hot water overflowing from expansion tank

Hi can anyone help, i have a pumped system where the hot water part works fine, however, when the heating is switched on a trickle of water immediately comes out the expansion oveflow and if left the expansion tank itself overflows about an hour later, i also get loads of trapped air in the rads and not always but often when i try to bleed them i find air being sucked into the rads rather than being pushed out. I have minimal understanding of diy but would greatly appreciate any advice or suggestions.

There could be several reasons, my guess is that you have a badly designed system, and you really need to have someone in the know inspect it. I'm not a plumber, so these are only my suggestions picked up over years of DIY.

1. It could be that the pump is on the return pipe to the boiler and not the flow. If this is the case, there may be a tiny leak somewhere that is letting in air when the pump is running. The air replaces the water in the system so the water level in the tank rises. If the system has been working previously perhaps you have a blockage somewhere that is restricting the correct flow. (don't bleed the radiators with the pump on as you are introducing air into the system and making it worse).

2. Where is the water level in the tank when the radiators are cold? It should be well below the level of the overflow. When the water in the radiators is heated, it expands and the water level in the tank rises. If it's too close to the overflow, the expansion of the water will make it overflow, then when the system cools the ballcock opens to let back in the water that has just been expelled. When the system heats up the process is repeated.

3. Are you getting over pumping? Water is being pumped up the vent pipe (the pipe that bends over the top of the tank and has an open end) and into the tank.

4. Air is being sucked into the system by the pump from the open vent pipe the air replaces the water and so the water is displaced back into the tank. To chek this, put the open end of the vent into a glass of water when the pump is switched on and see if it is sucked up the vent.

Whatever it is you need to get it fixed. Every time you expel water at some point it will be replaced with cold water from the mains. This will dilute your corrosion inhibitor and also introduce air into the system which causes corrosion, sludge and more air. It's a vicious circle.