There was only one person teaching, and by the end of the day Yun Xi’s throat was practically on fire, but when she saw those pairs of pure, earnest eyes, she felt that everything was worth it.
A few days later at noon, Yun Xi returned from the schoolhouse to find Yun Dahai already waiting for her at home. “Come, come, take a break, Xi’er. How have things been at the school these past few days?”
Yun Xi glanced at her overly enthusiastic father, wondering if he needed something from her. “Not bad, Father. Quite good, actually. The children in the village are all very obedient and they study very earnestly.”
“That’s good. Xi’er, that thing you mentioned last time… um… what was it called again? Oh, I remember now—earth dragon! Yes, that earth dragon thing. What’s it about? Can it really provide warmth?”
So that was all it was. Yun Xi let out a quiet breath of relief; she had thought it would be something troublesome. “Father, it’s called an earth dragon, and it’s used for heating in winter. A true earth dragon requires the wall to be hollowed out and built into a hollow double-layer structure—commonly called a ‘fire wall.’ A fire channel is dug beneath the wall, and the fire mouth is set outside under the eaves. Charcoal is burned in that opening, and the heat travels through the hollow wall to warm the entire room. But since our house is already built, we can only use an improved version.”
“And what counts as an improvement?” Yun Dahai looked at Yun Xi eagerly, fully intent on learning.
“The improved version builds a ring-shaped channel along all four interior walls. It doesn’t need to be very tall—thirty centimeters is enough. Then, outside the house, we build a fire channel. When the fuel burns, the hot air flows through the outer fire channel into the inner channels, allowing the heat to circulate smoothly. At the end of the fire channel, we add vents so the smoke can exit from openings under the foundation. That way, the whole room can be heated.”
Fire channels, inner channels… Yun Dahai was already getting confused. “I can’t quite understand it…”
“In that case, why don’t we build it in our house first?” Yun Xi suggested.
“Good, good! That’s much more convincing than any explanation. Xi’er, you know all this because over there… um… is this also how people keep warm where you’re from?”
“Of course not. This method’s been outdated for a long time. The people there have far more advanced ways of heating.” Yun Xi didn’t want to mention things like radiators or air-conditioning—they would take too long to explain, and she didn’t really understand all the details anyway.
Seeing that Yun Xi didn’t want to elaborate, Yun Dahai didn’t push further. This unfamiliar daughter of his already shocked him often enough. “Then what do I need to prepare?”
“Not much. Mud, big tiles, and lots of firewood—the kind that burns long.”
“That’s simple. We have everything right at home.”
Father and daughter got to work immediately. They first moved everything in the house aside to clear the walls. Yun Xi directed Yun Dahai as he mixed large batches of wet mud. There was no cement in this era, so mud had to do. In less than half a day, they had built the inner channels. Looking at the ring of mud channels, Yun Dahai was a little incredulous. “Xi’er, these can actually heat the room?”
“Not yet, Father. We still haven’t built the most important part.”
They carved a hole through the wall connecting the inner channel, then dug a deep pit outside the house, with several smaller pits inside it. “Xi’er, what are these pits for?”
“Oh, they’re for removing the ash from inside the pit. Otherwise, the ash from the fuel wouldn’t have anywhere to go. Father, you can start burning the wood in the pit now.”
“That’s all?”
“Yes, Father. We can test it now. If any part of the inner channel leaks air, we can patch it.”
Half doubtful, Yun Dahai lit the fire. Before long, the inner channel began to warm. Since the room was small and the weather wasn’t too cold yet, the temperature rose quickly. “Xi’er, it really is warm!” Yun Da-niang said excitedly.
Yun Xi smiled. At least they wouldn’t suffer through winter anymore. As for worrying about firewood—please, the mountain was full of it. She had nothing to worry about.