circulation problem?

[Drfeelgood1]

hello, this is the first time i have used this site so here goes, i have just started my central heating to check it is ok for the colder months ahead. what i have found is that the first three radiators that the boiler services are all gettiing hot but after that no heat into any of the others.
i have checked the pump which is a three speed model and its working fine on all three.
i removed the central cap and a little water dropped out and then ran the pump up.
the outlet pipes and inlet pipe to the boiler are both getting hot suggesting that the pump is getting the hot water around the closed system but for some reason not into any radiator after the third one.
i have worked through the radiators bleeding them , a bit of air out of the first three but the fourth is proving tricky, it will bleed out to water coming out but when a i crack it again its almost as thought its sucking air into the system for maybe two minutes, is this possible?
i have checked the clossed system refil tank in the loft and its full but when i ran the system just now i could here the sound of water running in the tank and its warm suggesting that its been spat into there from the heating system loop?
could anyone help please, cheers stu

 

[kevplumb]

bleed the rads with the pump off

normal valves or trv's

 

[palerider45]

it sounds like you may have a blocked cold feed/vent to system this would explain system pumping over into expansion tank. Look in tank when system on, if water is pumping out of the open vent pipe above tank then you have a restriction in the vent/feed connection before pump. This would also explain lack of heat to last rads and system not filling up.

 

[doitall]

Check the ballvalve isn't stuck for starters.

If you have TRV's check the pin isn't stuck by removing the head.

 

[bengasman]

Are the rads colder in the middle than at the sides when you feel along the bottom? First guess with this kind of problem is always corrosion.