Myson Apollo 50B boiler, serviced annually. Set to 'hi' which I think outpu ts water at 80 degrees. Nest 3rd Generation system. Gravity fed. Header tank, yellow cylinder in ai ring cupboard etc.
When we moved into the house we had an old Randall 103 programmer and no ro om thermostat which has since been replaced by the Nest system including th e installation of a cylinder stat and Honeywell 3 port mid-position valve.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 s hort 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 cont inuous operation on either heating or hot water or both (i.e. when it's bee n running for a while). It never did this before installation of the Nest s ystem. I cannot say for certain whether the short-cycling started at the ex act 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 work ing in it's on/off short cycling mode I have measured voltage across Neutra l and pin 3 (heating call-for-heat) on the Heat Link and with the system ca lling 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 b ehaviour 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 whe n 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 boile r controls the pump. Boiler off = pump off, boiler firing = pump runnin g.
I'm looking for some next steps. Thank you kindly in advance.