Monday, 10 December 2012

Steamy breathing

We've now got three new humidifiers in the house, each with a performance of 300 ml/hour, so if they're all steaming away they can put out 900 ml/hour, which should be enough to keep us at 50% relative humidity when it's bone dry outside. It may now be possible to over-humidify the house, so I'm just going back to the question of how much humidity is added to the house by other means. I know we have some plants in the house, but since we're watering them every few days with a single wine bottle, and the house is losing that much water every hour, they are not making a massive contribution. 

We can easily estimate how much humidity we breathe into the air. Our lungs are moist and at body temperature, so we can assume exhaled air is saturated and around 37 degrees C. After a little googling, and avoiding the contentious red herring of how many breaths we make a minute and the futility of trying to count your own breathing rate, I found this site on normal breathing.

Apparently 6 litres per minute for a 70kg adult. That's 360 litres per hour.

From this site on humidity and anaesthesia, just in case anyone is still conscious out there, they have figures for water content in mg/l at 20-degree room temperature and 37-degree body temperature: 18 and 44 mg respectively. These figures correspond with the g/kg figures I was talking about  in my humidity blog

If the air going in is at 20 degrees at 50% humidity, holding 9 mg of water per litre, it looks like a standard adult will add around 35 mg/l, a total of 12 grammes of water to the air per hour. Two adults and two children will add around 40 grammes. So this is something like 5% of the humidity we're loosing on a day when it's freezing outside and 20 degrees C inside. 

Another reason for humidifying is that apparently it makes the ventilation system exchange heat more efficiently. Presumably humid air carries more heat, so the heat exchanger will work better. I'm not sure how big an effect this is going to have. Stopping to think about this for a couple of seconds, once the air has been cooled ten or fifteen degrees, it's going to be saturated anyway, so it's only going to make a difference for the warm part of the heat exchanger. Perhaps the actual condensation of the airborne moisture in the heat exchanger improves the transfer.

But then the bells of legionnaires disease start ringing again.