The system saw a lot of fills and drain last year, so I can't be certain, but in general, the radiators have not been quick to fill with air. They probably get bled each autumn: I lose track.
What used to happen (long-term issue) is that the air would run around the system, get caught under the airing cupboard where there was a pass-under on the flow under another pipe and then every now and then there would be a GLUG sound and then a load of air would get to the H arrangement. Most would go up the vent, but some would get into the pump which would then make an airy sound.
My response was to re-design all the pipework in the 12x10 landing-room, remove several radiators (who needs a radiator in a draughty porch or in a loft?), remove a run that ran all the way around the landing for no apparent reason before going to a bedroom radiator, add a radiator in a drying cupboard, drop the pump a few inches to see if that helped, replace the lounge radiators with underfloor heating (on the same zone as everything else but done for technical and practical reasons), design the return to ensure reverse circulation was impossible, and put a Magnaclean on the highest point of the return to act as an air-vent point, and as a filter.
The GLUG sound no longer happens and the air seems to be finding its way into the vent. It is no longer usual for the pump to get air in it. Everything seems to be working as it should. The only exception is the running air sound and this small quantity in the Magnaclean (only with a hot system).
Even if the pump were able to cavitate as the water temperature increased, as you say, the resultant steam could not survive for long. Though, at 75°C, it should be okay at a 0.5m head inlet pressure, according to Grundfos data, the only risk being cavitation.
What used to happen (long-term issue) is that the air would run around the system, get caught under the airing cupboard where there was a pass-under on the flow under another pipe and then every now and then there would be a GLUG sound and then a load of air would get to the H arrangement. Most would go up the vent, but some would get into the pump which would then make an airy sound.
My response was to re-design all the pipework in the 12x10 landing-room, remove several radiators (who needs a radiator in a draughty porch or in a loft?), remove a run that ran all the way around the landing for no apparent reason before going to a bedroom radiator, add a radiator in a drying cupboard, drop the pump a few inches to see if that helped, replace the lounge radiators with underfloor heating (on the same zone as everything else but done for technical and practical reasons), design the return to ensure reverse circulation was impossible, and put a Magnaclean on the highest point of the return to act as an air-vent point, and as a filter.
The GLUG sound no longer happens and the air seems to be finding its way into the vent. It is no longer usual for the pump to get air in it. Everything seems to be working as it should. The only exception is the running air sound and this small quantity in the Magnaclean (only with a hot system).
Even if the pump were able to cavitate as the water temperature increased, as you say, the resultant steam could not survive for long. Though, at 75°C, it should be okay at a 0.5m head inlet pressure, according to Grundfos data, the only risk being cavitation.
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