I have a Fiorelli combi boiler and the pressure keeps going too high and too low depending on wheather the heating is on or off. When it's been off overnight it drops below 0.5 and flashes "F37" to indicate that the pressure is too low. I top it up to 1.0-1.5 and the heating comes on but it slowly creeps up to above 3.4 then we have the problem of it being too high and flashing "F40" The only thigng to do then is bleed a radiator to get it down again. It normally stays OK after that until we turn it off at night then have the same problem again in the morning. Does anyone know what could be the problem as we are topping up pressure and bleeding the radiator on a daily basis now.

I could not find your boiler (no model or serial# given) but you should have an auto vent valve on the pumps. Or a faulty air pressure switch, or a number of things going wrong here, including turning your system on and off. WHY??

Spares | Ferroli UK (http://www.ferroli.co.uk/spares/)

Type in your model and click on view diagram, or call one of the service numbers for additional help. They have some good tutorials on setting the system and leaving it alone at night.