Air-source heat pumps can both heat and cool, and precisely because of this, the design of the system becomes more complex. For instance, for the same system, the requirements for pump flow may vary when used for heating and cooling. A dealer previously told me: In a household heating and cooling project, there was no problem during heating, but during cooling, the pump flow was insufficient, which eventually led to the compressor cracking due to freezing. The customer had to replace the new unit.
So, how should the two-pipe supply system be designed more reasonably, or more energy-efficient? How should the pumps be selected? Regarding these questions, we invited Tang Zhijun, the technical director of Hailei RuiNeng, to clarify the issues raised by the dealers.
Which system is more reasonable, the primary system or the secondary system?
Question 1: For a 100-square-meter home heating and cooling system (with 5 to 6 air vents), is it better to use the primary system or the secondary system?
For small-scale home heating and cooling systems, it is recommended to use the primary side system. The primary side system is simpler and easier to install. Since the users of small-scale home projects have limited installation space and budget, the primary side system is more suitable. Users are more receptive and it is easier to promote.
The advantage of the secondary side system is mainly to increase the system water capacity and reduce water temperature fluctuations. Therefore, as long as the primary system ensures the minimum water capacity of the system, it can achieve the effect of the secondary side system. According to the "Heating and Ventilation Design Manual" edited by Lu Yaqing (2008 second edition), the minimum water capacity for cooling is 28L/1kw, and for heating it is 15L/1kw. It can also be estimated based on the building area, approximately 1.3 L/㎡.
Therefore, for small and medium-sized systems, even large systems, it is recommended to use the primary side system. By increasing the system pipes or adding series buffer water tanks to increase the system water capacity, it can achieve energy-saving and system stability effects.
How to select the pump?
Question 2: For a 100-square-meter home heating and cooling system (with 5 to 6 air vents), when using the secondary pressure-bearing system, considering factors such as cost, noise, and energy consumption, for the heating and cooling circulation pump of the home heating and cooling system, which power pump is more reasonable?
If using the secondary side system, it is recommended to use a shielded pump for the primary side and a multi-stage pump for the secondary side.
For a 100-square-meter two-pipe supply system, the main unit is approximately 6HP, with a cooling capacity of 15KW. The pump flow of the primary side is about 2.5m3/h. The maximum system resistance is the water resistance of the main unit, generally 4 to 5 meters. Therefore, a shielded pump is sufficient, which is quiet and energy-saving. For the secondary side system, the water flow is about 2.5m3/h. The water resistance of the air vents is generally 3 to 4 meters, and there are more local resistances and along-the-line resistances in the terminal system valves and pipes, approximately 2 meters. Considering future use, the system may scale up and get clogged or dirty, it is recommended to choose a pump of about 10 meters for the secondary side, which is more suitable for multi-stage pumps or variable frequency pumps.
The pump of the two-pipe supply system, especially the terminal circulation pump, is recommended to select a pump with a relatively flat characteristic curve. A flat curve pump is suitable for systems with a large flow adjustment range and a small change in head. For systems with multiple air conditioning panels, the system flow changes significantly when one air panel is open and all are open. A flat curve pump can adapt to this change and the system is more stable. While a steep performance curve pump is suitable for systems with high flow requirements but low pressure requirements. As long as the system flow of the primary system is stable, the main unit will not alarm with high pressure or low water temperature, and the primary side can operate stably.
Question 3: As shown in the following two figures, in the secondary system, for the heating and cooling circulation pump of the home heating and cooling system, sharing one pump is better or using separate pumps is better?
It is better to use two pumps, considering energy conservation and usage. Because: 1) The heating pipes are smooth pipes, so there is only local resistance along the way, which is relatively small, less than 0.5 meters, and can basically be ignored; the internal resistance of the fan coil unit is generally between 3 to 4 meters; 2) The temperature difference between the inlet and outlet water of the fan coil unit is generally 5 degrees, while the temperature difference between the inlet and outlet water of the heating pipes is generally calculated as 10 degrees. Therefore, the rated flow of the heating pipe pumps only needs to be half of that of the fan coil unit, and the rated head is also about half. So, using two pumps is more reasonable, but the installation and control will be slightly more complicated. Using one pump is also possible, and it is best to install a differential pressure bypass valve on the main pipe to reduce the wear on the heating pipes due to high flow rate and increase the safety and reliability of the system.
Question 4: How do you match the pumps for the heating system? If you add the fan coil unit, how should you match the pumps?
The heating pipes are smooth pipes, so there is only local resistance along the way, which is relatively small, less than 0.5 meters, and can basically be ignored; the internal resistance of the fan coil unit is generally between 3 to 4 meters, and moreover, the temperature difference between the inlet and outlet water of the fan coil unit is generally 5 degrees, while the temperature difference between the inlet and outlet water of the heating pipes is generally calculated as 10 degrees. Therefore, the selection of the pump's head for the fan coil unit is much larger.
Adding the fan coil unit, generally, the flow and head should be increased by at least twice. However, when using the heating system, the pump should be used for pressure reduction to prevent excessive flow rate from causing wear on the heating pipes and shortening the service life of the heating pipes.
Is the pump selection reasonable?
Question 5: As shown in the following figure, is it reasonable to use a 400W WELLO 200E pump in the primary system?
It is recommended to use the MHI202 pump. The original pump selected was the 200E pump, whose characteristic curve is steeply decreasing. Generally, it is not suitable for multiple fan coil systems. When used in the floor heating system, it has no effect. For multiple fan coil systems, it is recommended to choose a flat-type pump. Moreover, the 200E pump has a flow rate of approximately 9 meters at a flow rate of 2.5 m³/h. The resistance of the main unit + fan coil (the most unfavorable circuit) is about 8 meters, and adding the resistance of other valves, it is at least over 10 meters. Therefore, the head of the 200E pump is too small, and the MHI202 pump has high efficiency, energy saving, and low noise, and is more suitable.
In addition, in the system diagram, the fan coil units do not need to install three-way electric valves for each one. Even if the main pipe does not have a differential pressure bypass valve, only one three-way electric valve needs to be installed on the fan coil unit in the most unfavorable circuit. Having one more valve component means having one more failure point. Of course, for general small household systems, the main pipe does not install a differential pressure bypass valve, and only one three-way electric valve needs to be installed on the fan coil unit in the most unfavorable circuit to achieve the system pressure relief effect.
Question 6: For the two-source supply system, the pump flow rate is 2.9 cubic meters, the head is 22 meters. Is a 10-way water divider sufficient? The pipes use size 20, one way 10-15 square meters, the floor heating area is 130 square meters, and the main unit has an inbuilt pump.
If this pump is only used in the floor heating system, it is a bit large. Reducing it by half is about right.
The temperature difference between the inlet and return water of the floor heating is half calculated at 10 degrees. The water resistance, except for the main unit, includes the local resistance of some valves and the along-course resistance of the pipes. Generally, it will not exceed 10 meters. The pipeline design should be as simple as possible, reducing unnecessary valves and pipe diameter changes, and reducing the use of bends. This can be more energy-efficient.
How to select the fan coil units? Do you need water treatment?
Question 7: How to select the fan coil units based on the room's cooling and heating load or room area? How to calculate?
The selection of fan coil units is based on the cooling load. Calculate the unit cooling load, then calculate the cooling load of each room area, and choose the fan coil units according to the room load. The full heat cooling capacity of the fan coil units when the speed is medium.
The cooling load can be calculated through HVAC software for the building's load, or estimated. Use the recommended values in relevant HVAC design books for calculation.
Question 8: In Taiyuan, Shanxi Province, how to calculate the heating capacity of the floor heating system and the cooling capacity of the fan coil units in the two-source supply system? The maximum heating temperature of the floor heating is 45°C. Is it necessary for water treatment at such a low temperature? If water treatment is needed, what method should be used?
It is recommended to calculate the building's load using relevant HVAC software or directly adopt the recommended values in CJJ 34-2010 "Urban Heating Pipeline Design Code", and then look up the corresponding fan coil parameters.
Water will scale at 45°C, and even in the fan coil cooling condition, it will also scale. It's just a matter of speed. A general soft waterer will do. In cases where the water capacity is not large, you can directly purchase pure water from the market and add it all at once, which is more cost-effective.
