Night Ventilation...

One of the keys to passive house is air tightness and mechanical heat recovery ventilation... which is great most of the time time. But during hot days and cool nights it is often beneficial to get as much night time cooling (i.e., ventilation) as possible - which means opening the windows.

In our passive house the primary thermal mass is the water wall, and the house is designed to allow for efficient night ventilation of the house and water wall by opening two panels of the big north facing window near the water wall and the clerestory windows at the top of house. This creates a 'stack effect' with the hot air rising out the top of the house helping to pull cool air in the house at the bottom... across the water tanks in the process.

Last night was the first time we opened the windows to allow this night time cooling to happen (as today's weather was forecast to be very warm we wanted to cool down the house as much as possible before hand). While I'll work on a more detailed analysis with actual data once we have a sample size greater than one... I have some observations that amused me.

The last 6 days have gotten progressively hotter - both at night and during the day. On Sunday the high was ~ 25 °C whereas today it was ~ 38 °C and the low on Sunday was ~ 7 °C whereas last night it was ~ 17  °C. On Sunday, with a high temp of 25 °C and a low of 7  °C the house temperature ranged between 20 °C and 23 °C - quite comfortable, but with the temperatures increasing and no night time ventilation the house gained about 1 °C a day over the week. So on Thursday morning we started the day ~ 23 °C and ended the day at ~ 25.5 °C - or slightly over the target range of 20 - 25 °C.

The answer to this is to start the day a bit cooler - or night time ventilation.  So last night despite it only dropping to ~ 18 °C overnight, opening the windows brought the temperature down to ~ 23 °C again to start the day today. Ultimately the house performance would have been a lot better this week if we had done nighttime ventilation on Tuesday and Wednesday nights as well... as we could have avoided gaining a degree each night.

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