Pressurised hot water tank problem


I've got a pressurised heating system, ie no header tanks with pressure being provided by the mains water, which has a minor problem. The hot water tank has a vent pipe (which is fitted with a temperature/pressure fitting) which drips when the boiler is on. I have looked on the web and found that these tanks should be vented periodically by carrying out the following:
1.. Turn off feed to tank (mains). 2.. Open lowest hot tap. 3.. Open press/temp valve and leave open until glugging stops (several minutes). 4.. Turn on feed to tank (mains). 5.. Turn off hot tap. I have done this and the dripping stopped for a couple of days but it has now started again. Does anyone know how to resolve this problem as I cannot understand how the air in the top of the tank is being displaced by water.
Regards
Mike
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

absorb expansion/contraction as the water heats/cools?
If so, it may have a fault and the air has leaked out. Is here a thing like a tyre valve on it anywhere?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
No it doesnt have any sort of pressure chamber external to the tank its self. Any other suggestions ?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.