The boiler is 4 years old. When there is a demand on the boiler, it fires up for about 20 seconds but then shuts down for about 60 to 90 seconds before it starts up again for 20 seconds, and so on. Consequently if CH is demanded the feed is hot but the rads are very slow to heat. Eventually they all get up to heat. Can anyone advise why the boiler not staying on and therefore heating the rads quickly?