Ventilation Leak

Sam had an interesting issue with the mysterious appearance of water at his house in November. A narrow stream started appearing down a wall at the back of his house. Luckily it was on the outside. 

It came from the same place on the wall and seemed to be there all the time, although he could not confirm whether it was there while it was raining!

This seemed condensation-related, and evidently the cause was a problem with the seal on the exhaust vent meaning that instead of going outside, the exhaust air was going under the outer skin of the house. 

By November, it is usually colder outside than inside. The air is being cooled by the heat exchanger in the ventilation system as it leaves the house, and with at least a 10 degree temperature difference, and up to 50% relative humidity inside, this means the humidity of the exhaust air will usually reach 100% by the time it leaves. Since the ventilator does not have 100% efficiency, the exhaust air will always be a little warmer than the outside air, and have some moisture to deposit as soon as it hits something.

The something that was hit by the saturated and slightly warmer air seems to have been the inside of the siding, and the water found its way outside, where it dripped merrily away.