Friday, 26 August 2016

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.