A heat recovery ventilator includes four rectangular regenerative heat exchangers, two blowers, a rotating air switch all disposed in a compact rectangular housing. The regenerative heat exchangers are made of a pleated HEPA filter material. The HEPA filter material captures at least 99.97% of particles...http://www.google.com.hk/patents/US6289974?utm_source=gb-gplus-share專利 US6289974 - Integrated heat recovery ventilator HEPA filter using a HEPA filter material regenerative heat exchanger
