The flow of the extractor fan is several times more than the ventilation system. There was an option to get an extractor fan which also lets air in to replace the air that is being extracted, but we decided against it as it was likely to reduce the airtightness. The doors and windows are all carefully sealed, and the vapour barrier and outside layer of tyvek have been carefully installed to get an airtightness around ten times better than the average house being built in Japan. The extractor fan is not designed to these exacting standards. In fact during the airtightness test the extractor fan was taped over, which apparently is standard practice, but seemed to me more like cheating. Regardless of the test result, throughout the life of the building an extractor that sucked air in as well would have two holes in the wall rather than one, and twice as many gaps.
Anyway, the extractor fan wasn't on, and it didn't seem quite like a pressure issue, so I thought it was another problem with our front door. We've had problems with the key on our front door, making me wonder whether it is a big crooked door to go with our big crooked window that we all got from what I worry is a big crooked German. Then I remembered the ventilation system.
I used to clean the filter every month, at the same time as my monthly collection of the temperature data from the thermometers around the house, and my monthly inputting and uploading of power generation and consumption data from the solar panel monitor. Since they put in the new ventilation system with the bypass, I haven't actually cleaned the filter.
When I went into the machine room, which was up in the thirties due to the power conditioners in there and the boiler below, and the insulation around the room keeping that heat from getting to the rest of the house, I saw "FIL" flashing on the ventilation unit. I had heard stories of people with ventilation systems very happy with them until they open them to clean the filters and are attacked by swarms of insects. No insects swarmed out, but there were plenty in there, mostly dead, and several fat spiders scurrying around. One moth flew out. The vacuum cleaner took care of them all.
The filter for air coming from outside had evidently become rather clogged, leading to less air being pumped into the house than being pumped out of it and, in spite of a few windows being open most of the time, lower pressure inside than outside. This was enough to suck the front door in and make it harder to open. It took a few hours for the pressure to balance, but the front door now opens normally.
The filter for air coming out of the house, there to protect the other side of the heat exchanger, had grey dust growing from it like small drifts of snow.
Once a month seems to be the right frequency for cleaning the filter, especially in the summer!
