Evening all,

Pointers appreciated.

Myson Apollo 50B boiler, serviced annually. Set to 'hi' which I think outputs water at 80 degrees.
Nest 3rd Generation system. Gravity fed. Header tank, yellow cylinder in airing cupboard etc.

When we moved into the house we had an old Randall 103 programmer and no room thermostat which has since been replaced by the Nest system including the installation of a cylinder stat and Honeywell 3 port mid-position valve.

Around about the same time the boiler has developed the annoying habit of short cycling (turning off and on at intervals of between 30 and 60 seconds whilst the system is still calling for heat) after about 30 minutes of continuous operation on either heating or hot water or both (i.e. when it's been running for a while). It never did this before installation of the Nest system. I cannot say for certain whether the short-cycling started at the exact time of the Nest installation.

In an effort to solve this problem I have:

Balanced the radiators
Replaced the gas valve (service showed a slight leak)
Replaced the pump overrun stat
Replaced the 'hi' stat
Replaced the 'lo' stat
Replaced the thermal cut-off device

All with no effect.

With the heating turned up to 30 degrees to force it on and the boiler working in it's on/off short cycling mode I have measured voltage across Neutral and pin 3 (heating call-for-heat) on the Heat Link and with the system calling for heat this is at 240v as expected. When the heating is turned down the system stops calling for heat and it drops to 120v and then 0v. This behaviour is also as expected, although I don't know the significance of the 120v reading, if indeed there is one.

Crucially, with the system calling for heat the voltage remains at 240v when the boiler is in the 'off' phase of the on/off short cycling. In layman's terms (if my logic is sound) the system is calling for heat but the boiler is not complying.

A plumber mate has suggested my circulation pump but I'm not convinced. The pump L and N are connected to the boiler, which in my mind means the boiler controls the pump. Boiler off = pump off, boiler firing = pump running.

I'm looking for some next steps. Thank you kindly in advance.

Jon