We've got under-floor heating throughout that obviously hasn't been used much through the summer. Now we want to use it, but I'm struggling to get flow through the circuits. The boiler appears to be working normally and getting heat to the mixer valve of the manifold, but the only way I can get flow through each of the circuits is to turn the mixer valve to its coldest setting. As soon as I turn the temperature up on the valve the flow stops. It isn't much use having flow that is cold! The pump on the manifold also appears to be working correctly, the pressure is ok and I can't seem to get any air out of the sytem. Any one any ideas?