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post #1 of 42 Unread 1 Week Ago Thread Starter
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Forced air heater duct design

I was down at the boat last night checking on it, (hanging out in the man-cave!) And was having an closer look at the heater installation. I discovered that Jeanneau for some reason, had installed the heater so that it draws all of it's intake air from outside, rather than from the cabin space. This is may be fine if you are just taking a bit of the chill off a late season evening, but when the weather is below freezing as it was last night, it is far from efficient.

I like the idea of drawing 100% fresh air in some situations, but for winter operation I would like to have the option of recirculating the air, so my plan is to install a wye in the return duct with a damper that allows me to switch between the 2 branches. One branch will be fresh air and the other will be return air, hopefully with the option of a mixture of both.

The challenge is, where to draw return air from. The simplest thing would be to install a grille in the aft bulkhead at the back of the aft cabin, but that is not ideal because it will likely increase noise levels in that berth. Also, with the cabin door closed it will not be able to draw air from the rest of the boat. Ideally I would like a return duct that draws from the main salon, but I am not sure there is a route I can run a 90mm duct.

I am curious what kind of duct layout other boats have for their heaters. Is this idea of drawing 100% outside air instead of return air a common thing, or was Jeanneau just taking the easiest way?


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Re: Forced air heater duct design

IIRC its recommended to keep risk of CO poisoning down. That said neither of my boats have ducted intake air. One heater is in a cockpit locker that communicates to the saloon. The other is installed over the engine and while there is a duct into the engine compartment it also communicates with bilge air, etc.

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Re: Forced air heater duct design

Good question....

I have 3" ducts w/ 3 grilles. We sleep in the aft cabin athwarship head to port. The 1st grille... adjustable vanes which can be closed is on the starboard site. Thermostat is forward to port in the aft cabin

The duct has a Y which feeds the aft cabin then continues forward to below the aft end of the port settee... The duct has another Y which carries on thru the bulkhead between the salon and the head with an outlet grille on the port side of that bulkhead.

The heater is mounted below the aft cabin berth midship. It draws air from the compartment where it is mounted which is open to the aft volume below the after end of the cockpit (lazarettes). The aft cabin has a small transfer grille in the aft bulkhead above the opening to the heater compartment. I presume that conditioned air is pulled through the transfer grille as well as air from the lazarettes. That air is not interior condition air... but it's protected and not literally outside air.

The airtronic4 is very quiet and I can just barely hear it when it's on... especially when the fan is not going hard in the initial period as it tries to bring temps up to target temperature. I can hear a faint ticking of the fuel pump.... which is below where my head would be when I sleep. I can tell the heater is running! But it's not annoying.

If there is any wind noise... or slapping waves the sound of the heater is masked. At times it's so quiet I have to put my hand near the exhaust on the transom to see if hot combusted air is being emitted.

++++++

You could (I could as well) add a duct to the return air side of the heater... and run the duct to the grille so it would pull air from the air cabin (easy install)

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Re: Forced air heater duct design

Diesel combustion produces little to no carbon monoxide, so co poisoning is not a concern. Drawing outside air instead of return air would greatly increase the chances of drawing exhaust into the boat, be it from your own heater, or other boats heaters or generators if you are in a crowded anchrage or at a dock.

Unfortunately my Webasto is a bit noisy, although I don't know if it is normal as I have never had forced air heat before. It makes a fairly loud moaning noise when it is running at higher speeds.

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Re: Forced air heater duct design

The combustion air is drawn from inside the lazarette. Mu boat has no diesel fumes smells inside with the heater on... But it does produce diesel fumes at the transom.

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Re: Forced air heater duct design

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Originally Posted by SchockT View Post
Diesel combustion produces little to no carbon monoxide, so co poisoning is not a concern. ...
This is dangerous thinking. In fact, CO incidents in homes due to misfiring diesel furnaces are quiet common. One of my aunts was chased out of her home by a misfiring furnace, and the fire department confirmed dangerous levels. Building codes require that the return duct not be in the same room as the heater or furnace. Years ago I tuned furnaces, and assure you that CO is quite possible. In fact, you tune, in part, by reducing air until you see some CO in the exhaust.

The bottom line is that you need a CO monitor if you sleep with a fired heater in the confined space of a boat cabin. things can go bad MUCH faster than in a home.
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Re: Forced air heater duct design

I have a Webasto also. it was installed by the US Beneteau dealer with a return duct. when I was looking at the Webastco information it shows the unit installed with supply ducts only. they list using outside air as a feature of the heater in the brochure. we don't need to use it very often but I still wonder if the dealer installed it correctly. I like the idea of drawing in clean fresh air even if the heater has to work a little harder

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Re: Forced air heater duct design

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Originally Posted by overbored View Post
I have a Webasto also. it was installed by the US Beneteau dealer with a return duct. when I was looking at the Webastco information it shows the unit installed with supply ducts only. they list using outside air as a feature of the heater in the brochure. we don't need to use it very often but I still wonder if the dealer installed it correctly. I like the idea of drawing in clean fresh air even if the heater has to work a little harder
That is why I would like the option of fresh air or return air. When it is really cold it is very inefficient to heat up 100% outside air. Not only that, but air distribution will be less effective as well. When drawing in outside air you need to vent the same amount of air or you will be pressurizing the boat. Higher static pressure means less airflow. When using return air, there will be better air circulation as air migrates from all areas back to the return intake.

I guess it is an installers choice whether they use outside air or return air. Perhaps it is as simple as how easy it is to get outside air. On my boat the fresh air comes from a hidden opening under the transom step within a couple feet of the heater installation. On boats that do not have a ready place to get fresh air they may have to cut a hole and install a vent hood or Dorade. In that case they may opt for return air, or just draw air from the compartment the heater is located in.

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Re: Forced air heater duct design

A surveyor has told me that current ABYC "recommendations" call for outside intake air, even tho it might seem unintuitive.
(i have been scoping out how to install a small forced air furnace in our boat.)

Note that the exhaust and intake should not be close to each other, and further note that any assumptions about the skill and knowledge of the original furnace installer are often erroneous.

Unfortunately 'tis the way of the world.....

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Re: Forced air heater duct design

Quote:
Originally Posted by hpeer View Post
........while there is a duct into the engine compartment it also communicates with bilge air, etc.
I got a chuckle over "communicates". It's very true. I have heat pumps, which all recirculate air. The units all sit under seating, with grating in front of them to allow for return cabin air. Output is ducted to vents that are placed high in the cabin, presumably allowing for circulation. The intake is on the unit, no duct to this return air grating, so it draws all ambient air inside it's compartment, which is also open to the bilge. If there is anything smelly with the bilge, the heat or air conditioning effectively pumps it throughout the boat.


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