Most likely it is the element
Just replaced mine made out of the Same materials as yours after next door started having problems with it blocking,had a camera put down and...
Check boiler instructions for clearance and ventilation
Has it ever worked?Are any valves closed are the zone valves powering open
The flue can’t stay as it is now,it may be possible to reposition it but must be done by a gas qualified engineer as it will need testing
As muggles said it sounds like you are messing with the water secondary return pump, nothing to do with the heating
Is the water level in the cistern high enough
A thermostat/ control is a requirement on a installation
I would think it should be left in the on position
Easy to check put it in the off position and the heating should not come on on the remote stat
Looks like electric supply
Check how it has been connected,
It will as less restrictive but a lot of will depend on length of run and head water ,can you not run a tank feed cold ?
Not a fix for it ,just seems to be a design flaw
If you have a woodburner connected to a sealed system I would not use it till it’s been checked over ,
Slight leak ,get them out nice easy repair
Could be pressure related or a ventilation issue would not use till fully checked
If it’s a 2 pipe system it’s ok ,blank off unused pipes
Stitch drill then hammer and chisel
25 will be plenty
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