Can't read the air

Building culture changes around the world. People talk about "vernacular architecture" as if buildings are all having conversations with each other, but often the languages seem to be mutually unintelligible. 

Heat recovery ventilation is one area where a lot seems to be lost in translation. And there was plenty of hot air to start with.

In Japan, ventilation systems are usually called ２４時間換気 niju-yo-jikan kanki  i.e. 24-hour ventilation. It seems like a strange name, as if you would call your refrigerator a 24-hour refrigerator, or your roof a 24-hour roof! They have been mandated in new buildings since 2003, but a lot of people switch them off. So they are not really 24-hour after all. There are plenty of reports of people who have switched off their ventilation because it makes the house cold, but if you do that, as Last resort blog (in Japanese) warns, you're going to get condensation.

顕熱交換 ken netsu koukan  is heat recovery ventilation (HRV), while 全熱交換 zen netsu koukan is energy recovery ventilation (ERV) which also recovers moisture, and is probably the more common choice in Japan when any kind of heat is being recovered. Translation aside, this may be a shock for those who read the first law of thermodynamics and thought heat and energy were the same thing. My dictionary gives Ken netsu as "sensible heat". And that's not a very sensible expression in English where "sensible" usually has a different meaning. The opposite of "sensible heat" is not "foolish heat" but is "latent heat".

The Japanese language often uses abbreviations of English words that are not commonly used in English. For example PA is used for parking area, and IC for interchange, and you will find these driving on the expressways. KY is an abbreviation of kuuki yomenai, ie cannot read the air. Reading the air means reading a situation, so KY refers to someone who is out of touch with the rest of the room.

Japanese building plans with ventilation systems will show SA for supply air, RA for return air, EA for exhaust air and OA for outside air. These abbreviations are occasionally used in English.

There is a page here that:

proposes a housing environment that has a "Flow of healthy air" and is "Clean and safe".

It should be pointed out that inverted commas are used in Japanese for emphasis, so the inverted commas around "flow of healthy air" and "clean and safe" do not mean that somebody said it was healthy, clean and safe, but they're actually being sarcastic. 

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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.

Acknowledgment:
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.

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A breath of fresh air in a sea of thermodynamics

I remember a conversation between our architect and one of the potential contractors, and the question was what kind of heat recovery ventilation system we were using. I'd only just discovered that heat recovery ventilation was possible, and didn't dwell too much on this question. The answer was that we were using heat recovery ventilation rather than moisture recovery ventilation.

To recap:

• If you want a warm house, you need insulation.
• If you have insulation, the house should also be airtight. 
• If it's airtight you need mechanical ventilation with heat recovery.

Insulation, airtightness and mechanical ventilation with heat recovery represent a holy trinity of low energy building that cannot be violated. 

Without insulation, you're going to lose a lot of heat,  and you're going to get cold spots on the thin external walls, which will lead to condensation. Cold, expensive to heat, and damp!

Without airtightness, you're going to get humid air passing through the insulation and at some point that will lead to condensation. It will probably happen in the worst possible place: where you can't see it, but your structure can.

Without ventilation, you'll eventually suffocate, but way before that the humidity is going to get so high that you'll get condensation even where there is airtight insulation. 

Mechanical ventilation is best because any kind of natural ventilation will usually lead to too much or too little exchange of air, depending on how nature is feeling at a particular time. 

If you're going to have mechanical ventilation, you should put in a heat exchanger and then you don't need to throw away all the heat in the air.

...

So five years later I'm still learning things about ventilation. I've written a bit about our problems with heat recovery ventilation, but I know even less about the other kind: energy recovery ventilation, or moisture recovery ventilation. These are abbreviated to HRV and ERV for any fans of the TLA (three-letter acronym). 

In both systems the air leaving the house passes through a heat exchanger and there is a transfer of heat to the air coming into the house across a membrane. In fact it is an array of membranes and with a well-designed arrangement of cross flow and counter flow, you can recover over 90% of the heat in the air. The transfer of heat follows the third law of thermodynamics, from hot to cold, so this will keep the house warmer in the winter, and cooler in the summer.

The difference between the systems is in moisture. Heat recovery ventilation just transfers heat, while energy recovery ventilation also allows moisture to transfer. This will also move from higher to lower humidity, so will tend to keep the humidity out in the summer, and stop the house from getting too dry in the winter. 

So which one should you get? 

As usual there are different schools of thought:
A) A is definitely better than B
B) Only a bloody idiot would use A
C) There are good and bad points of both so in the end it doesn't make a lot of difference which one you choose

It's probably more fashion than physics, and I'm going to be writing about fashion soon!

But until that link works, you can read about the different kinds of heat exchanger from Zehnder America.