bleeder valve

[amp]

I have hot water baseboard heat. There's a bleeder valve on top of the expansion tank that I replaced years ago do to it being very coroded. I think this would make the system a self bleeding system. I understand the purpose of the valve, but I'm curuious..........should I leave this open a crack all the time to let any trapped air come out or should I only use this when I've drained the system and I'm filling it back up? Any thougts on this.......will water come out of this all day if i open the top of it a bit and leave it?

[Clutchcargo]

What type of expansion tank do you have. It sounds like the older type that should be half filled with air so that the water can expand. If it's a newer type, the water and air are separated by a rubber bladder. These need to be pumped up with a bicycle tire pump to the operating pressure of your system (usually 15 psi.)

[joed]

The expansion tank needs to be full of air NOT water. The air is what allows the water to expand. The air can compress. The can not. That valve is used to let into the tank when you open a drain valve to let the water out.
Once the system is stabilized and all the air purged out of the lines you don't need to add air until next time you drain the system for a problem or alteration.

[amp]

so it's used to let air in? joed, your reply confused me a bit. after i recently drained and filled my boiler (the lines as well as the boiler), there was a few air pockets in the system.........which were typically removed by draining the lines going through the house for a few minutes and then letting the boiler call for more water to accomodate. it worked out fine. so there was just a little air in the lines last night and i cracked that bleeder valve and a fair bit of air came out and then it squirted a little water. after doing this a few times the system seeds totally bled and is quiet as well as warm.
can you help me out with the my confusion. thanks!!!

[joed]

When water heats up it expands. Water is not compressible. The expansion tank is meant to hold an air cushion. Air is compressible. The air cushion gives the water some place to expand.

It lets air in when you are draining the system, not during system operation.

[amp]

joed,
thanks for the info...although i know how the system works......just curious on why my issue was pushing air out a bit and then a bit of water. I'm all set...........i think i'm probably not quite asking the question in the right way.
I userstand the properties of water as well, just cuious on why my system was acting the way it was.