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Old 01-09-2019, 12:01 PM   #1
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Diesel Heater exhaust question

We have a Webasto hydronic system on the boat we just bought, with the exhaust coming out the transom below the swim step. Just wondering if anyone with a hydronic or forced air system backs into their slip and if there are problems with the exhaust? I am guessing it will be louder with the sound bouncing off the dock right behind the boat, and I saw the warning about keeping fenders away from the outlet, but any other issues people have found?
Exhaust smell inside the boat?
Soot on the transom?
Starting the dock on fire?


The temperature is in the teens here and the boat is iced in pretty good, so the heater is running quite a bit.
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Old 01-09-2019, 12:04 PM   #2
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We have no problems. The exhaust cools off real quick as it leaves the outlet. Like you said - certainly wouldn't throw a fender over it.
It really doesn't smell much at all. It only stinks during startup because there's a fair amount of unburnt diesel in the exhaust. Once it's cranking, she ain't bad.

Also, no soot problems for us. Ours comes out the side and we can saddle right up to the dock without problems. We're in Boston, so ours puts out on those cold days too.
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Old 01-09-2019, 12:19 PM   #3
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We have a Webasto hydronic system on the boat we just bought, with the exhaust coming out the transom below the swim step.
Does the exhaust have a loop to prevent back flooding in a large following sea?
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Old 01-09-2019, 12:55 PM   #4
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Does the exhaust have a loop to prevent back flooding in a large following sea?

Not much of one. There is some sort of a drain loop at the low point in the exhaust tube that seems to be working OK. We had waves sweep the back deck numerous times when we brought the boat north last month so it can't get any worse than that, and when I opened up the unit to clean it there was no sign of rust or corrosion so I don't think it is really something to worry about.
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Old 01-09-2019, 01:00 PM   #5
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We have no problems. The exhaust cools off real quick as it leaves the outlet. Like you said - certainly wouldn't throw a fender over it.
It really doesn't smell much at all. It only stinks during startup because there's a fair amount of unburnt diesel in the exhaust. Once it's cranking, she ain't bad.

Also, no soot problems for us. Ours comes out the side and we can saddle right up to the dock without problems. We're in Boston, so ours puts out on those cold days too.

Thanks. Ours smokes white a bit on first start-up if it has been shut down for a day or more, but nothing beyond that.
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Old 01-09-2019, 02:10 PM   #6
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I've seen a number of them exhausted out the transom. The only time this was a problem was when they were mounted high and had a swim step that allowed people to get burns when brushing up against them. Otherwise, no issues.

I did see the result of one that had the exhaust out the side and it melted the fiberglass of another vessel it was rafted up to.
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Old 01-09-2019, 02:38 PM   #7
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Just a note of caution about testing you carbon monoxide sensors on a routine basis.
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Old 01-09-2019, 03:22 PM   #8
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Not much of one. There is some sort of a drain loop at the low point in the exhaust tube that seems to be working OK. We had waves sweep the back deck numerous times when we brought the boat north last month so it can't get any worse than that, and when I opened up the unit to clean it there was no sign of rust or corrosion so I don't think it is really something to worry about.
My exhaust outlet is a couple of inches above the swim step. During my major refit we added a more than 2' high exhaust loop in the cockpit, inside an SS enclosure. Not all that pretty but you really do not want to flood the Webasto in a following sea, it would destroy it. If you don't do much offshore boating then you can maybe avoid installing a loop.
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Old 01-09-2019, 04:24 PM   #9
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My exhaust outlet is a couple of inches above the swim step. During my major refit we added a more than 2' high exhaust loop in the cockpit, inside an SS enclosure. Not all that pretty but you really do not want to flood the Webasto in a following sea, it would destroy it. If you don't do much offshore boating then you can maybe avoid installing a loop.

I guess my point was that I didn't think it was necessary.
Worst case is the boat sinks, the heater is the least of your worries.
Next worse would be hours or days of following seas breaking over the stern of the boat and submerging the exhaust. That would be exactly the conditions we saw in the first couple weeks of owning the boat. Those conditions brought to light some deficiencies in how the aft deck drains, but didn't seem to bother the heater at all. If there is a drain in the exhaust line like in ours, it will take care of the water that gets in the line. You would have to deeply submerge the stern for an extended time for the saltwater to be able to harm anything in the heater.


I feel like this is an example of something that got mentioned enough on the internet that people started thinking it was a problem, but in reality nobody has ever actually had this problem. Lot of that going around, especially on boating forums.
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Old 01-09-2019, 05:46 PM   #10
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My loop was put in on the recommendation of a very good yard, who had seen examples of the problem. My engine's exhaust has a backflow preventer, and of course the engines will be running providing some counter pressure to a wave trying to run up the pipe. Imagine a wave running up the beach! But the Webasto will be off if my engines are running, as they provide the heat when running, so the Webasto is somewhat vulnerable.

Sure, the downside of no loop is just a destroyed Webasto unit if its gets a bellyful of sea water, and if it is mounted high in the ER or lazarette then the risk might not be all that high. I did not want to take the risk.
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Old 01-09-2019, 06:03 PM   #11
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My loop was put in on the recommendation of a very good yard, who had seen examples of the problem. My engine's exhaust has a backflow preventer, and of course the engines will be running providing some counter pressure to a wave trying to run up the pipe. Imagine a wave running up the beach! But the Webasto will be off if my engines are running, as they provide the heat when running, so the Webasto is somewhat vulnerable.

Sure, the downside of no loop is just a destroyed Webasto unit if its gets a bellyful of sea water, and if it is mounted high in the ER or lazarette then the risk might not be all that high. I did not want to take the risk.
diesel heater install 101, some sort of a back flow preventer. Or the furnace mounted well above the waterline.
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Old 01-09-2019, 07:52 PM   #12
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We have a hurricane heater, so pretty much the same concept. Our exhaust is on the starboard side maybe a foot forward of the transom. The only other thing I would caution is make sure no dock lines are near the exhaust, having done that once allready, we modify our dock line use when using the heater.
Also, we have a plug for the exhaust outlet pipe to prevent water ingestion but have never used it. Hope this helps.
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Old 01-10-2019, 11:32 AM   #13
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We have a hurricane heater, so pretty much the same concept. Our exhaust is on the starboard side maybe a foot forward of the transom. The only other thing I would caution is make sure no dock lines are near the exhaust, having done that once allready, we modify our dock line use when using the heater.
Also, we have a plug for the exhaust outlet pipe to prevent water ingestion but have never used it. Hope this helps.

Thanks. Our lines are all at least 2' above the exhaust with the swim step between so we should be good.
The first Hurricane I ever installed for a customer, he thought he would be helpful and drilled the hole for the exhaust himself over the weekend. When I got back to the boat on Monday I pointed out to him that he had located it directly below the cleat on the stern quarter. Luckily he knew a good glass guy.....
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Old 01-10-2019, 09:21 PM   #14
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We have a plug for our Webasto, also. I had one occasion when I taped it up - 40+ knot gusts from astern in big seas - before I found the plug in the pilot house.

We have has no issues with the exhaust. The unit itself has provided many opportunities for learning.
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Old 01-11-2019, 12:11 PM   #15
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So this has drifted from issues backing into a slip, to issues with following seas filling the exhaust. I like drifting.


Our heater exhaust pipe comes in through the transom, loops up a couple inches, loops down about a foot before coming back up and in the bottom of the combustion chamber. At the lowest point in the pipe there is a drain like the one pictured below.
Our conditions bringing the boat north last month were such that I know the exhaust was submerged, there were times we had several inches of standing water on the back deck from waves coming in the scuppers located more than a foot above the exhaust opening. The exhaust pipe probably filled each time, then drained out the bottom. There was no sign of rust or water intrusion in the combustion chamber when I opened it after the trip. I would bet that none of the water made it that far, and even if it did it would have to fill the chamber enough to spill over the heat shield to get anywhere near the important parts like the nozzle.
I used to be the guy that read all the advice on being safe and tried to do it all. Now I've become a lazy curmudgeon that enjoys questioning everything and thinking through "fixes" to make sure I only do the absolutely necessary ones. Leaves me more time for drinking coffee and yelling at people.







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