What's the story behind HVAC stuff using "nominal" sizes for filters and other stuff? I know some manufacturers list the actual size, but a lot of times it's just a rounded up number....what's the point? Is there some history behind this kind of like how a 2x4 isn't 2" x 4" anymore?
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HVAC measurements
- Thread starter DeeKay
- Start date
LS6 Tommy
Well-known member
Filters are smaller than the nominal label size, generally around 1/4" smaller than label size. Unless you have a specialized application, order by label size.
Tommy
Tommy
Filters are smaller than the nominal label size, generally around 1/4" smaller than label size. Unless you have a specialized application, order by label size.
Tommy
I know that, was just wondering what the history is behind using a nominal size.
metlmunchr
Well-known member
- Joined
- Sep 10, 2011
- Messages
- 1,278
Precision costs dollars. By making filters less 1/4" on each dimension, both the filter mfgr and the equipment mfgr have some leeway in mfg tolerances. If filters were on the stated size, the only thing it would accomplish is forcing equipment mfgrs to enlarge their filter racks to accommodate manufacturing variations in both the filters and their equipment.
Lots of people think everything that's manufactured should go together with precision location fits, but anyone who's ever been involved in volume manufacturing knows the public wouldn't pay for that precision in areas where it accomplishes nothing. The fit of filters in filter racks is one of those areas.
Lots of people think everything that's manufactured should go together with precision location fits, but anyone who's ever been involved in volume manufacturing knows the public wouldn't pay for that precision in areas where it accomplishes nothing. The fit of filters in filter racks is one of those areas.
LS6 Tommy
Well-known member
Precision costs dollars. By making filters less 1/4" on each dimension, both the filter mfgr and the equipment mfgr have some leeway in mfg tolerances. If filters were on the stated size, the only thing it would accomplish is forcing equipment mfgrs to enlarge their filter racks to accommodate manufacturing variations in both the filters and their equipment.
Lots of people think everything that's manufactured should go together with precision location fits, but anyone who's ever been involved in volume manufacturing knows the public wouldn't pay for that precision in areas where it accomplishes nothing. The fit of filters in filter racks is one of those areas.
Metlmunchr nailed it. There's no need to be that exact. Even with the seemingly loose tolerances that exist, air is a fluid and follows the path of least resistance. It's easier to flow through the filter than around it. There is very little bypass.
I have to duct tape the inlet side of all the filters on three units at work because the Engineering Department thinks the filters are "too loose" and they're letting dirt bypass the filters. The filters they want taped are pre-pre filters and pre filters. Then there's a set of HEPA filters that have built in seals. There is zero bypass before the HEPAs. Even if a spec of dirt got by both sets of pre filters, the HEPAs would get it.
Tommy
I totally get that they don't have to be exact to work for HVAC, It's not like these are filters that are going on a Turbine filter house or anything.
There's got to be some history here though,
These days It doesn't cost any more money to to make a filter that's actually 16"x25"x1" rather than 15.75x24.75x.75 (Aside from the tiny bit more material)
I guess back in the day when manufacturing wasn't quite as exact, probably not automated maybe filters did vary a little more to save a few bucks, but be honest have you guys ever actually gotten a filter that's not pretty much exactly a 1/4" under sized?
There's got to be some history here though,
These days It doesn't cost any more money to to make a filter that's actually 16"x25"x1" rather than 15.75x24.75x.75 (Aside from the tiny bit more material)
I guess back in the day when manufacturing wasn't quite as exact, probably not automated maybe filters did vary a little more to save a few bucks, but be honest have you guys ever actually gotten a filter that's not pretty much exactly a 1/4" under sized?