If air is trapped in the pipes of a hot water heating system, it can cause:

• A. Poor circulation, reducing efficiency
• B. Corrosion of ferrous components
• C. Damage to the seals and bearings
• D. All of the above

Answer: (D)

In a hot water heating system, trapped air can lead to several issues that hinder the system's performance and longevity. When air is present in the pipes, it disrupts the flow of water, causing poor circulation. This inefficiency means that areas of the system may not receive enough heat, which can lead to higher energy costs as the system works harder to maintain temperature.

Additionally, trapped air can contribute to corrosion, especially in ferrous components of the system. The presence of oxygen from the air increases the likelihood of oxidation, which can deteriorate metal parts over time and lead to leaks or failures.

Furthermore, the accumulation of air can create pressure fluctuations and water hammering, which can stress seals and bearings. This distress may result in premature wear and ultimately damage those components, leading to costly repairs and system downtime.

Hence, the correct answer encompasses all these detrimental effects, making the choice that includes all of these consequences appropriate.