Mystery of the Day

This is a screenshot of our ecobee temperature report from last night. We have a whole house fan that we turn on at night after opening a bunch of windows. The white line is indoor temperature, green line is outdoor. You can see that the two lines are decently well correlated through the evening because the fan is running, bringing in outdoor air into the house and as the outdoors cool, so does our house.

Around 3am the time on the whole house fan turns off, and here’s the mystery: within an hour the indoor temp jumps from 71º to 78º.

This is the mystery. Why? It’s 4am, the sun’s not out yet, the outdoor temperature continues to decline a little bit, and we’re obviously not running the heater. Why does it jump 7º?

I have noticed the whole house fan has not been as effective in this house as our older house and I’m trying to figure out why. Even if the outside temperature in the middle of the night is mid-low 60ºs, at best our house will be low 70ºs by the time I wake up, warmer than the outside air by a decent amount.

I don’t think it’s a sensor problem as the ecobee has two independent temperature sensors and both are usually within a degree of each other, and we have a couple of clocks with temperature gauges that register very closely as well.

This continues my obsession to lower our energy use and increase our comfort. But I’m really struggling to understand how to better optimize using the cool outside air to keep our house comfortable longer.