What Maintain the HW Pressure

Neil_5

I have HW boiler system, system set up and working well with 12 PSI @ cold and approx 18-20 when running. I have an automatic fill set at 12-15 PSI and a main cut off before it on the cold line.

I turn off that main and after 24 hrs my system pressure went down to 4-5 PSI. Am I right when I say the cold water pressure is resonsible for maintaining the HW system pressure?

I also notices that water came out of the vent on the back flow valve when the system is fired up, approx a qt of water. I turn the valve back on and for a quick second or less I heard a quick hiss and stop.

Why would the water come out of the vent line on the back flow valve?

Thanks,
Neil

Canuck_2

Where is your gauge located?

Aside from a defective valve, the relief port on the BFP discharged because heating system pressure was greater than the cold water supply pressure in the line, which you had shut. If your gauge is between the PRV and BFP, you may be getting misleading readings on your gauge.