If all the air goes through the filter you have now that is all you'd need. In fact, it is better to filter out the debris before it gets into the return duct at all, so it stays cleaner in there; the filter in the hall grill is a better place. www.heatpro.us energy businessmen's knowledge Yes, you really have to find out the MAKE and MODEL to get good answers. There IS more than one machine made.
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