Gas fume smell on my newly acquired Avanti 3485 in the cabin when the AC runs.-gctid396575

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I have just purchased an 89 Avanti 3485 Express Cruiser and am currently doing a complete shakedown. I am getting a stale gas smell when I run the AC in the cabin. It is almost like I am pulling air from the gas tank vent.

I have pulled all the engine room hatches in the cabin and there are no gasoline leaks and all hoses are fine including vent hose.

The fumes are not at explosive levels but are enough to give cabin occupants a headache after a few hours.

Anybody got any ideas?

[Nikko Guru Find all posts View Profile Close Nikko]

In order to produce cold air to feed to the cabin, the AC unit has to draw that air from somewhere. Ideally, that air should be pulled from the cabin space you live in (not just to assure clean air, but also for efficiency). It sounds like the return air is being drawn from a utility space instead, and perhaps from the one that holds your fuel tanks.

Your AC unit will either have a ducted return, or not. If it's a ducted return, there will be flexible ductwork connected to the suction side of your AC blower and run/mounted to a grill (or more than one). Those grills are where the air comes from that eventually turns into the cold air that the AC unit delivers and should be in the living space of the boat.

If you do not have a ducted return, the AC unit will collect the air it needs from the space it occupies. This will put that space in a negative pressure meaning it will draw in the air it needs via the path of least resistance. If that path of least resistance has a fuel tank in it - voila, gas fumes.

If this is the case, you need to do a couple of things. you need to provide a path of "lesser" resistance for the return air to flow to the AC unit. This probably means adding a big grill somewhere in the cabin and ducting it to the AC unit, or to the space that holds the AC unit. And most importantly, you need to seal the "old path" of least resistance - the one that causes your fuel tank "air" to mix with your living space air.

Hope that helps...

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Thanks for the reply.

The odor/fumes are migrating in from my engine compartment. Turning on the blowers for about an hour seems to eliminate the odor/fumes. My AC is under the starboard salon seating next to the head. It has two large inlet grills under the seat one facing the table leg and one facing out into the cabin. The AC unit has the intake on the backside of the AC where the cooling lets the inlet air in. After this air travels thru the coil it is then blown into the supply ducting and enters the salon and the front bedroom. There is plenty of clearance so air coming in the intake vents and inlet air can flow freely around and over the AC unit to get to the cooling coil intake. There is however a crevice at the floor and the back of the AC enclosure facing the head enclosure.

I am wondering if the air from the engine compartment is getting sucked in also, or is just migrating through the cracks and seams and getting distributed into the cabin. Now I can take a can of Instafoam and seal this potential source, but I cant help but wonder if I ran a 1.5 to 2 inch hose from the AC intake plenum out to the side of the boat to pull in a slight amount of fresh air might be a good idea before I seal this thing up. This would have the effect of slightly pressurizing the cabin which might help keep the engine compartment stench out. The only downside might be that the AC would be a little less efficient, having to cool or heat the raw outside air. Maybe not though since I am probably getting a good bit of raw yet odorized air coming in already from the engine area.

What do you think?

[capt._ron Seasoned Member Find all posts View Profile Close capt._ron]

Make sure the return air filter & the coils are clean

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I found a viable solution.

Cold, dense, and dry air produced in the cabin by a heat pump or AC, causes the warmer more humid air to migrate to the cabin even with the best efforts at sealing the cabin (to many little passages on a 35 foot boat). This is similar to osmosis of two dissimilar gasses with different weights. The solution was to put a small blower in the bilge running at about 1/4 of the capacity of the normal bilge blowers. This unit runs on AC power, cannot be heard and runs all the time during warm weather. This keeps odors from the bilge from building up and as a result the cooler cabin has no odors whatsoever. In the winter the warm cabin air wants to migrate to the cold bilge so no fumes are present even with the small blower turned off.

This completely solved my problem.