I have a combi BAXI boiler running 8 rads with TRV's. When the heating has been on for a while and heated the flat, I've noticed that the boiler will fire up for approx 10 secs every 3 mins. Is this normal? I would have thought it would fire for a longer time to heat more water, not do short bursts? Will my boiler have a shorter life? Is there something I can do to make it run more efficiently?