The maintenance cycle of valves is not fixed and is influenced by various factors, including the type of valve, usage frequency, working environment, material, and the quality of maintenance. Here are some general guidelines for the maintenance cycle of valves: 

I. General Maintenance Cycle 

1. Routine inspection and maintenance: 

For valves in general thermal systems, it is recommended to conduct a routine inspection and maintenance at least once a year. This includes checking the appearance of the valve, its sealing performance, operational flexibility, and whether the fasteners are loose. 

After the heating season ends, a comprehensive inspection of the valves should be carried out, especially for those that are frequently operated during the heating period, where the wear condition should be carefully examined. 

2. Professional Cleaning and Maintenance: 

Based on the usage of the underfloor heating system and the water quality conditions, it is recommended to have a professional cleaning every two years, including the heating valves, manifold valves, and pipes. In areas with poor water quality, it is best to clean once a year. 

During the cleaning process, all dirt, impurities, etc. in the pipeline should be thoroughly removed to ensure the normal operation of the valve and the entire heating system. 

II. Maintenance Cycle under Specific Conditions 

1. Frequent use: 

If the valve is used very frequently, the maintenance period may need to be shortened accordingly. For instance, in some places that require continuous heating, the valve may need to be inspected and maintained once every quarter or even once a month. 

2. Harsh working conditions: 

If the valve is exposed to harsh working conditions, such as high temperatures, high pressure, corrosive gases or liquids, then its maintenance period should also be shortened accordingly. This is because these environmental factors will accelerate the wear and aging of the valve. 

3. Special material valves: 

Valves made of different materials have different service lifespans and maintenance cycles. For instance, stainless steel valves typically have a longer service life, while brass valves may have a shorter one. Therefore, when choosing a valve, one should fully consider its material and the usage environment, and accordingly formulate a reasonable maintenance plan. 

III. Criteria for Maintenance and Replacement 

1. Leakage situation: 

If there is a leakage in the valve, whether it is minor or severe, it should be repaired or replaced immediately. Leakage not only leads to heat loss but also poses a threat to the system's safety. 

2. Operational Flexibility: 

If the valve becomes unresponsive or sluggish during operation, it might be due to the accumulation of impurities inside or the wear of the seals. In such cases, the valve should be cleaned or the seals replaced. 

3. Visual inspection: 

Regularly inspect the appearance of the valves to check for signs of rust, cracks, etc. If any such problems are found, appropriate measures should be taken promptly for repair or replacement. 

4. Service Life: 

Generally speaking, metal valves may experience aging and wear and tear after being used for a long time. Therefore, based on the material of the valve and its service life, a general replacement plan can be formulated. For instance, some valves may need to be replaced after being used for 5 to 10 years. 

In conclusion, the maintenance cycle of valves should be determined based on specific circumstances. When formulating the maintenance plan, factors such as the type of valve, usage frequency, working environment, material, and quality of maintenance and upkeep should be fully considered. At the same time, conducting regular inspections and tests on the valves, and promptly identifying and resolving problems, are crucial for ensuring the safe and efficient operation of the thermal system.