Saturday, 19 May 2012

Defrosting the ventilation

The Passive House lady was talking about defrosting inside the ventilation system, and this seems like it could be a problem on cold winter days. So far it hasn't actually been a problem, but I'm not sure why.

The problem relates to humidity. As we all know the capacity of air to hold water decreases with temperature. We can see this in the condensation on low spec windows and on glasses of beer as the cooling air deposits its moisture. We can see the opposite effect in hair dryers, as the hot air carries water away.

So in the heat-exchanging ventilation system, on a cold day, the air leaving the house drops from room temperature towards outside temperature, and the humidity will go from 50%, or whatever it is in the house, up to 100% if it drops 10 degrees. As the temperature continues to drop, with 100% relative humidity, the moisture will start to precipitate. Back in December, when the airtightness expert came to show us how the ventilation system worked, we could hear a gurgling sound as the air flow in the ventilation system was turned up, evidencing this jettisoned moisture.

It stopped gurgling when we put a loop in the hose, as it suggests in the Steibel manual, which the expert had apparently not read very carefully.

Anyway, what was worrying the Passive House lady was what happens when it's really cold outside, and the heat exchanger is taking the outgoing air below freezing. In this situation, the precipitating moisture is going to be ice, or perhaps snow, or is going to form icicles inside the pipes. Ice is a crystalline structure, and once it starts, it will keep growing. This could spread to take over the ventilation system, block the exhaust ducts and lead to a lack of fresh air within the building.

To try to get an idea of the size of this problem, we need to think about the amount of water that the air can hold, which goes up exponentially with temperature. Very roughly, the capacity of air to contain moisture is 4 grammes per kg of air at freezing. This halves or doubles with each 10 degree change, so is 2 grammes at -10, 1 gramme at -20. Going the other way, it's 8 grammes at +10, and 16 grammes at +20.

When it's cold outside, it's going to be cold inside the heat exchanger and the air could be precipitating moisture below freezing and ice could form. This may start to happen around -4 degrees (water needs to be below freezing to actually freeze). If it's 20 degrees inside, and the heat exchanger is 80% efficient, that's going to start happening at outside temperature around -8 degrees or something. Even if it's very dry inside, say 25% humidity, by the time the air temperature has dropped 20 degrees, the relative humidity will have doubled to 100%.

The total amount of moisture that air can contain at that -4 C is something like 3 grammes per kilogramme. As it was probably saturated (100% humidity) at 0 C, it was holding 4 grammes and will have precipitated 1 gramme. A kg of air is about 0.8 cubic metres, so if the pump is set to 120 cubic metres per hour, that's 150 grammes of water per hour. If the outside air gets colder, it's going to precipitate more.

Of course the air is moving. If the cross-sectional area is 100 square centimetres, or 0.01 square metres, that's moving at 12 km/h, so some of the precipitation is going to be carried away. That's not a very fast wind, and we've all seen icicles form in windier situations.

I've been monitoring the outside temperature, and it's been below -8 only half a dozen days, and then for an hour or so in the morning. It doesn't stay that cold here for very long.

In Central and Northern Europe, where this technology comes from, it does get that cold, and does stay that cold, so there are electric heaters within the system to keep the air above the dew point. Counter-intuitively, the more efficient the heat exchanger, the bigger the problem, which reduces their efficiency as electrical heating consumes a lot of energy. Perhaps our ventilation system also has one, although there was no evidence of excessive electricity usage on those cold mornings.

The airtightness expert seems to have been completely unconcerned with this problem, perhaps because it is not an issue in the Japanese climate. It may be that he's just not very good with numbers, unless they have yen signs next to them.