What can happen if the expansion tank fails?

If the expansion tank fails, it can lead to dangerous fluctuations in the pressure within the heating system. The primary function of an expansion tank is to accommodate the increase in water volume as it heats up, allowing for the thermal expansion of water without causing excessive pressure buildup. When this tank is compromised, the system can no longer manage the pressure changes effectively. This can result in over-pressurization, which may damage components of the heating system, lead to leaks, or even catastrophic failure such as ruptures.

The other options do not accurately describe the consequences of a failed expansion tank. Efficient operation would be compromised due to instability, excessively cold water is not a direct result of this issue, and frequent ignition of the furnace does not relate to the expansion tank's failure but rather to other system malfunctions.