Ground Cooling Neither Freezes Your Feet nor Produces Condensate! Believe It? Listen to the Experienced Craftsman Talk about the Comfort, Application and Boundary Conditions of Ground Cooling

2025-05-07

The ground radiant heating and cooling system with a heat pump heating and cooling unit as the heat source has currently become an option for the upgrade of heating and cooling air conditioners in the south. But before talking about ground cooling, let's first mention two off-topic but also popular topics. That is -
First: Will ground cooling actually freeze your feet?
A simple answer to this question is: In a ground cooling system (especially in high-temperature and high-humidity areas in the south), the ground cooling does not bear all the cooling load. There is an air handling unit for assistance or it is mainly responsible for cooling. Moreover, the ground temperature is recommended to be around 22°C and not lower than 20°C. Therefore, your feet will not feel cold.
Second: Will condensate water actually form on the floor?
A simple answer is: It is indeed very risky to use the fixed water temperature design of a conventional heating mixing water system for "ground cooling". If the fresh air dehumidification system malfunctions, the dew point temperature may exceed the ground surface temperature at any time. It is necessary to adopt a control form of multi-point collection of the dew point in different regions, and the water temperature should be variable. By controlling the ground temperature to always be higher than the dew point temperature and timely turning on the air handling unit for auxiliary cooling, and independently controlling the temperature and humidity, it is possible to achieve no condensation, as well as comfort and energy conservation.
After these simple answers, if you want to truly and systematically understand the knowledge about the application scenarios, system design, construction and installation, system control, after-sales debugging, etc. of the ground radiant heating and cooling air conditioner, let's talk about the comfort requirements, system design, etc. of the ground radiant heating and cooling today. (The following comes from the Qianliao open class of the Heat Pump Business School, and the speaker is Kovatu and Ye Lifang)
I. Comfort in Different Scenarios
Ground radiant cooling is not a new thing but a mature technology. It has been applied in Europe for decades, but it has just started in China, and the promotion scope is relatively small. Different engineering companies use different products and solutions, and there have been many failed cases. This reminds the promoters to conduct a comprehensive evaluation instead of just focusing on the right price.
Ground radiant cooling basically relies on radiant heat transfer. Since cold air always moves downward, the ground convection in ground radiant cooling is very small. If fresh air is supplied, such as from the corner of the wall or the ground, it can increase the ability of convective heat transfer.
However, if the convective speed is too high, it will go against the pursuit of comfort in radiant cooling. Therefore, the overall cooling capacity of ground cooling is much smaller (about 60%) than that of ceiling cooling.
The following are three different application scenarios of ground radiant cooling:

Commercial places, such as airports, shopping malls, etc.
Construction drawings of ground radiant cooling and heating in Beijing Daxing Airport. In these places, there is a large flow of people and a high level of activity, and basically people wear formal attire. So, the temperature of the floor surface in these scenarios can be lower, and people will not feel uncomfortable because people are mainly engaged in activities, and there are fewer people sitting still. A ground temperature between 19~20℃ is acceptable.
Office areas.
People mainly wear formal attire, but they mainly sit still, and the level of activity is not as high as in commercial places. The chest, back, and heart of people are closer to the ground. So, people have a higher requirement for the ground temperature than in commercial places.
If the temperature in a commercial center is 19℃, the temperature in the office should be at least above 21℃, specifically depending on the on-site feedback from people.
Home environment.
In a home environment, people usually wear slippers and home clothes. Especially, the feeling of comfort regarding the ground temperature is completely different when people wear slippers and shorts compared to when they wear leather shoes and long pants. Moreover, people mainly sit still at home. So, the requirement for the ground temperature at home should be higher than that in the office area.
If the temperature in the office area is 21℃, the temperature at home should be above 23℃. Of course, it is related to people's physical constitution and sensitivity to temperature.
Therefore, people's comfort requirements for the floor surface temperature in different scenarios are different. We cannot generalize and should design flexibly according to local conditions.
II. Boundary Conditions of Ground Cooling
Different Humidity Environments in Different Regions
The humidity is different in different regions, and the design requirements for ground radiant cooling are also different. For example, in northern regions, the humidity is low. Even if the airtightness of the office area is not good, the probability of condensation on the ground is very small.
The average humidity in Beijing in June is 54%. If the air temperature is 26℃, then the dew point temperature is only 17.6℃. At this time, the floor surface temperature can be set to 18℃, and the water supply temperature can be set to above 14℃.
In southern regions, it is different. The air humidity is high, and the floor surface temperature cannot be too low, otherwise, there may be a problem of condensation. When using low-temperature water for cooling in public buildings, one must be careful.
The average humidity in Shanghai in June is 83%, and the average humidity in Guangzhou in June is 85%. If the air temperature is 26℃, the dew point temperature is 22℃. Then the floor surface temperature must be higher than 22℃. At this time, the load is very small, and it cannot meet the cooling demand.
Therefore, when using ground cooling in the north, it doesn't matter if the building airtightness is a bit poor or the dehumidification ability is a bit weak. But in the south, the building airtightness must be good, and the dehumidification ability in the system must be strong. Otherwise, there will either be condensation or the water supply temperature will be increased, sacrificing the cooling capacity.
So, the risk of condensation for ground cooling in northern regions is relatively smaller. But in the south, it can only be used in scenarios with good airtightness in buildings such as offices or residences. The risk of using it in open commercial places is too high.
Different Heat Transfer Capabilities of Floors with Different Materials
In residential buildings, attention should be paid to the heat transfer capabilities of floors with different materials. Under the same water supply temperature, there is a big difference in the heat transfer capabilities of ceramic tiles, solid wood floors, and composite floors.

According to the test, when the water supply temperature is 18℃ -
The surface temperature of ceramic tiles is around 22℃;
If it is a 1.2 cm thick composite wood floor, its surface temperature is around 23℃;
If it is a 2 cm thick solid wood floor, its surface temperature is around 24℃.
If the room temperature is set to 26℃ and the water supply temperature is 18℃, then the temperature difference between the surface temperature of the solid wood floor and the air temperature is only 2℃, and the cooling capacity of the ground is about 15W, which is almost negligible. It can only be used in passive houses; when the temperature difference between the surface temperature of the solid wood floor and the air temperature is 4℃, the cooling capacity of the ground is about 30W.
III. Design and Application of Ground Cooling
The first form of ground radiant cooling is mainly using ground cooling.
This is suitable for northern regions or low-energy-consuming buildings. Because the recommended ground temperature in office places is higher than 20℃, and the recommended ground temperature in residential buildings is higher than 21℃. The European standard is not lower than 19℃.
At this time, the cooling capacity is between 30~40W, which can meet the cooling demand.
In conventional buildings in the south, the sensible heat cooling load must be above 60W. Even for ceiling radiant cooling, it is around 60W or even higher.
The second form of ground radiant cooling is a form with convection as the main part and ground cooling as the auxiliary part.
This system is an upgrade of the traditional two-in-one supply system or the convective terminal system. Ground cooling is used to solve part of the ground sensible heat problem, reducing the load design of convective cooling.
If the indoor terminal is an all-air system and the cooling load is, for example, 100W, ground cooling can provide a cooling load of about 20W. Then the load design of the terminal of the all-air system can be reduced by 20%. In this way, the diameter of the air duct can be much smaller, and the impact on the floor height will be smaller.
If the indoor terminal is a fan coil unit, its power can also be reduced, allowing the air handling unit to operate at a low wind speed for more time, which has a significant improvement in enhancing comfort and reducing indoor noise.
This method can be widely promoted and applied in the two-in-one supply system.
Of course, it is best to be equipped with a suitable fresh air dehumidification system in this system. Otherwise, the fan coil unit cannot be turned off during ground cooling.
For such a system, we should objectively evaluate its value and comfort. We should neither belittle it too much nor overstate it. After all, you get what you pay for.
The third form of ground radiant cooling is the application of offsetting the solar thermal radiation in front of large French windows.
Whether in a ceiling radiant panel air conditioning system or a convective terminal air conditioning system, it is difficult to effectively solve the local gradient temperature difference in the room caused by the solar radiation of large French windows.
For example, for a large French window of more than ten square meters, even if an air conditioning air supply outlet is set by the window, it cannot solve the solar radiation heat of such a large area. In this scenario, the best method is ground cooling.
The intense sunlight shining on the relatively low-temperature ground can effectively offset the heat of solar radiation. At this time, the cooling capacity of the ground may reach more than 100W per square meter, which is very effective in solving the indoor gradient temperature difference.
Ground temperature adjustment in conventional buildings in the south cannot be used as the main form of the cooling terminal. It is more suitable as an auxiliary cooling system. It is necessary to adopt a control form of multi-point collection of the dew point in different regions, and the water temperature should be variable.
If a conventional heating mixing water system is used for fixed water temperature, the risk is very high. If the fresh air dehumidification system malfunctions, the dew point temperature may exceed the ground surface temperature at any time. Ground temperature adjustment is just a low-investment upgrade form of the two-in-one supply system, which has a broad application prospect, but it should not be overly packaged into a conceptualized product.