It's our very old vented ch system with a Potterton neataheat boiler. Having attempted some system maintenance the boiler cuts out after a few minutes presumably because the hot water doesn't seem to be circulating. Sorry about the long post but I know you guys like to know all the details before suggesting any action. The system had been pumping over for some time and it was suggested that this may be due to a blockage around the junction of the 15mm feed from the expansion tank to the 22mm flow. We drained it down, cut out and replaced the pipework from 6" below the feed, past the vent and up to the pump, including a few inches of the vent and feed pipes. A few days Before starting I'd added Sentinel 400 to help get the crud moving. We then hired a power flusher, removed a radiator to plug it in, capped off the feed and vent pipes, removed the motor unit from the 3-way valve and set to CH, and cycled with water through all rads and individual adds for some hours, reversing the flow and dumping the water regularly. Then added Sentinel 800, cycled that for a couple of hours through all rads and through each rad for 10 mins. Finally flushed clean water through everything until it was clear. A valve at the pump was closed for much of the to force water through other rads, but ran with the valve open for some of the time. The removed rad will be replaced with a new plinth heater. We then reconnected the feed and vent, bled all the rads and pump a couple of times. Switched on and bled everything again. I removed the cap on the pump and its spinning well when the system is running. When the boiler runs the water doesn't seen to circulate - the flow and return above the boiler are similar temperatures and the flow to the pump is cold. Cam anyone suggest what might be wrong please?