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Old 20-07-2018, 19:38   #1
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Condensation Siphon

On my recently purchased boat I discovered the condensation pan for the master cabin HVAC uses a siphon system connected to the seawater discharge line, without a secondary path in the event of failure (!!!). I thought, "well that's going to suck when it fails ... add it to the list of things to fix".

It failed before I fixed it. Woke up to a growing puddle of water on the floor, draining into the bilge.

We tore everything apart to clean and inspect the parts and discovered the 1/4" one-way spring loaded ball check valve was letting only a microscopic amount of water through. I'd enter this thing into an RO watermaker competition if I thought there was such an event.

We reassembled the system sans check valve and cycled the system a few times to see if we got any backflow. The siphon line was already the high point in the loop, and we only got a couple tablespoons worth of water back into the drip pan when the pump shut off.

At a glance I'm thinking we're two thumbs up running without the check valve. But it's a $5.50 part from the manufacturer ($12.20 via Amazon!?!), so why not just replace it - the hose parts are already cut and ready to go. It's literally a drop in ...

What do the seasoned marine plumbers think?
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Old 21-07-2018, 10:43   #2
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Re: Condensation Siphon

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Originally Posted by LoudMusic View Post
On my recently purchased boat I discovered the condensation pan for the master cabin HVAC uses a siphon system connected to the seawater discharge line, without a secondary path in the event of failure (!!!). I thought, "well that's going to suck when it fails ... add it to the list of things to fix".

It failed before I fixed it. Woke up to a growing puddle of water on the floor, draining into the bilge.

We tore everything apart to clean and inspect the parts and discovered the 1/4" one-way spring loaded ball check valve was letting only a microscopic amount of water through. I'd enter this thing into an RO watermaker competition if I thought there was such an event.

We reassembled the system sans check valve and cycled the system a few times to see if we got any backflow. The siphon line was already the high point in the loop, and we only got a couple tablespoons worth of water back into the drip pan when the pump shut off.

At a glance I'm thinking we're two thumbs up running without the check valve. But it's a $5.50 part from the manufacturer ($12.20 via Amazon!?!), so why not just replace it - the hose parts are already cut and ready to go. It's literally a drop in ...

What do the seasoned marine plumbers think?
Leaving that line without a check-valve would be unwise. Unfortunately, however, they are prone to failure due to rusting of the springs. Replacements are available at US Plastics. US Plastics also has diaphragm type valves without springs that would work as well. Both are inexpensive. BTDT...
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Old 21-07-2018, 13:45   #3
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Re: Condensation Siphon

We had one of those on a prior boat. It's amazing how much water an air/con unit creates. Took out 5 gal out of the bilge when it failed one time.

Make sure to clean the drip tray periodically along with the tubing. We had it clog with crud and stop sucking up the condensate.

If we got stuck with another one, I would drill and install a secondary higher outlet and lead tubing to a small pan. In the pan, I would put a moisture sensor....but really, I wouldn't want that system again.

PS: if it's the $200 mermaid condensator, you can replace it with a drastically cheaper garden hose venturi siphon for a few bucks if it fails.
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Old 21-07-2018, 13:47   #4
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Re: Condensation Siphon

Little blue pump
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Old 21-07-2018, 14:34   #5
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Re: Condensation Siphon

I installed a Venturi unit(pond aerator) in the water line after it leaves the AC unit. No moving parts, sucks the tray dry.

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Old 21-07-2018, 16:06   #6
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Re: Condensation Siphon

That's why check valves should never, ever be used regardless of whether it uses metal or plastic parts. A properly maintained Venturi drain system will never fail except when you don't remember the 'properly maintained' phrase.
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Old 22-07-2018, 12:37   #7
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Re: Condensation Siphon

Thanks for all the great information!

It does appear to be the mermaid product, installed by previous owner. I think the short term solution is to pop in a rubber check valve and figure out how to improve the system in the mean time.
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Old 23-07-2018, 06:41   #8
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Re: Condensation Siphon

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A properly maintained Venturi drain system will never fail except when you don't remember the 'properly maintained' phrase.
That can be said for just about anything...if you don't plan to clean/inspect it every few days, don't be surprised when it clogs and you get water overflowing.
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Old 23-07-2018, 07:21   #9
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Re: Condensation Siphon

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That can be said for just about anything...if you don't plan to clean/inspect it every few days, don't be surprised when it clogs and you get water overflowing.
In a climate (Florida and the Caribbean) where it couldn't be any more favorable to clogging, i typically clean the pan and drain system annually and I've NEVER observed any potential problem. In fact, the only problem ever experienced was a result of a clogged and malfunctioning back flow valve. With them, the only uncertainty is WHEN, not if, it will fail.
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Old 23-07-2018, 07:53   #10
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Re: Condensation Siphon

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In a climate (Florida and the Caribbean) where it couldn't be any more favorable to clogging, i typically clean the pan and drain system annually and I've NEVER observed any potential problem. In fact, the only problem ever experienced was a result of a clogged and malfunctioning back flow valve. With them, the only uncertainty is WHEN, not if, it will fail.
I guess you were luckier than we were.
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