The volume of the house, again in the roughest of ballparks, is 500 cubic metres. A kg of air takes up about 0.8 cubic metres, so let's over-compensate for our overestimation of the amount of water that the air can hold, and say that a cubic metre of air can hold 4 grammes of water at freezing, 8 grammes at +10 and 16 grammes at +20. In a house we're not really interested in the weight of air, and the volume is going to be pretty constant.
If we start with 50% humidity at 20 degrees inside the house, that means there are 500 * 0.5 * 16g = 4kg = 4 litres of water in the air. If we imagine it's a steady zero degrees outside, also 50% humidity, and we switch on the ventilation system to shift 120 cubic metres in and out per hour, that's going to bring in 120 * .5 * 4g = 240 grammes per hour, and expel 120 * .5 * 16g = 960 grammes. A net loss of 720 grammes.
The humidity outside is going to make a difference, but even if the air is dripping with mist and it's 100% humid, we're still going to be losing twice as much water as we gain, around half a litre per hour. If it's bone dry, we lose almost a litre. Britain tends to be dryer in the summer and wetter in the winter, while Japan is the opposite, with humid summers and dry winters. In the summer, the opposite effect happens, so if it's 35° C outside, even if there's only 50% humidity when the temperature drops to the 25° C inside temperature, it will be saturated.
To maintain the humidity in the cold winter, then, we need to be emptying something like one wine bottle of water into the air in the house every hour. Of course, there are some sources of humidity within the house, for example bathing, washing clothes and cooking. If we use a tumble dryer, or hang out washing inside, this will help keep the humidity up. As humans respire and perspire, we're giving out water too. The air we breathe out from our moist lungs is saturated and above room temperature. That's why mirrors and spectacles steam up when we breathe on them. House plants can also keep the humidity up as the water we give them evaporates. This is all good, but I'm not really sure how big the effect is.
Burning fossil fuels gives off moisture, as the hydrogen atoms within the hydrocarbons combine with oxygen in the air. Our cookers are electric, so they don't help us.
The other place humidity is going to come from is the building materials. This is not such good news, if the building is drying out.
At the moment we have one small humidifier which gurgles away noisily and empties its 2 litre tank in about six hours, which is not going to keep up with the ventilation system's dehumidifying effect.
One option when we were choosing a ventilation systems was whether they maintain humidity going in and out, or ignore humidity. We chose one that ignores humidity, probably for reasons of hygiene as the moisture that it's passing from the outgoing air to the incoming air could contain bacteria. Legionnaires' disease has been known to thrive when moisture is circulated in a ventilation system. We usually just hear about this from hotels, rather than private houses. This may be because hotels have bigger systems, or maybe because it affects more people and is bigger news. Since this disease kills one in ten healthy people it affects, the stakes are high and caution is warranted.
The US Department of Labor offers some useful tips on designing HVAC systems to avoid legionnaires' disease. Very simply, if a system avoids bodies of water, especially any between 25 and 45° C, and only allows clean air in, it should be OK. Perhaps we could have followed these to make a built-in system to regulate the humidity safely. Getting another humidifier is probably much easier and cheaper though.
More precision (than you probably need or want)
| Temperature | Maximum possible water vapour grammes per kg of air |
|---|---|
| -10° C | 1.79 |
| 0° C | 3.84 |
| 10° C | 7.76 |
| 20° C | 14.95 |
| 30° C | 27.69 |
