When do you close the cap on the top of an air vent?

[rwh442]

With the mild weather the boiler use has been intermittent - run for one day, down for three days etc. I am somewhat confused on when to close the little red plastic cap on the air vent on my system. It is at the highest point in the system, supply side, a few feet from the boiler. I am under the impression that this should be closed when the system is not in use - but if there is a constant pressure at room temp (70 degrees, 10 psi) then the float should always be closed and hence no air? Always leave it open?

What happened, twice so far, is that I closed the vent when I shut off the pump when the water temp was around 100 degrees or so. Then came to fire it up a couple of days later (water was cooler - 65 degrees) and opened the vent and had a little hiss, gurgling in the boiler, fired up the pump and no issues. Pressure was still 10 psi. Normal?

I am thinking leave the vent open and with internal pressure there should be no air issues. Thoughts?

 

[in hot water]

close it only if it starts leaking. Then replace it with a new one

Look for a brand that includes a small check valve so replacement is a snap, not draindown needed.

They all leak at some point, usually a bit of dirt or crud in the system will stick the open and they will dribble or spray. But you really need to leave the cap lose so it can do it's job, or else just replace it with a coin vent or mini ball valve.

hr

 

[sgschwend]

Your air separator has a float in it so when the liquid reaches it the float will rise and seat the valve closed. You may be concerned about evaporation of your fluid, but this should not be the case with this part since unless there is air present the vent valve is closed.