The dry exhaust system on Dirona is well-built and reliable. The way it works is the stack heads up through the boat in a 5-inch pipe to release exhaust gas at the top of the stack. The exhaust pipe is enclosed in a larger pipe where it passes through the boat. This larger pipe provides an air gap to insulate against heat transfer. A fan at the bottom of the stack assists convection in getting this heat out of inner-to-outer pipe void where it exhausts through a stainless-steel pipe just above the boat deck at the side of the stack. The only downside of this is the exhaust for this cooling air is up over 200F so care needs to be taken near the exhaust pipe, but it’s otherwise a very reliable system and provides very safe temperature levels as the stack passes through the house.
To be extra safe, we measure the temperature in the stack outside of the outer pipe where there is another void between the pipe and the house. If the air in the inner pipe stops flowing, temperatures in this area rise. To nearly preclude risk of fire we alarm at a very low 140F, so we’ll see an exhaust cooling fan failure very early. We look at this as a very reliable and nearly fool-proof system and yet, look at the melted parts in the pictures below. The parts clearly were up over 200F and there was risk of fire. Why didn’t our temperature warning system trigger and warn us of this developing problem?
As a first test, we checked to see if the temperature sensor was working properly. We disconnected the fan and confirmed that the stack getting up over 140F triggers an alarm quickly. Of course we checked the fan and it’s running well. We checked the exhaust and it’s flowing well. Every measure we could come up with of the system had it working correctly and the alarms would certainly trigger if air stopped flowing. And we’d proved that the alarms would trigger quickly if the air stopped flowing.
It appears what happened is the fan, just once, started up running backwards. What we eventually found was the fan capacitor was down below 2.3uF when it was supposed to be 6.0uF. This capacitor helps AC fans start properly and turn in the correct direction.
If running backward, the fan will draw air from outside and exhaust it into the engine room. Pulling air down from outside through the exhaust stack is no less effective at cooling the stack than pushing engine room air out to the outside. Either way cools the stack fine so our fan fault warning won’t trigger.
Since we are now exhausting air into the engine room, the fan ducting is seeing 250F air, rather than engine-room temperature air. Neither the fan, nor the ducting and adapters can withstand those temperatures. We were able to re-assemble the ducting without the melted adapter. Understanding that one common AC fan failure mode is to run in reverse, it would probably be better if the components on the intake side were designed to withstand the temperature that results from reverse operation.
We replaced the capacitor and put a temperature sensor and alarm on the fan itself to quickly detect reverse operation. This is the first fault we have seen of this cooling assembly in 9,500 hours—the fan itself has been very durable and continues in use with a new capacitor.