Drained the system to replace a couple of trv's its an open system with a feed n expansion tank in the loft and runs off an oil fired boiler. after doing the trv's turned the water back on to let it refill but now only getting water out of the downstairs rads and upstairs there is little or no air coming out of the rads! left it overnight and tried again this morn still hardly any air coming out! its abig house with a very small feed tank in the loft, tried running the boiler but it just gets too hot n shuts off! Any Ideas???