I have a 20x25x4 filter box at my furnace. I can't find any filters that size in town. I asked my HVAC technician where to buy this size filter. He said he'd never seen any that size. The thickest he has seen is 2". He suggested stacking two 2" filters together in my filter box? Any thoughts on this??
Thanks.

Welcome aboard Hysu.  I have never seen or heard of a 4" filter either.  How about fabricating up a 2 or 3" spacer to sit in front or behind the filter?  Shouldn't be too difficult to do.

Your filters can be purchased.  Search on-line for them.

 

https://filterbuy.com/air-filters/20x25x4/merv-8/?utm_source=google&utm_medium=pla&utm_campaign=google pla#1-pack&gclid=EAIaIQobChMIqInF77fh4gIVxoSzCh2VqgeSEAQYASABEgII_fD_BwE

 

If this a new furnace the place you bought it from can tell you and they probably sell filters...The company we bought our furnace from sells the best filter that Lowes and Home Depot sells only much cheaper than either store...

I wonder if the original filter was one of those electronic/electrostatic types. The neighbors had something like that (not sure of their exact dimensions).

 

I don't see any problem with stacking two 2" filters. When changing them, you could probably rotate the filters by replacing the "first" (dirtiest) one with its' backer and putting the new one as the "backer". Of course that's my frugal upbringing showing.

 

I just saw a TV commercial for a business that sends you filters on a regular basis- https://www.filtereasy.com/. You don't even need to go to the Borg!

Cal's idea or Lew's are workable.  I prefer to purchase all that I can from local retailers.  I like to see what I am getting.  When a child, my grandparents most always bought from Spiegel catalog and sometimes it was damaged and returns costs were horrible.  And, you had to wait.  If you want to use normal filters, I would create something to ensure the filter sat as close as possible to the opening going to the furnace.  It helps keep the blower fan clean.  

I don't think stacking two filters is the way to go.  You will be drawing air through two filters instead on one.  A four inch filter is just one filter fabric same as one two inch filter.  It is folded in four inch pleats and increases the square inches (area) of the fabric but still just one filter.  I would use just one two inch filter and change it as necessary if you don't want to order the four inch..

Edited June 11, 20197 yr by HandyDan

2 hours ago, HandyDan said:

I don't think stacking two filters is the way to go.  You will be drawing air through two filters instead on one.  A four inch filter is just one filter fabric same as one two inch filter.  It is folded in four inch pleats and increases the square inches (area) of the fabric but still just one filter.  I would use just one two inch filter and change it as necessary if you don't want to order the four inch..

+1

As Dan said you will be restricting the air flow with 2 filters stacked and the first one will be the one doing all the work.  Roly