Short cycling Rayburn boiler

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Indeed it is a dual element temperature controlled dual stat, as the control stat reaches cut-off temp it switches over to the overrun stat and in effect continues to run the pump until the factory set cool down temp is reached then pump switches off - but there is one main difference on switchover - the burner will not (read as 'must not') light until control stat comes back into function and the pump still continues to run per demand - if the pump does not clear the residual heat within 30 to 60 seconds then viola!

...the saw tooth effect.
 
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I didn't appreciate that the boiler was cycling so rapidly as there is no time scale shown on the x axis.

The temperature drop when the boiler is off will depend on how pump overrun is controlled. Some boilers have a timer which runs the pimp fro a fixed time; others are thermostat controlled - the pump runs until the flow temp has reached a set temperature. The manual does not say which type is used, but I suspect, from the wiring, it is temperature controlled.

Think I said this before a couple of posts before. Pump overrun does not play a role here. The boiler has a continuous "go" signal because the room thermostat is not satisfied. Pump overrun will only come into play either when the room thermostat (eventually) is satisfied, or at the end of the day when the heating goes off.

So the sawtooth pattern is caused entirely by the boiler thermostat turning the burner on and off. And it is doing this because it only takes 30 secs to heat the 600ml of water in the heat exchanger to the thermostat high set point when the burner switches off. And it only takes 30 seconds to cool to the thermostat low set point when the burner comes on again. This is because the heat exchanger is being fed by return water which is below the thermostat low set point. In other words it is doing it because the deltaT is bigger than the thermostat hysteresis.
 
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Not quite, the pump is initially run by the boiler stat but switches over to a permanent live when the water reaches around 35c, the pump is then controlled by the p/live until the water temp goes under 35c when it switch's back under control of the boiler stat, I still think your pump is too small.
 

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