Holding Tank Venting

[Ken Coit]

I checked out the transom vent for Parfait's holding tank and discovered that the through hull was a standard W/C fuel tank vent. I have replaced it with a 1/2 inch bronze mushroom that the insects and salt spray will find more difficult to block. The hole in the transom needed to be enlarged a little and I didn't have the right tool, so it was a memorable installation.

I wonder if anyone has added a second vent to the holding tank and where they put it. Somewhere forward seems preferable in order to get good airflow through the tank. However, you really don't want to fill the tank with seawater, so it needs to be high or at least dry, yet outside. Any thoughts would be most appreciated.

Ken
CD/36 Parfait
Raleigh, NC



parfait@nc.rr.com

[John R.]

I checked out the transom vent for Parfait's holding tank and discovered that the through hull was a standard W/C fuel tank vent. I have replaced it with a 1/2 inch bronze mushroom that the insects and salt spray will find more difficult to block. The hole in the transom needed to be enlarged a little and I didn't have the right tool, so it was a memorable installation.

I wonder if anyone has added a second vent to the holding tank and where they put it. Somewhere forward seems preferable in order to get good airflow through the tank. However, you really don't want to fill the tank with seawater, so it needs to be high or at least dry, yet outside. Any thoughts would be most appreciated.

Ken
CD/36 Parfait
Raleigh, NC

A while back I built a new holding tank for our CD30 and I provided for two vents for cross ventilation. Even with two vents you must make sure they are large enough so they are not easily blocked. They need to be located as high as possible in the tank. That can be a problem because if you install them too high in a tank wall you can weaken the wall or tank seam.

They need to have a low restriction connection to fresh outside air. No sharp turns in the vent plumbing. The tank vent fittings should be easily accessible so they can be serviced periodically; which is next to impossible on the standard CD30 setup from the factory. Keep the tank as empty as possible all the time. A back flush of freshwater through the vent line once in awhile is a good idea but be sure there are no low spots in the vent line that will create standing water and restrict the venting. The vent fittings in the hull should have stainless or monel screens to prevent insects from entering the vents.

[Ken Coit]

John,

Thanks for the info. You somehow or another finally made the "no low spots" directions strike home. I have read that from Peggy Peal several times, but it never occured to me that the reason is that you do not want an accumulation of condensation or other liquid in the vent line as it will block the airflow. Duh!!

I have not installed a screen yet, but I plan to use some. I may try some plastic screening for starters as I have it on hand and it will be easy to secure. How have you secured your metal screen? Will it stay in place inside the through hull without being permanently fastened in some way?

Thanks again,

Ken

I checked out the transom vent for Parfait's holding tank and discovered that the through hull was a standard W/C fuel tank vent. I have replaced it with a 1/2 inch bronze mushroom that the insects and salt spray will find more difficult to block. The hole in the transom needed to be enlarged a little and I didn't have the right tool, so it was a memorable installation.

I wonder if anyone has added a second vent to the holding tank and where they put it. Somewhere forward seems preferable in order to get good airflow through the tank. However, you really don't want to fill the tank with seawater, so it needs to be high or at least dry, yet outside. Any thoughts would be most appreciated.

Ken
CD/36 Parfait
Raleigh, NC

A while back I built a new holding tank for our CD30 and I provided for two vents for cross ventilation. Even with two vents you must make sure they are large enough so they are not easily blocked. They need to be located as high as possible in the tank. That can be a problem because if you install them too high in a tank wall you can weaken the wall or tank seam.

They need to have a low restriction connection to fresh outside air. No sharp turns in the vent plumbing. The tank vent fittings should be easily accessible so they can be serviced periodically; which is next to impossible on the standard CD30 setup from the factory. Keep the tank as empty as possible all the time. A back flush of freshwater through the vent line once in awhile is a good idea but be sure there are no low spots in the vent line that will create standing water and restrict the venting. The vent fittings in the hull should have stainless or monel screens to prevent insects from entering the vents.

parfait@nc.rr.com

[John R.]

Ken,

There are three concerns with all vent lines and one is that low spots can accumulate moisture from whatever the source and become partially or completely restricted. Another problem is that 90 degree angles in the line retard good air flow. The third concern is debris from insects, backflow of solids from a holding tank, corrosion and blockage of screening such as a bronze screen.

Low spots must be avoided in all head plumbing if it is not in PVC piping. Not an easy task in a lot of boats.

The plastic screening you mention will probably suffice if it is not exposed to UV. The UV will disintegrate most plastics fairly quickly. Best if you use the stainless or monel. You'd be surprised how easy it actually is to find screening like that. Check industrial hardware suppliers near you, pool supply companies, plumbing shops, etc. Obviously any good marine supply house will have some. I saw some at Home Depot once, can't remember what department.

Easiest thing to do is just use a fuel vent fitting with the screen already in place. Best ones for this application are the Perko bronze ones that have 1/2" NPT thread on the connection end so you can attach any size hose connection you need. Another advantage of these is that they have a larger diameter interior than the ones with a cast on barb. If your boat is in saltwater it is best to strip the chrome plating from the fitting and use it in it's natural bronze finish as sooner or later the chrome finish will pit and peel, then look like crap. Easier to get rid of the chrome before installation.

John,

Thanks for the info. You somehow or another finally made the "no low spots" directions strike home. I have read that from Peggy Peal several times, but it never occured to me that the reason is that you do not want an accumulation of condensation or other liquid in the vent line as it will block the airflow. Duh!!

I have not installed a screen yet, but I plan to use some. I may try some plastic screening for starters as I have it on hand and it will be easy to secure. How have you secured your metal screen? Will it stay in place inside the through hull without being permanently fastened in some way?

Thanks again,

Ken

I checked out the transom vent for Parfait's holding tank and discovered that the through hull was a standard W/C fuel tank vent. I have replaced it with a 1/2 inch bronze mushroom that the insects and salt spray will find more difficult to block. The hole in the transom needed to be enlarged a little and I didn't have the right tool, so it was a memorable installation.

I wonder if anyone has added a second vent to the holding tank and where they put it. Somewhere forward seems preferable in order to get good airflow through the tank. However, you really don't want to fill the tank with seawater, so it needs to be high or at least dry, yet outside. Any thoughts would be most appreciated.

Ken
CD/36 Parfait
Raleigh, NC

A while back I built a new holding tank for our CD30 and I provided for two vents for cross ventilation. Even with two vents you must make sure they are large enough so they are not easily blocked. They need to be located as high as possible in the tank. That can be a problem because if you install them too high in a tank wall you can weaken the wall or tank seam.

They need to have a low restriction connection to fresh outside air. No sharp turns in the vent plumbing. The tank vent fittings should be easily accessible so they can be serviced periodically; which is next to impossible on the standard CD30 setup from the factory. Keep the tank as empty as possible all the time. A back flush of freshwater through the vent line once in awhile is a good idea but be sure there are no low spots in the vent line that will create standing water and restrict the venting. The vent fittings in the hull should have stainless or monel screens to prevent insects from entering the vents.

[Chris Scheck]

Does W/C mean Wilcox-Crittenden? It sometimes means "Water Closet" on boat plans. You can understand why "W/C fuel tank vent" had me baffled (pardon the pun)

Chris Scheck
RAGTIME
CD33 #117



cscheck@aol.com

[Joe Sankey]

On our CD30 the vent line for the holding tank runs along the starboard hull just below the hull/deck joint. It is attached to blocks which attache to the hull. The adhesive on these blocks is no longer working so the line sags. Could this create the condition you've described which would keep the vent from working? I have had the holding tank pumped, replaced the Y valve and the head, and still cannot dry flush the head to the holding tank. I am hopeful the line to the tank is not blocked. Any comments?
Joe Sankey
CD 30 Slow Dance

I checked out the transom vent for Parfait's holding tank and discovered that the through hull was a standard W/C fuel tank vent. I have replaced it with a 1/2 inch bronze mushroom that the insects and salt spray will find more difficult to block. The hole in the transom needed to be enlarged a little and I didn't have the right tool, so it was a memorable installation.

I wonder if anyone has added a second vent to the holding tank and where they put it. Somewhere forward seems preferable in order to get good airflow through the tank. However, you really don't want to fill the tank with seawater, so it needs to be high or at least dry, yet outside. Any thoughts would be most appreciated.

Ken
CD/36 Parfait
Raleigh, NC

sankey@gulftel.com