By 苏剑林 | April 22, 2025
Previously, in "A Preliminary Analysis of the Technical Principles of Zero-Cold-Water Water Heaters", we introduced the principles of zero-cold-water technology in detail. We concluded by pointing out that, at the time, only a device called "Aixiyi" on the market achieved the ideal design described in the article. I have been using it for the past two years. However, my unit recently malfunctioned, and since it cannot be integrated into Mi Home, I don’t really want to repair it. Furthermore, the newer versions of the "Aixiyi" devices have become increasingly expensive, giving off a bit of a "the dragon slayer has finally become a dragon itself" vibe.
Therefore, I decided to DIY a set of zero-cold-water devices that can be integrated into Mi Home based on the same principles. I am briefly recording the production process as follows.
With a Return Pipe
Of course, while I say "DIY," it’s actually just assembling various off-the-shelf components into a complete system. In fact, once you understand the principles from the previous article, the production logic is not difficult. The only challenge for non-professionals might be knowing how to search for and purchase certain parts.
First, let's consider the scenario where a return pipe exists. In this case, you only need four types of components: a circulation pump, a temperature control switch, a Mi Home relay (smart breaker), and a one-way valve.
1. Circulation Pump: You need a pure water pump that turns on when powered and stops when powered off. Do not buy a booster pump with automatic start-stop functions. I bought one from 1688 for about a hundred yuan.
2. Temperature Control Switch: A controller that stays closed at low temperatures and opens at high temperatures. I bought the XH-W1209, which costs a few yuan; any other with the same function will work.
3. Mi Home Relay: Buy a Mi Home-compatible relay with low-voltage input to remotely start the pump, which costs about twenty to thirty yuan.
4. One-way Valve: Any standard model, usually a few yuan.
Water Path Connection:
\[\text{Return Pipe} \quad\to\quad \text{Circulation Pump} \quad\to\quad \text{One-Way Valve} \quad\to\quad \text{Cold Water Pipe}\]
Circuit Connection:
\[\text{Mi Home Relay} \quad\to\quad \text{Temperature Control Switch} \quad\to\quad \text{Circulation Pump}\]
Note that the temperature control switch has a probe used to measure the water temperature in the return pipe. It can be directly taped to the corrugated pipe of the return line.
Start-Stop Logic:
XiaoAI or a wireless switch turns on the Mi Home relay, which powers the temperature control switch. If the temperature switch detects the temperature is low, it turns on the circulation pump to discharge cold water from the return pipe into the cold water pipe. Once the desired temperature is reached, the temperature switch disconnects the pump.
There is a small issue: the Mi Home relay does not provide a "stop" signal. This could be solved by adding another Mi Home-compatible contact sensor, but to simplify the system, we can just set an automation in Mi Home to turn off the relay automatically after 5 minutes (which is usually enough time to clear the cold water).
Without a Return Pipe
The scenario without a return pipe is more complex. it requires coordination between two parts: the "Circulation Pump" part and the "Return Device" part. In addition to requiring stronger hands-on skills, the only disadvantage of this solution is that the location where the return device is installed (usually under the bathroom sink or kitchen sink) must have a power outlet. Relying on batteries alone won't work.
The "Circulation Pump" part is installed at the water heater and requires the following materials:
1. Circulation Pump: Same as above—a pure pump that starts when powered and stops when power is cut.
2. Water Flow Sensor: Outputs a closed signal when water flows and an open signal when it stops. Cost is about twenty to thirty yuan.
3. Contact Sensor: Responsible for transmitting the water flow signal to Mi Home. This is typically made by modifying a Mi Home door/window sensor, costing fifty to sixty yuan.
4. Mi Home Relay: A Mi Home-compatible low-voltage relay to remotely start the pump.
Water Path Connection:
\[\text{Hot Water Outlet} \quad\to\quad \text{Water Flow Sensor} \quad\to\quad \text{Circulation Pump} \quad\to\quad \text{Hot Water Pipe}\]
Circuit Connection:
\begin{align}
\text{Mi Home Relay} &\quad\to\quad \text{Circulation Pump} \\
\text{Water Flow Sensor} &\quad\to\quad \text{Contact Sensor}
\end{align}
The relay handles starting and stopping the pump, while the sensor sends the water flow status to Mi Home.
The "Return Device" part is installed near the point of use and requires the following materials:
1. Temperature Control Switch: XH-W1209 or similar.
2. Normally Closed Solenoid Valve: An electromagnetic valve that is closed when power is off and open when power is on. Usually costs twenty to thirty yuan.
3. Mi Home Relay: A low-voltage relay to remote start the system.
4. Two union tee joints: A few yuan each.
Water Path Connection:
\[\text{Hot Water Pipe} \quad\to\quad \text{Solenoid Valve} \quad\to\quad \text{Cold Water Pipe}\]
Circuit Connection:
\[\text{Mi Home Relay} \quad\to\quad \text{Temperature Control Switch} \quad\to\quad \text{Solenoid Valve}\]
In this case, the temperature probe is used to measure the temperature of the hot water pipe and can be taped directly to the pipe.
Start-Stop Logic:
XiaoAI or a wireless switch turns on the Mi Home relays for both the return device and the circulation pump. The temperature switch powers up, detects that the temperature is low, and opens the solenoid valve. Since the circulation pump is already running, the cold water in the hot water pipe is pushed into the cold water pipe. When the temperature reaches the set point, the temperature switch closes the solenoid valve. At this point, water in the hot water pipe stops flowing, the water flow sensor disconnects, and upon receiving this signal, Mi Home turns off the relays for both the return device and the circulation pump.
Summary
This article briefly explained the logic and materials for DIYing a zero-cold-water system. For readers who have already dabbled in DIY smart homes, this shouldn't be too hard to understand; the only unfamiliar parts might be the temperature control switch and the solenoid valve.
If you search for "Mi Home temperature control switch," you can find ready-made ones that connect directly to the app, but they usually cost about a hundred yuan each. For scenarios without a return pipe where you might want to install multiple return devices at different points, it could get expensive. Of course, using off-the-shelf Mi Home temperature switches can simplify the system, so it depends on personal preference. As for solenoid valves, if you deal with water circuit automation, they are easy to find via search.
In short, readers who love to tinker can try following these ideas—after all, life lies in the struggle!