Our boiler's backflow preventer valve started to leak out of the vent around Thanksgiving. I read that it may have to do with system pressure and it may stop, which did stop dripping after a few days. Then around Christmas we have all 3 zones going with family in town and the dripping started again. I have a Solo cup under it and after 3 weeks or so the cup is full.

My question is, at this point is the issue probably either a old valve or crud effecting the valve inside? If that is the case, I would probably rather just spend the money on the valve to replace it unless it is not too time consuming to try and clean. What suggestions would you have for me? Thanks


This can happen if the pressure difference goes the other way, like if the water feed is shut off. Some common models like the Watts 9D-M3 have rebuild kits available, but they cost almost as much as a replacement valve, which is $30-35 in the US. (I just checked that at one of the big box stores here)

So the answer is replace it, though a plastic cup after 3 weeks is pretty slow. Note that since this particular model has (threaded) union connectors, if you get the same model as a replacement, you can swap it in without soldering.

Watts 9D-M3 back flow prevention valve

  • I wanted to verify that there isn't any damage being done by the constant leaking water. Is this correct? Obviously, I will replace the valve but I just wanted to make sure if I can't get to it for a few months that no damage is being done to the system. Thanks. – junta Jan 17 '18 at 20:08
  • Right. No damage if the water is contained. – Tim B Jan 18 '18 at 16:08
  • As of today, the leak has stopped. So it seems to come and go, but living in this house for the past three years, this winter is the first time there has been leaking. I understand that the pressure changes can cause this, but is the valve starting to fail and this is why the leaking is on and off? Is the best thing to just keep a bucket under it to catch any drips? Thanks again. – junta Jan 24 '18 at 2:48
  • Mine started leaking after I cut the water supply and drained the boiler to replace a gasket. Since it eventually stopped and I know what triggered the leak, I put the replacement I bought on a shelf and will replace it later. If it was leaking with no event to trigger, I'd just go ahead and replace it. It's not very expensive, and might be able to be replaced with just wrenches, as in the Watts model above. – Tim B Jan 25 '18 at 2:38

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