HVAC Glossary: Makeup Water

In the intricate world of Heating, Ventilation, and Air Conditioning (HVAC) systems, the term "makeup water" refers to the water added to a system to compensate for losses. These losses can occur due to evaporation, drift, blowdown, leaks, or other operational necessities. Maintaining the correct volume and quality of water is critical for the efficient and reliable operation of various HVAC components, particularly cooling towers, boilers, and closed-loop hydronic systems. This guide delves into the technical aspects of makeup water, its significance, sources of loss, treatment considerations, and practical implications for HVAC professionals.

Understanding Makeup Water in HVAC Systems

Makeup water is essential for maintaining the operational integrity and efficiency of HVAC systems that rely on water as a heat transfer medium. Without a consistent supply of makeup water, systems can experience reduced performance, increased energy consumption, and premature equipment failure. The primary systems requiring makeup water include:

Cooling Towers

Cooling towers are open-loop systems that reject heat from a building's cooling system to the atmosphere through evaporative cooling. As water evaporates, it leaves behind dissolved solids, increasing the concentration of impurities in the remaining water. Makeup water replenishes the evaporated water and helps dilute these concentrated impurities. Key factors contributing to water loss in cooling towers include:

• Evaporation Loss: The primary mechanism of heat rejection, where a portion of the circulating water changes into vapor and is released into the atmosphere. This is the largest component of water loss.
• Drift Loss: Small water droplets carried out of the cooling tower with the exhaust air. While drift eliminators minimize this, some loss is inevitable.
• Blowdown (Bleed-off): The intentional discharge of a portion of the high-solids circulating water to control the concentration of dissolved minerals and prevent scaling and corrosion. Makeup water replaces this discharged volume.
• Leaks: Unintended water loss from pipes, pumps, or the tower basin.

Boilers and Hydronic Systems

Boilers and closed-loop hydronic systems (e.g., chilled water or hot water loops) also require makeup water, though typically in smaller quantities compared to cooling towers. Losses in these systems are primarily due to:

• Leaks: The most common reason for makeup water demand in closed systems. Even small leaks can accumulate over time.
• Maintenance Activities: Draining and refilling systems for repairs or inspections.
• Evaporation: While closed systems are designed to minimize evaporation, some can occur in expansion tanks or during venting.

Makeup Water Quality and Treatment

The quality of makeup water significantly impacts the longevity and efficiency of HVAC equipment. Untreated or poorly treated makeup water can lead to a host of problems, including:

• Scaling: Precipitation of dissolved minerals (e.g., calcium, magnesium) on heat transfer surfaces, reducing efficiency and increasing energy consumption.
• Corrosion: Degradation of metal components due to chemical reactions with water and dissolved gases, leading to leaks and structural damage.
• Fouling: Accumulation of suspended solids, biological growth (e.g., algae, bacteria), or other debris that impedes heat transfer and water flow.

Effective makeup water treatment is crucial and often involves a combination of methods:

Calculating Makeup Water Requirements

Accurately calculating makeup water requirements is vital for efficient system operation and water conservation. For cooling towers, the total makeup water (M) is typically the sum of evaporation loss (E), drift loss (D), and blowdown (B) [1].

M = E + D + B

Where:

• Evaporation Loss (E): Approximately 0.85% to 1.0% of the circulating water flow rate per 10°F (5.6°C) of cooling range. A common rule of thumb is 0.001 gallons per minute (GPM) per ton of cooling capacity per degree Fahrenheit of temperature drop.
• Drift Loss (D): Typically a very small percentage (0.001% to 0.005%) of the circulating water flow rate, depending on the type and condition of drift eliminators.
• Blowdown (B): Calculated based on the desired cycles of concentration (COC), which is the ratio of dissolved solids in the circulating water to the dissolved solids in the makeup water. B = E / (COC - 1).

For closed-loop systems, makeup water is primarily driven by leak rates, which can be determined through regular monitoring of system pressure and makeup water meter readings.

Practical Implications for HVAC Professionals

• System Monitoring: Regular monitoring of makeup water consumption, water quality parameters (e.g., conductivity, pH, hardness), and chemical treatment levels is essential.
• Preventative Maintenance: Addressing leaks promptly, maintaining drift eliminators, and ensuring proper functioning of water treatment equipment can significantly reduce makeup water demand.
• Water Conservation: Optimizing cycles of concentration in cooling towers, implementing efficient water treatment, and promptly repairing leaks contribute to substantial water savings and reduced operational costs.
• Compliance: Adhering to local regulations regarding water discharge and chemical usage is critical.