New furnace supply plenum and filter Question.

[TOOMANYHACKS]

We recently had a new furnace installed and when I was in the progress of getting filters for the furnace I found a discrepancy between the size of the supply plenum (and filter) actually installed (16x24x1) and the size indicated in the Installation Instruction book (16x25x1). The natural gas furnace is a Concord model 95G1UH090CP16 (made by Allied Air) and is installed upflow in the basement. The supply is a side return and they only allowed space for a 1" filter so 4" media filters are out of the question. We will be adding a new central air soon in our 2 story home.

1) Is this under-sizing of the filter and plenum a problem that we need to address with the installing company?

2) Also, what is the best filter to use in the system? I have allergies and asthma so something better than the fiberglass filter is desired. We had a Neutron electrostatic filter in our last furnace and like the idea of washable filters. We made sure it was washed every month for the 20 years our last furnace was installed.

One of the techs on this site had a filter pdf that I read and it suggested the BoAir Electrostatic filter as the one with the lowest Arrestance of air flow. Is this good or what do you all recommend.

Thanks for your input on the above Questions.

[TOOMANYHACKS]

Correction to 1st post... I meant return plenum, not supply. Sorry. Multi-tasking not good... True it is the supply of air into the furnace but that is really the return Plenum and filter. :whistle::grin2:

[llskywalker]

Measure your return air opening where the filter goes. A filter that states 16x25x1 is actually 1/2" smaller all the way around, so it would actually be 15.5"x24.5"x1/2". Hope that makes sense. You would definitely want to make sure that the filter has low resistance to air flow. If you install a hepa filter or high filtration filter, it starts to make your blower work harder and will reduce your overall air flow, which wouldn't be good when you add the A/C. Have you looked into an electronic air filter? This is 4" electronic with a pre filter, no longer need to purchase any filters. You could also add a uv light. Good luck.

[TOOMANYHACKS]

Measure your return air opening where the filter goes. A filter that states 16x25x1 is actually 1/2" smaller all the way around, so it would actually be 15.5"x24.5"x1/2". Hope that makes sense. You would definitely want to make sure that the filter has low resistance to air flow. If you install a hepa filter or high filtration filter, it starts to make your blower work harder and will reduce your overall air flow, which wouldn't be good when you add the A/C. Have you looked into an electronic air filter? This is 4" electronic with a pre filter, no longer need to purchase any filters. You could also add a uv light. Good luck.

The way they installed the return plenum there is not enough space between the plenum and the furnace to get a 4 inch filter rack in. They would have to move the plenum further from the furnace to get more width between them for a 4" media filter or a 4" electronic filter.

I measured the inside filter opening and it is 15" high x 23.5 deep. The cheap fiberglass filter that they installed is marked 15x24x1 and fits snugly.

[llskywalker]

An electronic filter can be installed in the vertical section of the return air drop. It doesn't need to be installed directly next to the furnace as long as all the return air goes through it. Filter size looks to be correct based on your measurements. The other thing that you can do is angle a 2" or 4" filter in the return drop before it goes into the furnace.

[udarrell]

Okay if you want to do it by the ACCA manual D book:

A 90,000-input gas furnace @95% efficiency is 85,500-Btuh output @ 60-F temp-rise * 1.1 is 66; 85,500-Btuh output / 66 is 1295-CFM for heating.

1295 / 300-fpm velocity required by M.-D & Hart&Cooley Engineering Data, is 4.22-Ak sq.ft of actual filter open-air-area for 1260-CFM; or a 36X24 filter.

Or approximately, 'two' 26X26 Return air filters each filtering half of the air volume of the home; seems outrageous, right? It is a matter of applied math, so I don't argue with it...

Hart&Cooley engineering also claims that a filter velocity of over 400-fpm will exceed the performance of a media type filter; other places they clam 500-fpm.

There are claims that a pleated filter can start at 500-fpm velocity, however a 1" deep pleated filter as it loads will end-up having an excessive pressure drop...

The more 'filter area' you can provide the better...