Friday, 2 September 2016

Forgot to boil the water

In the jumble of pros and cons for ventilation systems recovering heat and moisture, the moisture advocates point out the extra energy in the water. This Polaris site (in Japanese) tells us the different heat contained in dry and humid air: apparently air at 20 degrees centigrade has 11.9 kCal of heat at 80% humidity, but only 9.2 kCal at 30%. He doesn't say how much air contains that much heat, perhaps a kilogram, or a cubic metre, or your living room? And he doesn't give a reference point for the heat, whether it is per degree, relative to zero degrees centigrade or the total heat relative to absolute zero. And how do we understand those numbers anyway? I usually only think about calories in food, and whether it's 11.9 or 9.2 it's still only about half a jelly baby.  

Whatever the actual meaning of these numbers, humid air is going to hold more heat than dry air, since the extra water in the air is holding more heat. 

But not only does the water in the air hold more heat, it also needs to have been vaporised. Most of us are familiar with water vaporisation, as it happens when we boil a kettle. Although 100 degrees is the boiling point of water, it doesn't all suddenly turn to steam when it reaches that temperature. It takes some extra heat to turn it from one phase to another. While it takes one calorie to raise one gramme of water by one degree, it takes over five hundred calories to turn a gramme of water into steam. 

This is not just true of the water in a kettle, but of any water that is evaporating. For example the water in clothes hung out to dry inside, or water that has been poured on a plant, or water in our skin. Water evaporating from skin is our main mechanism for losing heat, and our sense of temperature depends largely on how much heat we are losing. This makes humid places feel hotter, because less moisture will evaporate into humid air than into dry air. 

According to the ever-reliable Engineering Tool Box saturated air at 20 degrees centigrade has almost three times more energy than dry air relative to dry air at freezing, so the above figures would be more like 12.1 kCal per kilogram at 80% humidity and 7.5 at 30%. That's 60% more heat. 

The consequence for ventilation is that if you have a system that is exchanging heat but not moisture (HRV), then in the winter you're going to be losing a lot of energy in the airborne water vapour you expel as you bring in dry air. And in summer you're going to be bringing in a lot of heat to the house embodied in the humid air. 

So an energy recovery system will lose less energy, and is probably a good idea if you have dry winters or humid summers.

Of course you're unlikely to be choosing between 30% and 80% humidity. In the summer you may have 80% humidity and want 30%, and in the winter you're likely to get 30% humidity, but probably wouldn't want as much as 80%.

However, as the Polaris site points out, energy recovery systems transfer moisture back into the house. They usually do this across a paper membrane. Along with that moisture you can also get some bacteria and odours, which often makes people reluctant to use energy recovery ventilation in kitchens, bathrooms and toilets, putting simple extractor fans there instead. For example Mitsubishi suggests a dedicated extractor fan for the kitchen, and offers a system with additional drying, heating and direct ventilation options for the bathroom. 

Since kitchens, bathrooms and toilets are the places you usually want to extract air from, while supplying fresh air to bedrooms and living spaces, you may end up only using heat recovery ventilation for a fraction of the house, and any efficiency gains are lost, perhaps along with improvements in interior humidity. Unless of course you can recover heat and moisture without letting anything else through.

Special thanks to Ben Shearon for asking questions that lead me to investigate this topic.

Note from Wikipedia: In SI units, cs = 1.005 + 1.82H where 1.005 kJ/kg°C is the heat capacity of dry air, 1.82 kJ/kg°C the heat capacity of water vapor, and H is the specific humidity in kg water vapor per kg dry air in the mixture.