Friday, 23 September 2016

Cross leakage and cross contamination

So the big issue with Energy Recovery Ventilation is that along with the moisture, other things are going to be transferred from the exhaust air to the incoming air, compromising its freshness.
This is also called cross leakage, or how much of the outgoing air will end up coming back in again. It even has an acronym: EATR (Exhaust Air Transfer Ratio).

Air refers to US ASHRAE standards on cross leakage. Exhaust air is classified into four different groups: Class 1 air has low contamination, for example from office spaces, classrooms or corridors. Class 2 air has moderate contamination, for example from rest rooms, dining rooms, warehouses. Class 3 has significant contamination, for example kitchens, beauty salons, pet shops. Class 4 air has highly objectionable fumes or potentially dangerous particles, for example paint spray booths, laboratory fume exhaust or kitchen grease exhaust.

The US standard states that less than 10% cross contamination is acceptable for class 2 air. This seems like a lot, but in practice you will never get 0% contamination, even with a heat recovery system that is not trying to transfer moisture. Energy recovery systems can get as low as 1%.
Mitsubishi has a report on their Lossnay ventilation system with evidence from a test in 1999 that their membranes are fine enough to prevent bacteria from passing from exhaust to incoming air. They have more information about their systems in English here.

If the membranes are this good, then perhaps we should be using ERV after all, and they should be recommended for kitchens and bathrooms. 

Another compounding factor with kitchens in Japan is that a lot of stir frying, deep frying and grilling seems to take place in Japanese kitchens, and there is usually an extractor fan with three or four times the ventilation needed for the whole house. More about kitchens later!

Green Building Advisor has a useful comparison of ERV and HRV, with a nice aside: "...assuming, of course, that the designer or installer hasn't made any blunders. (Sadly, this can be an optimistic and risky assumption.)"