The check valve should down line from your pump so as to keep "prime" in your pump-line if the pump loses power/ shuts down.

My check valves are horizontal and place as to have the plug drop down (with gravity) for a good seal. One is 2 feet IN FRONT OF the pump, the other 6 feet BEFORE pump.

Personal note: make sure the thing is "pointing" the right direction (most have arrows indicating direction of water flow.)

LP

It can be placed vertical or horizontal depending upon your situation. I have them in both positions.

If horizontal, make sure the hinge of the flap is on the top. Also, use a level and make sure the check valve is level horizontally. If it slopes forward, the flap will not completely close and water will leak through the flap.

My check valves are after my pump but before my bio-falls. This prevents my biofalls from emptying back into the pond if the power goes out.

As LP stated if you need to use the check valve to keep the pump primed, then put the check valve before the pump.

My filtration systems are gravity fed so my pumps stay primed.

By putting the check valve underwater ,do you mean below water level or actually in the water?
Both of mine are above the water level of the pond.

Ok Gorillabuilder you are correct for most of the time. There are two types of checks valves. Swing and spring check valves. Most people use swing type valves so I will address thoses first. Swing check valves work the best in the vertical position. Depending on what the flapper is made out of will determine weather or not the check valve can go under water. If it uses a rubber flapper and a metal weight will work under water better than with a plastic weight in it. Under water a check valve should only be used in the vertical position. There is no such thing as a swing check valve that doesn't leak to some degree. The better ones will drip water thru them the poorer ones will leak faster. The more water that the valve has to hold back the better it will work. A spring valve doesn't use the weight of the water to make it work. Ones that are put under water are called foot valves. Sequence in the past reccommended a foot valve. Most submessible pumps use valves under water.
Mike

I have a 2" pvc with a flap. I don't know what you mean by weight. All I see is a flap.
Ponddoc has said to install it below the water level.
Wetwebmedia crew said it's ok above or below but will go with above to work on it.
Sequence says below.
Dolphin says above.
Mike says if it has a weight? it can go under.
Anyone else want to weigh in.

gorillabuilder,
how about calling the company that makes your pump and ask them?

I did. Dolphin pumps. They said slightly above the water level but what good does their information do when they just make pumps? Sequence makes pumps also and they said below the water. A bunch of other pump makers and pond experts weighed in and my point is everyone is all over the board. Some say below, some say above. They also make a point to make their opinion in bold. Signifying importance.
The only reason I care is I want to have my line exit a hole I have with flexhose and I'd have to install the checkvalve below the water a bit, to make the bend.
I didn't know if I did that, if the water level of the pond would cause issues with the flap keeping the line primed up to the pump.
I'm just trying to find a concensus of opinions. So far, out of like 6 sources, it's split 50/50.

Gorillabuilder check valves use a disc or plate on the back side of the rubber flap to keep the flap flat and to put some weight on the flap to seal the rubber. Some will use a plastic disc and others will use a metal plate. The better ones will use a stainless steel plate. The metal being heavier ussually works better. Now put both under water and now weight of both is lightened. So I would not use either in the horizonal position under water. The key to either working is the weight of the water in the pipe going to the pump. The only weight of water that is put on the flapper is the amount of water in the pipe that is above water level. Than as water leaks thru the check valve the weight on the flapper decreases and it leaks faster. But to answer your question. It will work provided the pump isn't off for too long. By too long I mean the amount of time it takes for the water to leak by the check valve. This is all the water going to the pump and the water going out of the pump that can drain back into the line.
Mike

Gorillabuilder check valves use a disc or plate on the back side of the rubber flap to keep the flap flat
The key to either working is the weight of the water in the pipe going to the pump. The only weight of water that is put on the flapper is the amount of water in the pipe that is above water level. Than as water leaks thru the check valve the weight on the flapper decreases and it leaks faster. But to answer your question. It will work provided the pump isn't off for too long. By too long I mean the amount of time it takes for the water to leak by the check valve. This is all the water going to the pump and the water going out of the pump that can drain back into the line.
Mike
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Mike, ok, yes, I have a plastic 2"check valve with a plastic weight. I will have it in the vertical position. My question is above the water level or below the water level.
Various sources have given me opposite opinions. Should the check valve be above the water level...vertical, or below the water level, vertical.
I have a concrete pond, 3 feet deep so I have the option of installing it vertically, above or below the pond water level.

I wouldn't make a big deal out of it except that I saw various companies that sell external pumps, insist on it being above the water level or below the water level.

Gorillabuilder the answer is it doesn't matter as long as it holds water. What the different pump makers are saying is exactly that. Sequence says below water because they want you to use a foot valve which is a spring loaded valve. The spring loaded valve is a better valve but is harder on the pump. It has to go under water.
Mike

Gorillabuilder,
Is you pump self-priming?
If it is not, do you have a priming pot?

it's not self-priming, and yes it has a priming pot.

I asked a handful of sources and I specified I have an external pump with a pvc flap check valve.

Based on the info you have given and assuming the check valve is being placed on the suction side of your pump, If you are installing your external pump above the water line, you will need your check valve below the water line before your priming pot. If your check valve is above the water line, over time, air pockets can form in the line which can cause you to lose prime.

How much below water level can you place the check valve? Your concern about the water level would be the water level dropping in the pond due to evaporation, etc. is valid if your check valve is not enough below water to compensate for different levels. This is another way air can get into the line. This is why some companies state to place the check valve one foot below water level. Your water level could drop a foot before it causes cavitation to the pump.

You need to keep the run of flex hose to the pump horizontal or sloping down to the pump. You do not want to start the run horizontally and then go up and back down. At the highest point an air pocket will form at that highest level.

If you can monitor your pond water level and keep the check valve always below that level by refilling the pond when it loses water through evaporation, etc, you should be fine.

If you are placing the pump below water level then still keep all lines as horizontal or sloping down towards the pump as possible to prevent air pockets from forming.You would not need a check valve in this situation as the pump would be in a " flooded" situation.

I hope this has helped.

Instead of water level, I should have stated water line as everything I am describing involves external pumps outside of the water- not below water. I do wish we had an edit feature on this site.
Sorry for any confusion.

I went to the Dolphin website and I agree that the website stating to put the check valve just above the water level is confusing.
The only reason I know to put it above is to make it easier to service. If you use a check valve with unions, servicing is easy wherever you put it. If it were me, I still would put it below.

I am sorry but I must disagree with lsst. Almost all non self priming pumps will not be hurt by pumping some air thru them. Since this is on the suction side of the pump there is no need to worry about air pockets. This will only happen on gravity feed or drain systems. Yes it is a little easier to prime the pump with the check valve in the water but the pump can easily be primed with the check anywhere on the suction side of the pump. I would never rely on a check valve for priming a pump in any case. Check valves will fail usually fairly quickly. Once they start to fail you will have major problems priming the pump. That is why I always install a valve before the pump and always close it for priming and for cleaning the basket. The only reason a check valve is used to prevent the pump from losing prime when the power goes out.
Mike

Mike, based on first hand experience I agree that swing check valves leak. I have several installed both horizontally and vertically and when the pump is shut off they do not stay sealed. This was from the beginning so I know crud was not built up on the seal.

I can now see why gorillabuilder is frustrated. After my last post I asked several pond installers here and also did an internet search. I too, am getting almost a 50/50 split. 50% state below water level due to air and 50% claim above water .

On another forum, A well known pond supplier for koi ponds stated "the check valve can be above water level and a swing check valve works better if it is above water level. It gives a tighter seal. It also works better if it is mounted vertically but will work when mounted horizontally but you have to position it so the hinge on the flapper is at the top. They have instructions written on them".

At this point, if I was gorillabuilder, I would install the checkvalve the way he needs to make his bend. I would add unions to the check valve so that if it does not work the way he has it set up he can move and reuse the check valve without having to buy a new check valve.

Isst, I agree that all check valves leak to some degree. The better ones will only leak drips and the cheaper ones will leak pretty fast. Vertically they work better but they still drip. Where they work the best is when there is a pipe full of water that is trying to empty back into the pond, than the vaccuum created by that water trying to exit the pipe pulls the flapper closed tighter and that helps to seal it off. This can't happen when the valve is installed below water. That is why a foot valve should be used there. The spring keeps the rubber seal tight. But I would still install a closeable valve on the intake of the pump. Since most pond installers don't work with external pumps they don't know what they are talking about. Than you look at the fact that most external pumps that are installed above pond level are self priming pumps and now you have less people that know what needs to take place. You need to talk to people that were installing external pumps years ago before low speed self priming pumps to find someone who knows about these things. Having installed over fifty non self priming pumps above pond level they can be installed in either location but work better above pond level but a closeable valve is a must.
Mike

I'm glad Mike got in here because he does have experience with so many setups. It is frustrating to those of us who have experience with only one or at most two ponds to try to decide if information from a single source is good in our situation. While I have only two ponds in action right now and the pumps are submerged I have had reason to put in check valves at one time or another and they rarely prevented what turned into a catastrophic backwash when power failed. I understand that probably wouldn't happen with an external pump but I have to go with what I had at the time. I finally gave up on the checkvalves and redesigned to include a stand pipe. The pipe sucked air and halted any backwash. Why am I bringing this up? It's only to show that not every solution is right for every design. Sandy