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by tempestn
1222 days ago
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That's not always feasible, as the built-in filter is usually integrated with the air handler. The add-on will generally have to be in the inlet ducting, so it will come first. I'm not sure that approach would even reduce cost though; much of the benefit of the large filter is that it has way more surface area, and so it lasts longer before restricting airflow too much. If you put a 1" filter in front of it, now you'll have two filters you need to replace regularly. You might preserve the life of the larger filter a bit, but not enough to make up for the cost of the whole extra filter. So I think I'd prefer to either just use a 4" alone (which is what we indeed do), or have the 1" as a backup, and basically use its condition to test whether anything significant is getting past the main filter. It would very rarely need to be replaced, as the primary filter should be catching everything. |
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I wouldn't use a prefilter with any real pressure drop thought. Why not? Well...
In theory a two-stage filter is ideal, because you can cycle the filters through: swap the (mostly clean) post-filter over to the pre-filter during filter changes, optimizing both filtration level and using the full capacity of each consumable filter element. This is the procedure when changing the ISS water filters, incidentally.
There's a downside, of course...
Essentially it's the same as series and parallel resistors, so for two filter stages in series (to achieve the same rated pressure drop) you need double the rated size for each of the stages, therefore 4x the total filter area and size. In practice, nobody really wants to install that in their basement.
Some of the Chinese positive pressure systems have seemingly the ultimate low-consumables design: a washable stainless prefilter, washable electrostatic filter, two stages (supports cycling) of HEPA filter, and last a refillable granular activated carbon stage. Spent activated carbon could be used as a soil amendment, or returned to a local facility for regeneration into new activated carbon.
Very low consumables, but very costly up-front.