In a previous article, "Smart Home: A Brief Analysis of the Principles of Zero-Cold-Water Technology for Water Heaters", we introduced the principles of zero-cold-water systems in detail. We concluded that at the time, only a device named "Aixiyi" on the market implemented the ideal design described in the article. I have been using it for the past two years. However, my unit recently malfunctioned. Coupled with the fact that it cannot be integrated into Mi Home (Mijia), I didn’t really want to repair it. Furthermore, the newer versions of "Aixiyi" devices are becoming increasingly expensive, giving off a bit of a "the dragon-slaying youth eventually becomes the dragon" vibe.
Therefore, I decided to DIY a zero-cold-water system that can be integrated into Mi Home based on the same principles. I have briefly recorded the production process below.
With a Return Pipe
Of course, while I say "DIY," it is actually just assembling various off-the-shelf components into a complete system. Once you understand the principles mentioned in the previous article, the logic is not difficult. However, for non-professionals, some components might be hard to find or purchase if you don’t know the specific terms to search for.
First, let’s consider the case where a return pipe is present. This situation only requires four components: a circulation pump, a temperature control switch, a Mi Home smart relay, and a check valve.
Circulation Pump: You need a pure water pump that turns on when powered and stops when power is cut. Do not buy a booster pump with automatic start-stop functions. I bought mine on 1688 for about a hundred yuan.
Temperature Control Switch: A switch that is closed (conductive) at low temperatures and open (disconnected) at high temperatures. I bought the XH-W1209, which costs a few yuan. Others with the same function will also work.
Mi Home Smart Relay (Switch): Buy a low-voltage input relay that can be integrated into Mi Home to remotely start the pump. These cost about twenty to thirty yuan.
Check Valve: Any standard model will do, usually costing a few yuan.
Water Circuit Connection: \text{Return Pipe} \quad \to \quad \text{Circulation Pump} \quad \to \quad \text{Check Valve} \quad \to \quad \text{Cold Water Pipe}
Electrical 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:
Xiao Ai or a wireless switch turns on the Mi Home relay, which powers the temperature control switch. If the temperature control switch detects a low temperature, it turns on the circulation pump to discharge the cold water from the return pipe into the cold water pipe. Once the target temperature is met, the temperature control switch disconnects the pump.
There is a minor issue here: the Mi Home relay does not have a "stop" signal. This could be solved by adding another Mi Home-compatible contact sensor, but to simplify the system, we can simply set an automation in Mi Home to turn off the relay automatically after 5 minutes (5 minutes is usually enough to clear the cold water).
Without a Return Pipe
The case without a return pipe is more complex. It requires the coordination of two parts: the "Circulation Pump" and the "Return Valve." In addition to requiring stronger DIY skills, the solution introduced here has one (and perhaps the only) drawback: the location where the return valve is installed (usually under a washbasin or kitchen sink) must have a power outlet. Relying on batteries alone will not work.
The "Circulation Pump" part is installed near the water heater and requires the following materials:
Circulation Pump: A pure water pump (on when powered, off when not). Again, not a booster pump with automatic start-stop.
Water Flow Sensor: Outputs a "closed" signal when water flows and an "open" signal when water stops. Costs about twenty to thirty yuan.
Contact Sensor: Responsible for transmitting the water flow signal to Mi Home. This is usually modified from a Mi Home door/window sensor. Costs about fifty to sixty yuan.
Mi Home Smart Relay: A low-voltage input relay to remotely start the pump.
Water Circuit 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}
Electrical Circuit Connection: \begin{gather*} \text{Mi Home Relay} \quad \to \quad \text{Circulation Pump} \\ \text{Water Flow Sensor} \quad \to \quad \text{Contact Sensor} \end{gather*}
The relay handles the pump’s start and stop, while the sensor transmits the water flow signal to Mi Home.
The "Return Valve" part is installed near the point of use and requires the following materials:
Temperature Control Switch: XH-W1209 or similar (closed at low temp, open at high temp).
Normally Closed (NC) Solenoid Valve: A valve that is closed when powered off and open when powered on. Usually twenty to thirty yuan.
Mi Home Smart Relay: A low-voltage input relay to control the valve.
Two Union Tees: A few yuan each.
Water Circuit Connection: \text{Hot Water Pipe} \quad \to \quad \text{Solenoid Valve} \quad \to \quad \text{Cold Water Pipe}
Electrical Circuit Connection: \text{Mi Home Relay} \quad \to \quad \text{Temperature Control Switch} \quad \to \quad \text{Solenoid Valve}
In this setup, the temperature probe measures the temperature of the hot water pipe and can be taped to the corrugated pipe.
Start-Stop Logic:
Xiao Ai or a wireless switch turns on the Mi Home relays for both the return valve and the circulation pump. The temperature control switch powers up, detects a low temperature, and opens the solenoid valve. Since the circulation pump is also running, the cold water in the hot water pipe is pushed into the cold water pipe. Once the target temperature is reached, the temperature control switch disconnects the solenoid valve. At this point, water stops flowing in the hot water pipe, the water flow sensor disconnects, and upon receiving this signal, Mi Home turns off the relays for both the return valve and the circulation pump.
Summary
This article briefly explains the logic and materials for DIYing a zero-cold-water system. For readers who have already tinkered with smart homes, it should not be difficult to understand. The only unfamiliar components might be the temperature control switch and the solenoid valve.
If you search directly for "Mi Home Temperature Control Switch," you can find units that integrate directly with Mi Home, but they usually cost over a hundred yuan each. In a scenario without a return pipe where you want to install multiple point-to-point return valves, this can become quite expensive. Of course, using a finished Mi Home temperature switch can simplify the system; it depends on personal preference. As for solenoid valves, they are easy to find once you look into automated water control.
In short, interested readers can try this approach. After all, life lies in the "tinkering"!
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