Posted by Harold Kestenholz - Hydronic Network on March 03, 2001 at 15:25:07:
In Reply to: Vaillant Boiler Problems Resolved posted by Eric on February 28, 2001 at 10:57:20:
Lest anyone read this post and believe that bleeders should always be left open, leaving bleeders open all the time can be a cause of a problem as well as a sometime solution. The cause of this problem was air in the system; once the air is out, you do not want it back in - an open air vent can permit air to enter as well.
It is better to read the manufacturer's engineers instructions and understand that the air vents are there to let air out conveniently for a week or so until the air is out and then be shut until air enters again due to a leak or draining and refilling of the system.
The caps are not necessary to keep dust out of the vent while in the box. If they were not ever meant to be closed, they would not be included with the vent; nor would the instructions that come with them tell you to shut them.