Caleffi backflow preventer cutaway
July 25, 2014

What causes nuisance dripping of backflow preventers and how is it correctable?

We get quite a few calls about and returns of perfectly good 573 Series backflow preventers because double-check atmospheric vent backflow preventers are not fully understood. Usually they have debris/dirt buildup in the seals, which is easily corrected by cleaning. If a picture is worth a thousand words, is a video worth a million? Bob “Hot Rod” Rohr gives you a look under the hood on how a double-check atmospheric vent backflow preventer works.

Categories in which appears
Blog post comments
Dan Steinhauer
January 30, 2016

I have this backflow preventer on my boiler system, it was installed about 16 months ago, the vent port has been dripping since day 1, I contacted installer and he told me that it should quit on its own, however, now it is a steady stream, installer just says, well it's doing its job, but I am wasting all that water, the vent should be an air vent mostly, right? I have taken it apart and have found no debris and it works great when disassembled, once I put it back together it sticks, it almost seems like it is getting hung up on the walls of the unit. We do have have pretty weak water pressure in the house, I am wondering if maybe it not pressing the seal all of the way, any other thoughts or ideas would be appreciated, thank you!

February 02, 2016

Dan, it sounds like your weak water pressure, compared to the pressure in the boiler system, could be the reason for the dripping. If you close an isolation valve downstream of the backflow preventer, and bleed off the pressure, does the dripping stop? In other words, increase the pressure differential across the backflow.

April 06, 2018

How long should a back flow preventer last and did the replacing of the pressure relief valve cause contaminants to flow into back flow preventer causing it to now leak?

Greg Tubbs
April 23, 2018

Any demand of water flow through a back flow preventer can cause debris to be carried into the valve body. No matter if you are flushing a system to remove air or if you have an expansion tank fail and the relief valve trips. What causes the double check to float is when there is a pressure fluctuation on the incoming supply. This will allow the checks break contact just long enough for debris to get wedged in the narrow sealing surface and the valve leaks through the vent. The longevity of a back flow preventer will always depend upon the water quality and weather or not you have problems with water hammering. The checks can be removed and cleaned most of the time but, it takes time to take the valve apart, clean it and re-install it. For most it is less time consuming to just replace the leaking valve.

November 18, 2018

I have a, Conbraco Sprinkler Backflow Preventer, Size 1", Mod# PVB, Ser# VP728. Before I had chance to blow the water out of the lines during an unexpected frost the outside Backflow started gushing water.
I hoping to find a repair kit since after searching the web high and low I could not find my same model. I live in Woburn, Massachusetts and have searched Loew's and Home Depot for these parts. I might have to replace the entire Backflow. I've included an attachment, displaying my unit.
Is there a Watts Regulator Backflow repair kit that I could replace these worn out parts or an entire unit?

Cody Mack
November 21, 2018


I would suggest contacting Apollo / Conbraco in regards to repair parts for your application.

Cody Mack
Caleffi North America

Scott Kronwall
January 14, 2019

I have a 573 series back-flow with pre-adjustable filling unit on my boiler system. Our future retirement home is in sand country in Northern Wisconsin where this is located. Last weekend I cleaned out the pre-adjustable unit and it was quite full of crud. My guess is that the back-flow is also needed to be cleaned. One of the videos I watched suggested replacing the gaskets. I currently live in the Minneapolis-St.Paul area, where can these gaskets be acquired in this area? Look forward to any information that is available.

January 21, 2019

Hi Scott,
There are a number of wholesalers in your area; we suggest you contact our local Representative Midwest Sales & Marketing at 1-800-246-8440 to find out the most convenient location for you.

January 17, 2020

Is it possible that debri build up within the back flow valve can cause boiler water to be leaked into the cold water supply under heavy water usage, causing the boiler to slowly lose water pressure ?


In reply to by Mitch

April 21, 2020

It is possible for debris to get into the seats of the check in the back flow and water would drip out of the vent port of the backflow preventer. Water would not flow backwards to the domestic side. If you are losing pressure due and you see water coming out of the vent port than it is a sign there is debris in the check valve. If you don't see water coming from the vent port than it could be a leak within the system or a failed expansion tank.

March 26, 2020

I have this 573 backflow preventer but it is leaking at the unions (checked with dry hands and always wet). The guy who fitted it just keeps applying more tightening torque but to no avail. I get sudden puddles (will now check to see if it is from the valve) and a constant drip from the unions. I cant have this leaking non stop as I am wanting to finish my basement. Are there any suggestions?


In reply to by Frank

April 21, 2020

The union connections have the fiber washer that is designed to provide seal to prevent leakage. It is important when installing them to not overtighten them or you will crush the washer and it will not seal no matter how tight you get the connection. The replacement washer part number is R50065. I would order two new sealing washers and then separate the unions and remove the old washers, wet the new washers and install them tightning the unions hand tight. Once hand tight you can give them another 1/2 to 3/4 turn using a wrench to snug the connection. Do not over thighten them. Evaluate the connection and snug up as necessary but it is important to not crush the washer. If you have any questions call Caleffi tech support anytime.

December 19, 2020

This video was very helpful to understand what the funcion is. Last night we had to shut off main water supply line because a lot of water was gushing out through vent. I am attaching a picture. Do you think cleaning the valve will solve my problem?


In reply to by YAIMA PEREZ

December 30, 2020

Yaima, Cleaning the valve may solve the issue if it is caused by debris in the check valves. This product typically will not vent water unless there is a surge in pressure or water hammer that causes the product to perform its design function, to prevent water flow in the incorrect direction. You will want to look for issues in your system that caused this to occur.