Heat exchangers for a boiler and a furnace work in the same way. The only difference is with that air is the heat transfer medium for a furnace and water is the heat transfer medium for a boiler. Typically, the heat exchanger is above the burner that heats the air or water. If the heat exchanger is cracked, it will result in some major problems for the gas boiler and your health.
If the heat exchanger is cracked, you would see water spilling onto the floor. The water flows though the exchanger as the water is heated before being directed to the radiators or baseboard heating sources. If the exchanger has a crack, the water will seep out of the crack and onto the floor.
Heat exchangers that have a small crack may not leak water onto the floor, but can leak water onto the burner that is used to heat the water that flows though the pipes. When the water drops down onto the burner, it will extinguish the fire. Once the burner fire is out, it is no longer heating the water and you will have no heat coming from the radiators or baseboard heating sources.
As with any appliance that uses gas for heating, a gas boiler with a cracked heat exchanger can be deadly. The crack in the exchange allows the combustion gases to escape into the house. Carbon monoxide poisoning can fill up a room or an entire house. If you don’t have a carbon monoxide detector, you would not know the gases are escaping into the home.
Some reasons for a cracked heat exchanger include thermal stress, manufacturer defect and fatigue of the metal because of having too little water in the boiler or from overheating. If the boiler would freeze and thaw out, the expansion and compression could cause the heat exchanger to crack. In some cases, a bad pump will not move the water through the boiler system, which causes stress on the empty heat exchanger and it will crack under the stress.