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Old 10-04-2009, 08:38 AM   #1
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Backflow Preventer - M2 or M3?


So my Watts 9D backflow preventer valve started leaking out the vent. My question is, how do I know which kind to replace? I found a Watts BFP at the big box store, but it seems too small. I see online there are now M2 or M3 versions, mine just says 9D on it. Any way to see which size I need? I think maybe 3/4", only because the 1/2" size seems like the ends are too small - the nuts that screw on either side of mine are > 1".

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Old 10-04-2009, 08:53 AM   #2
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Backflow Preventer - M2 or M3?


There is a DIY plumbing forum at the top of this page, post there.

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Old 10-04-2009, 09:12 AM   #3
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Backflow Preventer - M2 or M3?


sorry, figured since it was on my boiler it fell under HVAC, thanks.
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Old 10-04-2009, 10:22 AM   #4
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Backflow Preventer - M2 or M3?


We have a couple boiler gurus here, check back later.
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Old 10-04-2009, 10:49 AM   #5
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Backflow Preventer - M2 or M3?


The nuts. Will be about 1" even on a 1/2" BFP.

You want the vented model. The 9M is not vented.
Before changing it out though. make sure it is defective or leaking without another reason.
Also, most are rebuildable.
I seldom do it because its quicker to change it out on a service call.
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Old 10-04-2009, 11:06 AM   #6
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Backflow Preventer - M2 or M3?


Thanks. I went and got a 1/2" cuz that's all the big-box store had, wanted to just get one for size, and it's too small, like i thought. I ordered a 3/4" online, should fit hopefully.

The original was vented. Maybe it just said 9D because it was very old? Not sure.
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Old 10-04-2009, 12:20 PM   #7
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Backflow Preventer - M2 or M3?


9D is vented. 9M is not

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