Holding Tank Solar Vent

[51RD55]

Peggie, Being in total agreement with the fact that the holding tank needs oxygen to produce aerobic decomposition, my husband has an idea that he asks your opinion on.
The existing vent to our holding tank on our 31' Hunter is the usual 3/4" hose. He would like to run a second vent to the top of the holding tank with a hose running through my anchor locker to a solar vent. Of course he would have to adapt the solar vent to accept this hose. The idea here is to actively bring air into the tank and have it passively exhaust through the existing 3/4" vent. Is this a sound idea? If this concept a good one he has the resources to put it together? Thank you very in advance and look forward to your response.

 

[Peggie Hall HeadMistress]

First one bit of information that can come in very handy when you buy hoses: It's the INNER diameter that determines hose sizes (fittings sizes are always stated as the OUTER diameter, btw). And the standard size for vent lines--ALL of 'em, fuel, water and waste--is 5/8". So if yours really is 3/4" hose...you haven't made the same mistake a LOT of people make by assuming the outer diameter is the size, it means that a PO had to have either specified 3/4" when he bought that tank or replaced the original vent fitting with a larger one.

Now to answer your question: It could work, but 'twould work a lot better if the fan PULLED air through the tank instead of pushing it. If the line going forward is a 1" line no more than about 5 feet long, you wouldn't even need the fan OR the second vent line because your head would always be into any breeze when you're underway or on an anchor or mooring. However, whether either solution will work depends on how long and straight the vent line(s) would be. If ya'll would like to discuss in more detail than is practical in a forum, give me a shout.

 

[thinwater]

a. I suspect the holding tank gases will kill the solar fan in short order. This is why engineers always blow (instead of suck) in cases like this where corrosive gasses are involved. Additionally, explosive gases could lead to an explosion; pulling with an electric fan from a holding tank (only explosion-proof components are allowed in the holding tank, and an attached hose is considered to be in the tank) is forbidden by code (risk of methane explosion). So a savvy surveyor would flunk the solar fan as a serious hazard.

b. Suck vs. blow. Picture the tank, with 2 small openings. For every cubic foot that enters, one cubic foot leaves. I can promise you, based on a career doing designs involved with gas flow, that there is no difference in flow patterns, whether you pull or push in a tank ventilation example. How does the end of either hose "know" which end the pull is coming from? It does not. There is only a pressure differential. "Pull" in only different when there are multiple poorly sealed openings, like a house.

 

[Peggie Hall HeadMistress]

Weeellll...I've known a lot of people over the last 25 years who've installed double vents with exhaust fans...typically a computer "muffin" fan...haven't heard of it causing any explosions yet. Maybe because methane is an anaerobic gas which isn't being generated in an aerobically maintained tank....???

As for pulling vs pushing.... In the days when houses had whole house, aka "attic" fans (my previous house had one, this one doesn't and ohhh how I miss it!), when you needed to air out smoke from the burned beans, you opened a couple of windows and the fan pulled fresh air through the house and up into the attic. Most houses are at least as well sealed as the average marine sanitation system with a toilet on which the joker valve hasn't been replaced in years and vented loops with no air valves.

However, you may have a point about gasses, especially from urine, corroding the fan...

 

[thinwater]

Weeellll...I've known a lot of people over the last 25 years who've installed double vents with exhaust fans...typically a computer "muffin" fan...haven't heard of it causing any explosions yet. Maybe because methane is an anaerobic gas which isn't being generated in an aerobically maintained tank....???

As for pulling vs pushing.... In the days when houses had whole house, aka "attic" fans (my previous house had one, this one doesn't and ohhh how I miss it!), when you needed to air out smoke from the burned beans, you opened a couple of windows and the fan pulled fresh air through the house and up into the attic. Most houses are at least as well sealed as the average marine sanitation system with a toilet on which the joker valve hasn't been replaced in years and vented loops with no air valves.

However, you may have a point about gasses, especially from urine, corroding the fan...

I agree that it is very unlikely, but I am sure about the code. No electrical devises that are not explosion proof in a sanitary waste tank. Same code on dry land too.

The best answer, which you have expressed so clearly, so many times, is large natural ventilation lines.

 

[Peggie Hall HeadMistress]

The best answer, which you have expressed so clearly, so many times, is large natural ventilation lines.

True in most installations...but on large boats on which a tank is sitting on the keel in a 3' deep bilge, 6" diameter vent lines wouldn't provide sufficient passive ventilation because they'd have to rise too steeply and too far, and neither anaerobic nor aerobic gasses rise....they lay like a blanket on the surface to be pushed out the vent by incoming flushes. In those situations, some kind of mechanical intervention becomes necessary. Aeration does work when installed, operated and maintained properly, but everyone is looking for something simpler that requires less maintenance.

 

[51RD55]

Thank you very much Peggis and thinwater. I'll follow the advice in your book Peggie and stay clear of installing a solar vent. Have a great weekend!

 

[Ron20324]

That's not to say you still could not use a passive vent, but still may not be worth the effort. I put a 1.25 hose on my tank, and it is only about 20 inches long. Good improvement.

 

[Gunni]

Ron; did you also change out the tank and thru-hull fittings to give you a full 1.25" bore?

 

[Ron20324]

Ron; did you also change out the tank and thru-hull fittings to give you a full 1.25" bore?

Yup. Not easy, but not hard, eiher.

 

[Peggie Hall HeadMistress]

Install a new tank fitting using a Uniseal The UNISEAL Warehouse - Home of the Original Uniseal and 6" piece of PVC that becomes the hose fitting. Seal the original fitting with a threaded plug wrapped in plenty of Teflon tape (both are available from any hardware store). Replace the "vent" thru-hull with the larger size open bulkhead or "mushroom" thru-hull.

 

[Gunni]

Rgr. Thanks