Saturday, 19 April 2014

Water is not going to collect in the summer... or is it?

Back to the mystery of the dripping ventilation system, at first I was worried about the lack of a drainage channel for condensate from air coming into the house in the summer. We've had enough problems with the channel for water dripping from the air leaving the house in the winter. 

If it's very hot and humid outside, as the air comes into the house and through the heat exchange element, it's going to drop in temperature, and as it does so the relative humidity of the air will go up and over saturation, then water will start dripping from the air. 

For example, it could be thirty-five degrees outside, 80% humidity, and twenty-five degrees inside. As the air comes in from outside through the heat exchange elements, it's going to drop from 35 degrees to about 26 degrees. If the heat exchanger were 100% efficient it would drop to 25 degrees, but it's around 90% efficient. The dew point for air at 35 degrees, 80% humidity is 31 degrees. It's going to hit that temperature inside the heat exchanger, and water is going to start precipitating.

At first I thought this would be a problem, but I was assured that it would be fine since the water is going to head towards the chamber of the ventilation system where the temperature is the same as outside, and the extra heat will allow the air to absorb the moisture. 

But then I realised that actually it is going to be a problem, because water is going to be accumulating there, and while it might be fine in a steady state if you add some water to hot air, this is not a steady state. Humid air is constantly coming in, and water is going to be added to that chamber and will not be able to leave, so sooner or later it is going to push the humidity up and over 100% and water will start dripping. 

Hot and humid may be difficult to imagine in Europe, where the ventilation system was designed and is usually sold, but it does happen elsewhere. Japanese summers have both heat and humidity, and with discomforting frequency in some places. According to an article about Heat Index on Wikipedia, in Dhahran, Saudi Arabia on July 8, 2003 the dew point was 35°C while the temperature was 42°C.

However, just looking at the temperature and humidity here in Matsumoto in July and August 2013, the outside dew point never went above the inside temperature, and the closest it got was still three and a half degrees lower. Over those two months, the highest the dew point reached was twenty-two and a half degrees, and the lowest the temperature got inside was twenty-one degrees, ten days earlier. So it's possible to imagine that we'd get a saturation problem, but in practice when the temperature goes up the relative humidity drops and we're safe. This part of Japan is acclaimed for having dry summers, so I'm sure the story would be different if you were closer to the Pacific coast, not surrounded by mountains which do a good job precipitating humidity from the air themselves.


I know pictures add to the readability of a blog, but that equations reduce it. So here's a picture of an equation.
(TD is the dew point, f is the relative humidity, T is the temperature.)