We have been workiing on this for 5 weeks so far. HVAC tech had no idea. I thought we had it fixed. Last night added water to system, brought pressure up. it worked great for hours. Every radiator heated, system came on and off as needed. Woke up this morning cold. Same thing, runs for only 3 to 5 minutes then shuts off. Gets to high limit as if pump is not circulating to cool it down. ARGH! it was working last night. I bled all the radiators again, pressure has not changed and return is cold. PUMP< PUMP< it has to be the pump? Last night it was obviously running...........last night I did the victory dance, thinking it was water and pressure. ARgh....ideas? Think the pump might have a mind of its own??