Backflow preventer valve going off when shower is turned on

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Old 09-07-07, 11:20 AM
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Angry Backflow preventer valve going off when shower is turned on

I have a backflow preventer valve before the hot water heater. When hot water in one bath is turned on the backflow valve goes off. I have also noticed that the cold water pressure in this bath is very poor, even though it is connected to the same pipe as another faucet. Can it be that the mixing valve for this bath is not working properly? Why would this cause backflow preventer to go off?

Also every time backflow preventer goes off I have to open it up and push the cilinder back in order for it to stop leaking. Is this how backflow is suppose to work?

Thanks!
 
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Old 09-07-07, 04:41 PM
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I'd say you have a main cold water line issue where it is clogged with rust perhaps (clogged on the incoming side from the backflow preventer - and when downstream water is turned on, you create a pressure drop at the backflow preventer which causes the leak. Backflow preventers behave this way. You must not have enough pressure inside the device to maintain the needed pressure inside the device.

I am presuming that your hot water pressure is also poor? And is pressure poor everywhere or just in isolated fixtures so I can reassess what I have just said.

Hmmm. Re-reading your post I am of the opinion the bath sink faucet pressure is good? If so, maybe the discharge of the bathroom faucet does not yield all that many gpm's even though the pressure appears good due to behavior of an aerator. In other words, you could be deceived. Does the backflow preventer weep worse when the tub is running or the shower? Try this test: Run the bathroom faucet hot and cold at same time while you catch it in a container (like an empty quart of gallon jug) where you can do a timed test. Then do same test with tub on 50/50 hot/cold full volume mix, and then also do it with the shower. Then let me know which of these fixtures puts out the most volume of water in the same amount of time.

Is the cold water line that comes off the main line in the bassement, before or after the backflow preventer? Also, what about other fixtures like cold water on kitchen sink and also toilet(s).
 

Last edited by ecman51; 09-07-07 at 04:52 PM. Reason: Added last 2 paragraphs
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Old 09-07-07, 07:19 PM
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ecman51, thanks for a quick reply.

When I take the backflow preventer out there is no rust or any debree at all. Al piping in the house is brand new.

So far this has happened only when using one particular bathroom fixture, and it happens when switching from tub to the shower. This fixture has a very poor water pressure on cold when the hot water is turned off. I have another faucet running off the same line and it has great pressure.

I did have trouble with this fixture before. It seems as if it creates an air pocket and the pressure is terrible. This is brand new fixture as well. The tests you described cannot be ran because the faucet is a single lever with constant pressure and I am only able to adjust the temperature. I wish I would have paid more attention when buying this junk. I will try to run various tests on it and let you know what I find.

What about the backflow preventer - is it suppose to close back up, or will it remain open until it is fixed? This seems a bit strange. In case it happens and I do not notice it, it could create a huge mess!

Thanks for the help!
 
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Old 09-08-07, 10:30 AM
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If the tub/shower water is turned off, doesn't the backflow preventer stop that weeping out the middle drain hole?

I need to review your both posts to respond or give any other ideas, in light of fact I now know you have good new pipes and something equivalent to a Moen Posi-temp mixer valve.

But something that comes to my mind already is that you may want to remove that cartridge and flush out the hot and cold lines feeding the mixer. If you need help with this procedure let me know. This way you will know if blockage is in either line, or both lines or if it's in the mixer valve/cartridge itself. We have to get your pressure back to norm before we can solve your backflow preventer issue, is my guess.
 
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