I have an internal grant vortex 26 combi around 8yrs old.....the boiler is working ok but I have noticed that the burner varies in times it takes to ignite? Normally the Riello burner fan comes on and ignites within 5 seconds but sometimes the fan will come on and it takes up to 40 seconds to ignite......is this normal as I thought the burner would have 'locked out' after that amount of time?