No water/Air from radiators upstairs when pump on??

[Kelvian]

I have a problem, when the heating is turned off I can bleed all the upstairs radiators and the main bleed from the back of the immersion tank. But when the pump is on and the heating is on, if I try and bleed the upstairs radiators I can open them and nothing comes out they don't hiss and no air at all comes out?
I then after a couple of hours get boiling into the expansion tank presumably as I am full of Air, if i set the room thermostate above about 23deg C i get boiling.
Any ideas?

 

[gas4you]

What system do you have?

Sounds as though you have the pump fitted to the return, in which case what you are getting is normal.

The system should be bled when the pump is off and rads are cold.

 

[Kelvian]

Standrad Gravity fed system, but i noticed I have the pump straight out of the boiler, then it Tee's off one goes to the two way valve splitting for rads and immersion tank and the other goes to the 22mm dia vent pipe to the f&E tank, at this point about 5cm from the Tee for the two way valve is another tee and the 15mm pipe from the bottom of the F&E tank.

Where does the water go if the rads won't bleed when the pump is on?

 

[gas4you]

I doesn't really go anywhere, but is being 'held in' by the sucking action of the pump being on the return, hence why air can sometimes be sucked in the system if the pump is on the return and you bleed the rads whilst it is running.

 

[Kelvian]

If the pump is fitted on the return side, which makes sense as the arrow on the pump points into the boiler, how does the water boil up into the vent pipe, as it has gone right round the system pass the two way vale and then up the vent pipe and expansion pipe?

 

[gas4you]

The vent is just a high point that any air in the system is theoretically released from.

The cold fill pipe acts as an expansion pipe, so when the system heats up and the water expands, the 'extra' water expands up this pipe, hence the water level rising in the F&E tank by about 1"-2" when system is hot.

 

[Kelvian]

So I don;t understand why it boils, I drained the system about three weeks ago and filled with sentina X400, left it three weeks and drained system completely.
Then re-filled and added sentinal X100 inhibitor, re-filled system and bled until there was no air in sytem, I.e. heating was off and all rads filled nicely.
What causes boiling if not the thermostats as I checked the old emersion tank one was switching the hot water off, then I replaced with new and checked and it switched it off, room thermostate I replaced and checked , yet the water boils, it does it especially if the heating is off overnight and comes on first thing in the morning , within an hour its boiling.
Any ideas?

 

[gas4you]

What is boiling?

The HW in the cylinder, or the water in the system. If the latter I would check the boiler stat.

 

[Kelvian]

Its just boiled again as we speak, it is boiling in the system not the imersion tank, I will check the boiler thermostat. it has four settings and is set on 4 I am going to knock it back to 3 as a start.