Kettling when only part of CH system is on?

[howdoyousaynejc]

We live in a 2 story house that has an underfloor heating system downstairs (separated into 4 zones each controlled by its own thermostat) and the usual radiator set up upstairs, controlled by a single thermostat.
We also have an unvented cylinder for storing hot water and a Logic system boiler heating all 3 'mini-systems'.

The issue is that when only the underfloor heating and/or the HW is being heated up, the boiler runs normally and quietly as it should. But as soon as the upstairs radiators are turned on (only or in combo with HW/underfloor), after 10-15min the boiler starts to slightly kettle. So I bled the radiators but they seem all to be OK with no air trapped in them.
The kettling even goes away in a couple of minutes if the underfloor and radiators upstairs were running at the same time and the radiators turn off, leaving only the underfloor heating on.

So I'm puzzled, as the kettling noise is definitely coming from the boiler, but only when the upstairs radiators are on - which makes no sense to me. Anyone have any ideas?

 

[Ric2013]

Agreed. Makes no sense. Unless you have a boiler that is modulating the flame and increasing the heat output for the increased load or the noise is something other than kettling?

 

[howdoyousaynejc]

The boiler is set to economy on the temperature knob, so I was thinking that it could be that and when both CH systems are running it's struggling a bit, but the pressure is still hovering at 2 bar and the kettling noise still persists when only the upstairs heating is run.

Could it be the pump in the boiler, as we have another pump for the underfloor heating by the manifold?

 

[Gasmk1]

Sounds like heat not getting away quick enough when only doing rads

 

[Ric2013]

Boilers are more likely to kettle with reduced flow than with increased flow, but if it kettles when running radiators regardless of what other circuits are in use, this does not make much sense from that point of view.

I do see your point about how the UFH pump might assist a weak internal pump and increase flow across the heat exchanger, but if the internal pump is working well enough to give sufficient flow when the cylinder is being heated then it should still give enough flow across the heat exhanger when heating the cylinder and the radiators at the same time.