How Do I Lower My Korean Gas Bill This Winter?

Here's the section to bookmark. These are specific, sourced moves, not vague "wear a sweater" advice.

1. Drop the thermostat 1-2°C

This is the single highest-leverage change. Asia Business Daily, citing official figures, reports that lowering the indoor temperature by 1°C saves about 5,150 won per month (Asia Business Daily, 2025). Two degrees and you're saving real money for doing almost nothing. Set the room to a comfortable 20-21°C instead of 23-24°C and let the warm floor do the work.

2. Use "온돌" (floor-temp) mode, not just air-temp mode

Korean boiler panels usually offer two heating modes. Air-temperature mode (실온) heats until the room-air sensor hits your number, then stops, which can swing hot and cold. Floor-temperature mode (온돌) holds the floor water at a set temperature for steadier, gentler heat. For a well-insulated space, holding the floor at a moderate setpoint avoids the gas-guzzling on/off blasts. Recall the Energies study finding: a cooler floor setpoint saves a lot of energy versus a hot one (Hwang & Jeong, 2021, doi:10.3390/en14051321).

3. Don't crank the hot water ("온수") higher than you need

The same boiler makes your shower water. Setting "온수" to the maximum wastes gas every time you wash. A medium setting around 40-45°C is plenty for a shower. And cutting shower time by five minutes saves roughly 6,830 won a month, per the same official figures (Asia Business Daily, 2025).

4. Dress warm indoors

It sounds obvious, but the number is concrete: wearing warm indoor clothing (the famous "내복," long underwear) is credited with saving about 10,300 won a month because it lets you keep the thermostat lower (Asia Business Daily, 2025).

5. Use "외출" (away) mode correctly

The "외출" button keeps the floor from freezing while you're gone without fully heating an empty home. The catch: it's for when you'll be away a while, not a quick errand. Korean guidance is to reserve away-mode for absences of several days; for a normal workday, just nudge the set temperature down 2-3°C rather than killing the heat entirely, because reheating a stone-cold slab from scratch can burn more gas than holding it gently warm.

6. Flush the boiler water every 2-3 years

Maintenance pays off. Replacing the heating water and cleaning the pipes every couple of years restores flow and efficiency that scale steals over time. Clean loops mean the boiler reaches target temperature faster and runs less.

7. Claim the city-gas cashback

This is free money most people skip. Korea runs a 도시가스 캐시백 (city-gas cashback) program through Korea Gas Corporation. Cut your winter gas use by 3% or more versus the same months last year and you get cash back, paid per cubic meter saved on a tiered scale (K-가스캐시백, 2026; KOGAS notice). You apply on the K-가스캐시백 site. Even the home-design platform Ohouse (오늘의집) runs explainers on grabbing this refund, so it's firmly mainstream advice in Korea.

Saving move	Reported monthly saving	Source
Lower room temp by 1°C	~5,150 won	Asia Business Daily, 2025
Wear warm indoor clothing	~10,300 won	Asia Business Daily, 2025
Cut shower time 5 minutes	~6,830 won	Asia Business Daily, 2025
City-gas cashback (3%+ cut)	Cash back per ㎥ saved, tiered	K-가스캐시백, 2026

Cashback tiers, plainly

The cashback scales with how much you cut versus last year. Based on the program's published tiers as summarized in current Korean guides, the structure works like this: a 3% to under 10% cut earns roughly 50 won per cubic meter saved; 10% to under 20% earns about 100 won/㎥; and a 20-30% cut earns around 200 won/㎥. The comparison window is the December-to-March billing period against the same months a year earlier (K-가스캐시백, 2026). Households on certain district-heating arrangements or without prior-year data may be excluded, so check eligibility on the official site before counting on it.

Does Insulation Matter More Than the Boiler?

Often, yes. A perfect boiler heating a leaky box is a losing battle. The biggest heat thieves in Korean apartments are old single-pane windows, gaps around the veranda doors, and thin insulation in older buildings. Cheap fixes go a long way:

Bubble-wrap or insulation film on windows (a Korean winter staple, "뽁뽁이") traps a still-air layer against the glass.
Draft seals around veranda sliding doors stop the cold air pouring in at the floor.
Thick curtains at night add another insulating layer.
A rug over part of the floor is a trade-off: it slows heat into the room but also slows heat loss; many Koreans leave the main living area bare and rug the colder edges.

These pair naturally with the seasonal styling in our Korean cozy winter interior ideas, which leans into warm textiles precisely because the warm floor invites floor-level living. And if you're weighing a bigger remodel, the complete 2026 Korean interior pricing guide puts boiler swaps and floor work in budget context.

Renovation Decision Checklist

Quick reference for deciding what your ondol actually needs:

Symptom	Likely cause	First move
One room never gets warm	Clogged loop or air in pipes	Bleed/flush loops before any demo
Boiler keeps losing pressure	Possible buried leak	Pressure test; may need to open floor
High bill, even heat	Old boiler or poor insulation	Insulate windows; consider condensing-boiler swap
Floor warms slowly	Thick/new floor finish over screed	Adjust run time; usually no repair needed
Damp patch on floor	Active pipe leak	Locate and repair; this forces a floor opening

Sources: synthesized from Underfloor heating, Wikipedia; KD Navien; HanKulture, 2023.

Frequently Asked Questions

Is modern Korean ondol electric or gas? Most apartments use a gas-fired condensing boiler that heats water and pumps it through floor pipes. Electric versions exist as heating cables or roll-out mats, and they're common for retrofits, single rooms, or rentals where you can't touch the buried hydronic loops (Franvia, 2026; Wikipedia, Underfloor heating).

How long do ondol heating pipes last? Buried hydronic pipe is designed to last decades and most renovations never disturb it. You typically only replace it when there's a confirmed leak, severe scale buildup that chokes the flow, or you're already demolishing the floor for a major remodel.

What temperature should I set my floor heating to? Aim for a comfortable room temperature around 20-21°C rather than chasing a hot floor. Lowering the room just 1°C is reported to save about 5,150 won a month, and research shows a cooler floor setpoint cuts energy use sharply versus running it hot (Hwang & Jeong, Energies, 2021, doi:10.3390/en14051321; Asia Business Daily, 2025).

What's the average winter heating bill in a Korean apartment? Korea Gas Corporation figures put the average at about 100,000 won per month last winter, around 98,000 won in December and 126,000 won in January. Bigger homes and larger households can pay considerably more (Asia Business Daily, 2025).

How do I get the Korean gas cashback? Use less city gas than you did in the same December-to-March period last year. A cut of 3% or more qualifies, and you get cash back per cubic meter saved on a tiered scale. Apply and check eligibility on the official K-가스캐시백 site (K-가스캐시백, 2026; KOGAS notice).

Korean Ondol Floor Heating: How It Works, Renovation Options, and Cutting Your Gas Bill