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What could cause my heating system to get "stuck" in a state where the boiler will no longer come on in response to the room thermostat, and I can only seem to "fix" it by turning the property power on and off again?

I have the following home heating components:

  • Worcester Bosch Greenstar RI boiler.
  • Siemens RWB2E controller.
  • Honeywell T6360 room thermostat.
  • Honeywell V4073A Motorised Midposition Diverter Valve.
  • Hot water tank with thermostat.

How I get the system "stuck"

My start position is as follows:

  • Set room stat to max (so switch is on).
  • Set controller heating AND hot water to "Twice" but not in active time period.
  • Turn house power on and off (to "reset" the system).

The I do this:

  • Wind controller clock to first period of day.
  • Boiler fires.
  • Wind to end of first period.
  • Boiler stops.
  • Turn heating to OFF on controller (from Twice)
  • Wind controller clock to second period of day.
  • Boiler does not come on, but should for heating!
  • Playing with the room stat does not cause boiler get "unstuck" and fire.
  • Heating will not work again until power reset.

Summary: If the controller clicks off when Heating and Hot Water are ON, Heating will never come back on.

Does heating work on its own? Yes. If I reset the system and have Hot Water OFF, turning the clock will bring the boiler on correctly for the Heating. I can turn the clock through many cycles and the boiler will come on AND respond to the room stat.

I've recently had the boiler replaced. The room stat is also new, as is the motorised valve.

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There is a forced delay when the thermostat shuts off. This is so that poorly designed thermostats don't cause the heating system to constantly cycle on and off. –  Brad Gilbert Nov 18 '13 at 0:35
    
@BradGilbert which component are you saying has the forced delay? –  bentayloruk Nov 18 '13 at 8:35
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