June 2026: Risk of carbon monoxide poisoning in cabins when using generators and heaters

This investigation insight is for: 

• skippers, owners and operators of recreational vessels 
• diesel heater installers who are installing heaters on recreational vessels 
• Maritime NZ maritime officers, investigators, and technical advisors. 
  
  

Background 

In the winter of 2025, two men decided to stay overnight on their boat in a South Island lake. 

They used a diesel heater to keep the cabin warm overnight, and a petrol generator on the back deck of the boat to allow them to run their satellite broadband service. The next day they were reported overdue and a rescue helicopter was deployed. One man had died and the second was dangerously sick but was able to be stabilised. 

The toxicology report for the deceased showed high levels of carbon monoxide in his blood, which played a significant part in his death. 

Investigation 

The investigation found that a number of factors contributed to the event. 

At room temperature, carbon monoxide is about 53% denser than air, so it will sink and settle in confined spaces like a cabin on a boat. Adequate ventilation prevents such settling. 

It is possible for heaters such as diesel heaters of the type used in this cabin to create a negative pressure situation that draws external air into the cabin. This can occur when:  

• the heater is drawing air from inside a cabin and pushing it through to another space 
• the air pressure in a cabin increases as air warms, causing the total air volume within the space to decrease. 

Testing was conducted to simulate the situation that occurred in Safe Cove to determine if fatal concentrations of carbon monoxide could build up inside the boat cabin. The tests found that: 

• when only the diesel heater was operating, there was no significant increase in carbon monoxide levels in the cabin 
• when only the petrol generator was operating, there was no significant increase in carbon monoxide levels in the cabin 
• when the diesel heater and the petrol generator on deck were both operating, carbon monoxide in the cabin rose to fatal levels in 3.5 hours. 

In this case, the negative pressure created by the diesel heater drew carbon monoxide from the petrol generator along the starboard gunwale shelf and into the cabin via a wire access hole. 

Safety reminder 

In this incident, the combination of a petrol generator running in an open deck space and the use of a diesel heater inside the cabin caused a significant increase in carbon monoxide levels in the cabin. This raises two important safety concerns that boaties should be aware of. 

First, any equipment that generates or might generate carbon monoxide can be a risk. 

Second, carbon monoxide can creep into a cabin from an external space in unexpected ways. 

People installing diesel heaters must be aware of the possible dangers of running them in small spaces at the same time as running portable generators nearby. 

The use of carbon monoxide detectors provides an additional layer of protection enabling early warning of elevated carbon monoxide levels and reducing the risk of undetected exposure.  

People using marine diesel heater installations should consider installing carbon monoxide alarms to minimise these risks.