Air filters

The house has been built with an internal membrane to keep it airtight and all the air drawn in comes through ducting on the roof - in most houses it comes in round the edges of windows and doors - to an MVHR (mechanical ventilation and heat recovery) unit which filters it, warms it in a heat-exchanger, extracts excess humidity, then pumps it (very quietly) into the living spaces. The air is extracted from 'working' spaces - the kitchen, bathroom and shower - where cooking and showering has probably added a bit more warmth and humidity. The MVHR refilters the air circulating in the house and keeps it at optimum temperature and humidity.

Before 'used' air is pumped back out it goes through the heat exchanger which extracts its heat to warm the air coming in from outside.

Today the MVHR unit had its six-month service which included replacing the external and internal filters. The one below is the removed external one, which shows what town-dwellers (and presumably anyone near a road) breathe into our lungs when we fling open the windows for some 'fresh' air - though this is six months' of grime. The one above is the internal one, which shows how clean air indoors gets contaminated by dust and, in this case, the sanding and diy I've been doing over the last six months.

I had no idea what to expect and was a bit surprised by the insects and seed-heads on the external filter. I was pleased to know that it's OK to vacuum them in between the six-monthly changes to improve airflow and keep it cleaner.

The engineers were great, answered all my questions and showed me how to do the next filter change myself. It was interesting walking round the house with an expert, sniffing the air in different rooms and discovering that I was able to detect the difference in air quality on the landing, which doesn't have its own clean air inlet, and in the bedrooms, which do. 

They'll be back in a year for the MVHR unit's next service.