Lost pressure and refilled with filling loop. Since then boiler seems to ignite on demand as expected but temperature within rises quickly, boiler turns itself off, temperature drops equally quickly; this repeats continuously and radiators struggle to get warm. Have tried bleeding air from system but little improvement noticeable. What can be going on?
Checked pump and is operating fine, pressure on outlet normal. Shutting off the crossover valve I can heat radiators one at a time; albeit they heat up quite slowly.
Should add boiler is in attic so at highest point in system; could air be trapped in return pipe at boiler? If so how to blee