Wednesday, 2 December 2015

Lesson 7: Ventilation

The first attempt at teaching a course is always a much bigger lesson for the teacher than the students. As in warfare, the first victim of a lesson is often the battle plan. 

Three times I've found that a single lesson has turned into two, and the course feels like it is being resized and re-dimensioned. I thought we were almost half way through the time, but only a quarter of the way through the plan, but checking back with the plan I now realise that the lesson I had prepared on insulation included thermal bridges, and the lesson on air and water included ventilation, which were four lessons on the original plan, and I'm more or less where I thought I would be. 

The ventilation lesson began at the realisation that houses must be both insulated and airtight to have low energy loss. If an insulated building is not airtight, three things will happen: you will get condensation within the walls, you'll lose heat with the leaking air, and you'll get fresh air in the house. The last one doesn't sound so bad, and in fact you need fresh air in a building. I asked the class why, and how much we need. 

The most obvious is to provide oxygen. Although the most obvious it is not the most serious. We need around one litre of air per second per person to avoid a build up of carbon dioxide. Relatively low levels of carbon dioxide can lead to lack of concentration. 

You need about three litres per second to remove nitrous oxides given off while cooking, and 3.5 litres per second do remove unpleasant odours from the house. Then you need about 6.5 litres per second to be safe from the volatile organic compounds that are used in paints, varnishes, glues and other chemicals in building and its furniture. The biggest need for ventilation is to stop a build up of moisture, for which we need about seven litres per second per person. The air we exhale is close to 100% relative humidity, since we are mostly water, and close to body temperature, since we are warm-blooded, so we're putting out a lot of moisture. 

This means a design ventilation of 8 litres per second, or 30 cubic metres per hour per person. Taking an average of 35 square metres per person this means about one complete air change every three hours, or 0.34 air changes per hour. 

Air can be changed by opening windows, using air vents, using extractor fans or mechanical ventilation systems. 

Natural ventilation, whether by windows or vents, is cheap and easy. It works either on pressure difference or temperature difference. Opening windows on the north and south of a building will create a through draft. Opening windows upstairs and downstairs causes air flow through the stack effect. This can be calculated, but not in my lesson! 

Natural ventilation will usually provide too much or too little ventilation. There is also a risk that someone will get in and steal your telly if you leave the windows open. And it may not be a good idea if it's raining outside. Extractor fans are relatively cheap and will provide a more steady rate of ventilation. Mechanical ventilation systems are more expensive to install but will provide steady ventilation, and also allow heat recovery. 

So how much heat will be lost by ventilation? Using a few assumption, on a winter's day when it's freezing outside and 20 degrees inside, we lose heat at a rate of almost 200 watts per person. At first this doesn't sound like so much. 

We looked at heating degree days. For example, the graph below shows in blue how hot it was outside and how many degrees each hour we have to heat up the air by. 

As far as heating energy is concerned, a temperature difference of one degree for ten hours is the same as a temperature difference of ten degrees for one hour. 

You can add these up over the year, and for Matsumoto it's a total of 80,000 kelvin hours per year, or 80 kilo kelvin hours per year. Incidentally this is about the same as for Manchester. So you can work out that ventilation for a year is going to lose 768 kWh. This is about a month's electricity bill, so it looks like quite a lot. 

Finally I explained mechanical ventilation with heat recovery (MVHR), which turns out to be a very cheap way to heat your house, as long as you have gone through the expense of making it airtight and well insulated.

Also in today's lesson I gave out some questionnaires, which the school hands out in the middle of the term. Everyone seemed happy with the course the way it is, except one person who wanted me to make the calculations easier! I'm doing my best.