By 苏剑林 | January 04, 2023
If a household uses a single centralized water heater for hot water, we often need to let the cold water run for a while before hot water arrives. If the wait time is long, it can significantly impact the user experience. So-called "Zero Cold Water" (ZCW) is actually a method to pre-drain the cold water from the hot water pipes to achieve a (nearly) instant hot water effect. In fact, ZCW is not a high-end technology, but it hasn't become universal in households yet, possibly due to outdated concepts or misunderstandings. However, as people's demands for quality of life increase, ZCW is indeed becoming more popular.
This article provides a simple analysis of the implementation principles of Zero Cold Water technology, including the pros and cons of various solutions and reference ideas for DIY implementation.
Before We Begin
To start the article, it's necessary to correct a common misconception: Zero Cold Water is not intended to save money; it is intended to improve the quality of life. If your mindset is primarily focused on saving money, then the following content might not be of much value to you, as ZCW technology has little to offer in that regard.
(Most) ZCW technologies do indeed save water, but saving water doesn't mean saving money. The extra investment for ZCW is usually much greater than the saved water fees. A simple calculation clarifies this: assume every shower requires letting cold water run for one minute. I measured it, and my hot water pipe outputs 8 liters per minute under current pressure. Assuming a family of four, that's 32 liters of water drained per day. Calculated at 3 RMB per ton, the daily water cost is 0.096 RMB, which is about 35 RMB per year. Currently, the lowest configuration for ZCW accessories is around 700 RMB. The price difference between a ZCW water heater and a non-ZCW version of the same brand and configuration is often around this level or higher. Thus, the extra investment for ZCW is enough to pay for 700/35 = 20 years of water. This doesn't even account for the extra energy (gas or electricity) consumed by the ZCW system. Therefore, saving money through water conservation is unrealistic.
So, ZCW technology does not save money. You could say it's for water conservation, or more accurately, to improve the living experience. It primarily reduces the time spent waiting for hot water before a shower, especially in cold winters when shivering for a minute or two waiting for hot water is not only uncomfortable but can lead to catching a cold and is a waste of time.
Technical Analysis
To keep things simple, let's simplify the problem and assume there is only one hot water outlet in the house. Its plumbing can be simplified as follows:
Simplified household plumbing schematic (assuming one usage point)
After a long period of not using hot water, the water in the hot water pipe essentially cools down to cold water, as shown below:
The water in the hot water pipe has turned cold
As mentioned at the start, ZCW is about finding a way to pre-drain that cold water. The first question to solve is: where do we drain it to? There are three main answers, which we will analyze one by one.
The Return Pipe
As shown in the image, the return pipe is essentially a "copy-paste" of the hot water pipe, which is connected at the usage point. If you buy a ready-made ZCW water heater, it usually comes with a dedicated return pipe interface. You connect the hot water pipe, cold water pipe, and return pipe to their respective interfaces. The hot water pipe and the return pipe form a small loop, allowing the cold water in the hot water pipe to be recycled back.
Adding a return pipe can be understood as an extension or duplication of the hot water pipe
If you are DIY-ing with an existing non-ZCW water heater, there won't be a return pipe interface. We can connect the return pipe to the cold water inlet at the heater end. Since water won't circulate on its own, we need a booster pump (also called a circulation pump or return pump) to push it. This pump can be installed at the start of the hot water pipe or the end of the return pipe:
The booster pump can be installed at the start of the hot water pipe
Or it can be installed at the end of the return pipe
In fact, a ZCW water heater mainly just has the booster pump built-in. Once the booster pump starts, it pushes the cold water in the hot water pipe through the return pipe, forming a water cycle. Eventually, the entire hot water pipe and the return pipe will be filled with hot water:
During hot water circulation, the entire return pipe is heated, which involves some waste
Obviously, the hot water in that return pipe segment serves no actual purpose. Although we emphasize that ZCW is for experience rather than saving money, it's still better to save a bit of gas/electricity if it doesn't affect the experience. An improvement here is to find a way to heat only the hot water pipe and not the return pipe. A simple method is to add a timer to the booster pump, setting a duration just long enough to heat the hot water pipe, but this duration is difficult to control. A smarter method is to add a thermostatic check valve at the connection point between the hot water pipe and the return pipe (the flow direction is from the long side of the trapezoid to the short side, and it closes when the temperature reaches a certain set value):
Adding a thermostatic check valve can prevent the return pipe from being heated
Finally, to prevent water from the return pipe from leaking into the cold water pipe (which could cause parts of the cold water line to get hot and might trigger the water heater when only cold water is needed), another check valve (non-thermostatic) can be added where the return pipe connects to the cold water line:
Adding another check valve makes it perfect
This concludes the analysis for a single usage point. In real life, there are usually multiple hot water usage points. We can arrange the plumbing in several structures to ensure every point gets hot water quickly (pumps and valves omitted for simplicity, showing only basic plumbing):
Main-line plumbing layout, commonly known as a "small loop"
Idealized plumbing layout, commonly known as a "large loop"
Plumbing layout with similar effects to the "large loop"
The Cold Water Pipe
If your home renovation didn't involve pre-installing a return pipe, the above solution is out of reach. A very clever idea at this point is to "drain into the cold water pipe" by connecting the cold and hot water pipes at the outlet with a check valve:
Adding a check valve lets the cold water pipe act as a return pipe
Most ZCW water heaters on the market that claim to function without a return pipe use this solution. It does work as a makeshift, but it introduces two problems. First, the check valve allows water from the hot line to flow into the cold line, meaning that if you use cold water, it might trigger the water heater and release hot water. This leads to common complaints about "hot water flushing the toilet" in non-return pipe ZCW setups. While this might just waste some gas/electricity, a more serious issue is that during circulation, the entire cold water line can become hot:
It may cause the entire cold water line to become hot
This isn't just about wasting energy. Some usage points are sensitive to temperature. For example, the optimal operating temperature for an RO (Reverse Osmosis) water purifier is around 38°C; exceeding this can cause irreversible damage, and hot water circulation temperatures are usually above 40°C. To solve this, an upgraded version replaces the check valve with a thermostatic check valve:
Replacing the check valve with a thermostatic check valve prevents the cold water line from getting hot
The thermostatic check valve ensures the cold water line temperature does not exceed the set value. Compared to a pure check valve, the experience is significantly improved. Recently, some new or high-end ZCW water heaters have adopted this solution. However, this theoretically only solves the hot-cold-water-line issue but cannot solve the problem of cold water usage potentially starting the heater. To completely solve both, the thermostatic check valve should be replaced by a "normally closed thermostatic solenoid valve" with the following characteristics:
1. In the default state, the valve is closed, so the cold and hot water lines are disconnected, preventing the heater from starting when cold water is used;
2. When the temperature exceeds the set value, the valve is closed, ensuring the cold water line temperature doesn't get too high;
3. When the temperature is below the set value and the booster pump is on, the valve opens, allowing the cold water in the hot water pipe to flow into the cold water line.
It's easy to imagine that such a solution requires the "normally closed thermostatic solenoid valve" to have a (wireless) linkage with the booster pump:
Replacing the thermostatic check valve with a normally closed thermostatic solenoid valve linked to the pump is the most ideal
Furthermore, we can add an external remote control to provide a wireless link to the solenoid valve, allowing manual on-demand activation of the ZCW:
Adding a remote control is even better
Currently, the author has investigated only one brand, "Aixiyi" (爱喜易), that offers DIY ZCW accessories using this solution.
If you are DIY-ing this, what is the biggest difficulty? It's "electricity"! The "check valves" and "thermostatic check valves" mentioned earlier have non-electric implementations, but a "normally closed thermostatic solenoid valve" with wireless linkage must use electricity. Where is this typically installed? Usually under the sink or inside the showerhead. There are typically no power outlets in these locations, so it must be battery-powered. Since the solenoid valve acts as a wireless receiver for the pump and remote, it consumes relatively more power. Batteries generally don't last long, so a way to reduce power consumption is needed, which is challenging but has existing reference solutions that we won't detail here.
If there are multiple usage points, the most perfect solution is to equip each hot water point with such a solenoid valve, achieving "point-to-point" zero cold water. This is the most gas/electricity-efficient solution, but it also has the highest hardware cost. To save costs, one could install only one solenoid valve (or thermostatic check valve) at the furthest hot water point. The downside is that for the points closest to the heater, the entire hot water line still needs to be heated. For showers, adding an accessory directly at the showerhead is too ugly; fortunately, most bathrooms have matching sinks with hot water, so it's usually installed under the sink or a showerhead with built-in components is used.
The Sewer
Actually, without a return pipe, the simplest solution should be to drain the cold water directly into the sewer, exactly like a person manually letting the water run:
Draining to the sewer is also a solution worth considering
Compared to draining into the cold water line, its main feature is eliminating the need for a booster pump, and the hot and cold water lines are completely isolated. The only drawback is it doesn't save water. We can calculate whether the cost is worth it. Taking the aforementioned "Aixiyi" ZCW kit as an example, the booster pump alone sells for about 350 RMB (there are more expensive and cheaper ones on the market, but considering the linkage features, this is a fair estimate). According to our initial estimate, this money is enough to "waste" water for ten years or more. Furthermore, for a DIY enthusiast like you, wouldn't you update the solution within ten years anyway?
So, putting aside "environmentalism" or the concept of "water conservation," draining directly into the sewer is likely the lowest-cost and easiest-to-DIY solution (subtracting the pump linkage). Of course, if the reader is a staunch supporter of environmental protection, then only the first two solutions can be considered.
Article Summary
This article introduced the principles of implementing zero cold water for water heaters and analyzed/compared several DIY solutions for the reference of interested readers.
In summary, the goal of ZCW is to improve the quality of life, not to save money. In terms of implementation, ZCW can be achieved with or without a return pipe. I personally feel that with the "normally closed thermostatic solenoid valve" solution, the experience of a non-return pipe setup is not inferior to that of a return pipe system. Therefore, I believe ZCW is worth getting (though if you buy a ready-made ZCW heater, you should at least replace the check valve with a thermostatic study). Finally, regarding the increased gas/electricity usage mentioned by some users: ZCW definitely uses more gas/electricity than a non-ZCW setup, but if the increase is excessive, it's usually due to improper use—such as leaving ZCW on 24 hours a day. That's like leaving the air conditioner on 24 hours a day when no one is home and then complaining that it's useless and expensive; there's simply no reasoning with that logic.