The other day I turned of the electricity to the house for a short period. This resulted in the boiler and associated central heating controls being turned off as well. The boiler is a heat only Valiant Eco
415 that also provides heating to water in a tank.
On switching back the electricity the boiler appeared to come on OK and for a few days has been heating the hot water as per the schedule set by the controller.
Today I got a bit cold working outside and decided a few hours of central heating may be required. After around 15 minutes the radiators were only just warm. A quick check of the pump, controller settings, wireless thermostat etc. showed no obvious problems.
I did notice that the boiler was in its short cycle inhibit mode.
Watching the boiler cycle I found that it wasn't modulating down. The boiler came on full burner, quickly got to an (internal) water temperature of 60C and then entered its inhibit firing mode of five minutes that is designed to stop short cycling. What the boiler normally does in come on full burner and then as the temperature gets closer to the target temperature it progressively modulates down to slow down the rate of temperature rise and it tries to maintain the target temperature without switching off the burner.
In the faulty mode the water in the boiler was reaching 60C for a very short time and then left to cool down for 5 minutes with a return feed to the boiler at around 35C - hence only slightly warm radiators after
15 minutes.
The fix turned out to be simply turning of the mains to the boiler and controller for around 30 seconds and then turning back on again. I assume the software in the boiler crashed when the electricity was switched back on a few days ago.