RSF 82-ET intermittent heating and hot water?

My ravenheat RSF 82-ET boiler has had intermittent problems providing DHW and CH in the last few days, it is now firing up for about 30 seconds then nothing. I noticed one of the relays on the main circuit board was arcing and charred, this has been replaced (albeit with an alternative part) and the boiler worked immediately and lasted for 2 days. I have now seen the same arcing in the new relay, does anyone know what is the purpose of the relay and could there be anything else in the circuit to cause this.

leah
December 2011