The filters in our furnace aren’t actually in the furnace: they’re in the duct running out the top of the furnace, which means the furnace manufacturer can’t (or won’t) offer much help in choosing the right size filter.
The manual has a chart that’s quite useless: if the nominal air flow is x cubic feet per minute, the furnace filter should be y×z inches. That’s nice. How am I supposed to know the nominal air flow?
It looks like our filters are 20×16 inches, which is – I think – a pretty standard size. But it doesn’t match any of the suggested filter sizes in the furnace owner’s manual, so I’m a little confused. I suppose we’ll have to pull out one of the old ones & measure it directly.