Backflow preventer issue

[Teppei Iharada]

I’m having an issue with my back flow preventer that I’m hoping someone can help me out with. I installed my sprinkler system last year. My main line to the house is 3/4” but I bought a 1” PVB (conbraco Apollo) because the poly pipe in ground is 1”. The issue I’m having is when my sprinkler turns on I hear a click and water starts spewing out from the back flow preventer. It then takes about 30secs or so and the water will stop coming out from the cap and then the sprinkler system will start working properly. I have 9 zones in total and when I test out the system it’s almost guaranteed that the back flow will trip and water starts coming out from the cap especially at zone 1. What do you guys think the issue is. Do you think it’s because my main line is 3/4” but the back flow is 1”?

 

[WorthFlorida]

The backflow preventer works on pressure, it doesn't work like a check valve. By going from 3/4 to 1" you created a venturi effect. The means at the back flow preventer the water pressure drops as it picks up speed through the preventer. This drop in pressure opens the valve as if the pressure from the supply (input) is lower than the output. The valve closes as the back pressure builds up and the flow from the supply slows down when all sprinklers in the zone are putting out water. In your case zone 1 may have the most sprinklers or the longest run.

The prove this you need a spigot after the BFP with a garden hose. Do not turn on the sprinkler system but do open the spigot with a hose nozzle. You may notice it won't happen. A fix could be a ball valve after the BFV to provide enough flow resistance by partially closing the ball valve, or you can live with it. No harm unless it sprays water onto the siding of the home.