Gusts blowing out / downdrafting diesel heater chimney

[Tessellate]

Originally Posted by rphdiego

Why not just install it as designed? Its a 'balanced draft' heater and should have a second 3" flue coming out the bottom and back up to the deck. I sold these new back in the 90s and installed the 120 model in my 30ft sloop. Never had any problems.

That's a bit of an oversimplification, my heater is designed to operate either with or without a dedicated air inlet pipe. The Sig Marine manual briefly mentions an optional air inlet pipe but most of their stack specifications are related to min height, barometric damper and fuel+air mix.
(and it's less common to use an inlet pipe, since it's quite complicated to find space for that on a small boat).

The flue design requirements I'm reading say min stack height inside the boat is 48" (which I just barely make) but they recommend 5-6'. They say straight runs are ideal but 45 degree bends are okay as long as you have 2' straight pipe immediately above the heater (we do). They don't show or mention extensions above deck, but since I'm borderline on the overall stack height that seems worth considering.

The one thing we don't have is the barometric damper (which Sig Marine and Dickinson recommend in all installations). So I think I should fix that first. I've just been skeptical because we already have a healthy draft column afaik (flame above burner ring and no issues when it's not super windy) and I don't see how a slightly stronger draft can resist 20 kt gusts coming straight at the flue cap. We'll see. Thanks for all the input!

[CarinaPDX]

I suspect the problem is a lot simpler than already discussed. Consider that your cabin is basically a closed system, but with air coming in somewhere to replace the air going up and out the chimney. The hot air rising in the chimney creates a small decrease in pressure (vacuum) and in calm conditions that will draw the outside air in elsewhere. A gust hitting the exhaust head increases the pressure, and if the air inlet does not experience the same increase it can lead to backdrafting - hence putting a dedicated air inlet of similar design to the exhaust and in close proximity prevents that, and is why Sigmar designed them that way. More commonly, opening a companionway hatch inside a dodger where the boat is facing into the wind can lower the cabin pressure and cause a backdraft (station wagon effect) and has been an occasional problem with my Sigmar 120 (without a balanced draft system). The solution is to close any vents on the downwind side of things, and ideally rotate a cowl to face into the wind and thus create a positive cabin pressure, which can be improved by the optional exhaust fan. So set aside concerns about the heater for a moment and consider how the air is entering the cabin and whether that increases or decreases the cabin pressure in gusts.

Greg

[ussvdharma]

Suggest you obtain one of the wind driven rotation fixtures for the top of you Charlie Noble. I had same problem and this cured. OR is your Charlie Noble to short? That also can be a problem

[01kiwijohn]

We have a Dickinson, cruising Alaska and the PNW.
The two 45 bends haven't been a problem. We don't have a baro damper either.
Do you have enough air getting into the boat? Not enough air coming in will reduce the draft. We have a 10" x 8" vent in the hatch boards, under the dodger, which is always open. Often, we'll open windows,or the hatch, to reduce the inside temperature. We also have an 18" chimney stack ( offcut) that we just slip into the stack when we're anchored.
Had it blow back just once? In 10 years.

[Tessellate]

Originally Posted by CarinaPDX

I suspect the problem is a lot simpler than already discussed. Consider that your cabin is basically a closed system, but with air coming in somewhere to replace the air going up and out the chimney. ...

Yes, that is something discussed in the manual, and I always forget to test creating more positive pressure. That is a difficult thing to do to run around turning your dorades to different angles to capture the gusts though. As I mentioned at times we'll have no wind and then the next moment a 15-20 kt gust, from several possible directions in a 180 degree range. Sometimes current is contributing to turning our boat beam on to the wind.

We're not creating negative pressure by keeping the companionway open (since we're trying to heat the boat). We have 4 dorades which are always providing air into the boat, but it's difficult to turn them into the wind when the wind isn't coming from the bow or any consistent direction. Opening a hatch window may be a better bet, but we have done that at times and I'm not sure if it helped or not.

[Sailormantx]

Something I haven't tried yet: a computer fan or even a decent sized engine room blower blowing in from one of your ventilators. That would help in maintaining a positive pressure inside regardless of wind direction relative to the ventilator cowl. The downside is increased electrical consumption, a big engine room blower will be several amps.

[sailor1924]

I have a Fab-all Sigmar 170 heater.
I used to experience back drafting problems; burner explosions, noxius cabin fills and blow outs.
I have been using the Vacustack for three years in the Northeastern US, and been through several storms with high winds and very high gusts the Vacustack has completly stopped downdraft issues. I was hit broadside while tied to the dock and heeled over 15 degrees and no blow out or back puff.
I also have the factory draft assist fan which is only used to start up the heater and once the flue heats up is shut down the fan alone could not overcome downdrafts.
This year I installed a seperate 3" air inlet because I want to improve the combustion characteristics of the burner and not introduce cold air below, not because of downdrafting problems.
I am very satisfied with the Vacustack and recommend it.

I am a satisfied customer and have no affiliation with the company or any dealer.

[Dickinson_Tech]

Originally Posted by Tessellate

Thanks. I'm not sure I understand the issue with the diagonal bends - seems counterintuitive. If anything I'd expect the angles to improve the deterrence of any gusts making their way down the pipe. If that's not the case, perhaps it's because the bends decrease the strength of the draft, by making it take longer for the hot air to rise (possible cooling more in the process)?

Of course, moving the hood for a straight run is not easy. A prior owner installed it, and although it looks like a straight run would be possible from the interior, I think on deck that would put the cap so close to centerline / boom that it might interfere with the boom vang.

I can try a different hood, extension above-deck and barometric damper. (knowing which of those is most effective would help since these parts add up in cost quickly! Getting all 3 will be over $250)

Hi Tessellate,

I'm one of the technicians at Dickinson Marine. I came across this post and thought I'd chime in with my 2 cents.

The bends in the pipe do make it somewhat harder for downdrafts to rush down the chimney, but by the same token they inhibit the upward draft that is the most effective way of keeping wind out. Ideally I would reconfigure the chimney to a straighter run but I understand that's not always possible.

Lengthening the chimney above deck would be a good way to increase draft, although it doesn't sound like this will be entirely practical in this case either. Any chimney sections over approx 1' above deck would likely need some tethering against the wind making it harder to take on & off as required.

Running the fan whenever there is the risk of downdrafts would be one easy way to strengthen the draft, although this will increase fuel consumption somewhat and you may need to run the heater a touch higher than normal to achieve the same heat inside the cabin.

As others have already mentioned a barometric damper would help as well, as it draws air that would otherwise have to pass through the burner directly into the chimney and thus maintains a consistent draft. The barometric dampers work best installed in the first 1-2 feet of the chimney pipe above the heater.

I'm not familiar with the VacuStack caps so I can't say whether or not they'd work with our system, but it might be worth a try as a last resort. I'd make sure to install a barometric damper too though, as otherwise the amount of air passing through the burner would likely be difficult to regulate.

You can always contact me on here or by email if you think I can be of any help.

James

[mvweebles]

I too had a Sigmar 170 with down-draft issues in breezy San Francisco. I lived aboard and tried extending the pipe to a total of 3-feet above deck-level for a total of 7-feet of straight pipe; adding a blower fan from Sigmarine into the combustion chamber, and the H-Cap. None worked consistently. Only thing I didn't try was Sigmarine's recommendation to rig a second pipe to feed the burner to balance the air - I simply was not willing to install a second 3-inch stove pipe in my little boat.

For anyone who has not had a diesel heater blow-out due to down-draft, it's a scary experience. Diesel continues to stream out but not ignite, but it does vaporize. Makes a smelly mess. Pretty dang dangerous too.

I wish I had a solution for you. I liked the heater a lot when it was just cold but not windy. But I would not leave it running at night or when I was away from the boat.

Peter

[Tessellate]

Just wanted to follow up on this for future reference: adding the barometric damper seems to have completely solved the issue! (without needing to switch caps or extend the chimney higher)

We've had the heater on in 25 to 30 knot gusts and it didn't downdraft or go out. One thing I didn't understand about the barometric damper is it seems to actually deflect the pressure of the gusts.

When the gusts hit our chimney stack, the damper opens wider, giving a path for the air to exit without blowing out the heater. (this may not be the correct way to describe it - another way to put it would be that the damper dynamically adjusts the draft, and when you have a gust providing too much air to the combustion stack, it opens wider which reduces the draft).

Anyway, barometric damper is indeed important.