Pre filter between MVHR vents and MVHR unit

[Russdl]

3 months since it was last attended to but it’s time to change the pre filter again, it’s fan noise that alerts me more than any diary considerations.
 

When the fan speed automatically increases in the mornings and evenings it’s generally inaudible but as time goes by it starts to become noticeable. 
 

The pre-filter is collecting so much airborne dust! It’s also letting the tiny flies through, but I can see the route they are (involuntarily?) taking so I’ll endeavour to get that sealed up before the refit. 
 

(I ran the hoover over the centre section to see how thick the dust and dirt was).