Gas Boiler firing for 5 seconds then switches off for 15 seconds repeatedly

Hi
I noticed my my gas boiler doing the following this morning
When central heating was selected the temperature selector in the hall was set to about 16C the boiler would fire for about 5 seconds then stop for 15 seconds then it would fire for 5 and stop for 15, it seems to do this often. I've read that one point of inefficiency in gas boilers is short burn time, durning the start up phase not all the is gas burned therefore you don't get optimum heat from it. If my boiler is doing a start up 4 times a minute then it must be chucking a lot of unburned gas out the flue. The boiler is an "Ideal classic 270FF" with a "classic LX" programmer. Is there anyway I could increase the burn time and hopefully reduce the start ups.
Thanks Kev
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On Sat, 24 Feb 2007 03:53:05 -0800, purlpekenny1964 wrote:

Is the system heating up OK or is it slow to warm up?
Rapid cycling like this suggests poor circulation: either pump not working (or running very poorly) or some blockage in the pipework ('Y' in-line strainer/filters, if fitted, are prone to blocking up). Alternatively you may have thermostatic valves on all the radiators and they've all shut down, but if the system is calling for heat with the room thermostat set to 16C I'd guess the house is too cold for the TRVs to be closed unless they're all set really low.
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