My backflow preventer for my furnace recently started leaking from the vent. I replaced it with a new one and it too leaked from the vent only worse. There is only 20 psi of pressure in my line and the backflow preventer is rated up to 175 psi. Could I have possibly gotten something inside the backflow preventer that isn't allowing the seal to be made or is something else possibly going on?

Are you talking about a boiler, not a furnace?

My backflow preventer for my furnace recently started leaking from the vent. I replaced it with a new one and it too leaked from the vent only worse. There is only 20 psi of pressure in my line and the backflow preventer is rated up to 175 psi. Could I have possibly gotten something inside the backflow preventer that isn't allowing the seal to be made or is something else possibly going on?

This almost sounds like a water heater T & P valve problem. Are we speaking about a water heater that 's leaking out of the Temperature and Pressure Valve? Back to you, Tom

It is not "back flow preventer". It is Temperature and Pressure relieve valve. If it leaks, is means you have either: Too High water pressure or Too Hot Water Temperature. Check both. Adjust accordingly.

If it's indeed the T & P vaslve that's leaking let us know if you have a sprinkler system or a check or backflow preventer installed? (see image) If this is so you will have to install a expansion tank, )see image) on the cold water supply of the water heater. Let me know. Tom