Tuesday, 26 June 2012

Bypassing the heat

In the past I have been vehemently opposed to bypasses. Now I am delighted to have one in my house. Perhaps this is mellowing with age. More likely it is because the bypasses I hate are strips of concrete laid around towns encouraging more cars and trucks to speed through countryside, while the one in my house fits neatly as a part in the ventilation system.

I had assumed the ventilation system was fitted with the bypass, and shocked when I realised it was not. Six months later, on Friday 15th June, they came to put one in.

The ventilation system, rather crucially, has a heat exchanger. It's not perfect, but it means that if it's freezing outside, and 20 degrees inside, then the heat from the expelled air will be transfered to the incoming air, so rather than an icy draft, the fresh air comes in at around 18 degrees. This is great in the winter. It's also good news on a hot summer's day when it may be 35 outside, in the shade, and 25 inside. In this case the heat will be exchanged the other way, dutifully following the second law of thermodynamics, and fresh air will come in at perhaps 26 degrees.

Either way, the temperature comes into the house at more or less room temperature, keeping that temperature stable.

In the summer, all the things that help keep the house warmer in the winter, like solar radiation, heat from appliances and boilers, and even human bodies giving off 100 watts each, are now risking overheating the house.

On summer evenings, it tends to cool off and the trick of the bypass is, rather than exchanging heat, it directly expells the warm air from the house and draws in cool air from outside. I looked over record temperatures fo r Matsumoto and among the figures was the number of nettai-ya or "tropical nights", where the temperature stays above 25 degrees. The record was  two nights in a year, back in 1983. Past performace is no guarantee of future climate, but pretty much every night throughout the summer is going to drop to, or below, comfortable room temperature, and if we can keep the sun out during the day, the house is not going to get too hot. As long as the laws of physics hold up, that is.

The only question is, what exactly do the two temperatures in the settings mean? One is the "inside temperature", with a default setting of 22 degrees. The other is "outside temperature", with a default setting of 10 degrees. The people who came to fit the system had absolutely no idea.

It seems their boss was against this kind of bypass. I'm not sure why but one reason I heard was a fear that the pipes in the heat exchanger would start to get moldy from condensation if they didn't have air flowing through them. As the bypass is not going to be working the whole time, this seems like an unwarranted worry.

It would be nice to be sure but I guess that if the inside temperature is above the higher temperature, then the bypass will switch on when the temperature outside is below that. But if the outside temperature gets below the lower temperature, then it will stop bypassing and go back to heat exchanging. It seems like we're getting cool air coming in at night, and at the end of the month there may be some actual results to show.