Exhaust Cooling Fault

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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.

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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?

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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.

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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.

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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.

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7 comments on “Exhaust Cooling Fault
  1. Hector Cyre says:

    James; Just wondering – why an AC fan on a boat with tremendous DC capacity – and, I presume, an alternator or generator putting out plenty of power any time the engine is running? Could the solution be to change to a DC fan, eliminating the issues associated with AC fans?

    • jan-kees says:

      on our barge we use a 240 V fan, because of a following
      1) the cost……and AC fan is 1/2 to 1/3rd of the cost of a DC fan
      2) larger capacity with an AC fan
      3) with the fans I use ( and am installing on two friends boats) we can set the speeds to 3 settings, making sure that the Delta T never goes over 25F

      • Jan covered most of the reasons why we chose to go AC for our engine room fans Hector. However, if do decide to go AC, then you need some backup AC source should the primary system fail. On Dirona, we have 120V inverter, backed up by the generator, backed up by a 240V inverter with a transformer that can supply the 120V system so we are fairly well covered. Another advantage of an AC fan is they last very well compared to comparably priced DC fans. We currently have 9,800 hours on the stack fan and it ran 9,700 hours with no service. I’ve now changed the capacitor which is around $5 in single unit purchases and, it’s otherwise likely got a few thousand more hours in it. DC fans tend to fail far earlier and lower priced DC fans can be very problematic.

        Any fan can have failure modes so, regardless of what you use, it’s wise to have an over-temperature alarm. The configuration we have is an over-temp alarm in the outer portion of the stack to detect fan failure or cooling system blockage. We also have an over-temperature alarm on the cooling fan to detect reverse operation. This latter wouldn’t be required on a DC fan but is probably a good idea in any installation.

  2. Paul Wood says:

    Hi James, is it possible to wire in a sensitive (differential) air pressure switch to detect reverse airflow which might give peace of mind.

    • Certainly something has to be done for peace of mind and it needs to be tested to make sure it works. You suggestions sounds workable to me but I ended up thinking that a temperature sensor on the fan outlet was easier and cheaper and perhaps more accurate. On circulation fans, the difference in head between working, off, and even operating in reverse are measurable fut fairly slight. Temperatures is cheap to read and the differences between reverse and forward are likely around 100F so it’s easy to detect. The way I have it set up, if the fan exhausts goes up above a very low 150F, we alarm. I tested it and, if the fan is running backwards,it alarms almost immediately. Of course, I changed the capacitor as well.

  3. Steve Cellini says:

    “Understanding that one common AC fan failure mode is to run in reverse…”

    What?? ;) Who knew.

    (Thanks for enlightening us!)

    • Yes, the way many AC fans work is two opposing stator windings are connected directly to the alternator current source The other two stator windings are each 90 degrees from the first two and they are connected to the alternating current source through an capacitor. The capacitor shift the AC current waveform 90 degrees so all four windings “pull” the armature around in the same direction and the AC motor starts up reliably turning in the right direction. If the capacitor fails or get weak, the motor may sometimes start running the other way and, if it does, it’ll happily keep running that way.

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