wrote:
Boiler does start as per times set (Twice: once in morning and once in
evening, two hours each time). Water tank gives hot water. But all
radiators in the house are cold.

This began after holes were drilled right above Thermostat that
dropped lots of fine dust on Thermostat and surely quite some got in.

Previously when turned Thermostat was giving 'kt' sound when room
temperature was crossed. No such sound any more.

Sounds like it is knackered or possibly you drilled through its
connection wire. Investigate with a multimeter and see.

--
Cheers,

John.

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