Thread: Strange furnace filter

Get a load of this bizarre and totally ineffective filter on a 1981 Lennox furnace. The return is on the left.

Lennox filter.jpg

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To the best of my recollection its called a 'hamock style filter'.

I see them every once in a while John. I tell the buyers that they will be a real pain in the ass to change. Most of the time, the seller has a box of the filter material close by and when the filter needs to be changed, you have to roll out the filter material and cut a new section.

I particularly like the 2-3" void between the return and filter.

Originally Posted by Nick Ostrowski

...I particularly like the 2-3" void between the return and filter.

Yes, that gap is why I described it as ineffective. Well, the whole thing is ancient and rusted/corroded and needs to be replaced anyway.

Lennox was going for a larger area of filter with the hammock style so you would have to change it less frequently. Twice a year the instructions said. It was cheaper and maybe easier to change than the double "A" frame filters above the fans that used to be around.

They are a mild pain to replace but it's not that bad. I think they discontinued this filter system with their furnaces around 1990.

An equally crappy filter is the type that is the blue horse-hair material that has no frame around. It gets its rigidity from three metal rods that you have to manually run through the filter.

The minute you go to remove the filter most of the dirt falls out right beneath the blower.

I do caution buyers that the high efficiency filters with the tight pleats can and will restrict air flow and they'll need to change them every month. I've heard of fan blowers shutting off and even burning up and needing to be replaced due to restricted air flow. I typically recommend a middle-of-the-road quality filter as it will still offer decent filtration and air flow.

Originally Posted by Nick Ostrowski

I do caution buyers that the high efficiency filters with the tight pleats can and will restrict air flow and they'll need to change them every month. I've heard of fan blowers shutting off and even burning up and needing to be replaced due to restricted air flow. I typically recommend a middle-of-the-road quality filter as it will still offer decent filtration and air flow.

Good advice. In these parts, given that the majority of return air openings and chases can be (and often are) marginally sized or (frequently) under-sized, we often recommend the customer use the blue fiberglass throwaways. Not as effective in catching smaller particulant, but restricted airflow can cause more problems than many realize, including compressor failure due to liquid return (slugback) to the compressor, moldy duct work and supply grills, black discharge from supply grills and more.

In negative pressure systems, it can also prevent condensate drainage through the drainline, picking up the water and slinging it into the blower section or even the supply plenum, often leaking into the living space.

As for the filter design, yet one more stroke of proprietary genius (sarcasm) from Lemmox. They show no signs of slowing down.

Reduction of air flow over the heat exchanger on high eff. furnace due to the wrong air filter can shorten the life of heat exchanger, so I have read.

Originally Posted by Kevin Wood

Not to bust your bubble Bob but I have seen more restriction from plain bottom of the line filters as there are no pleats what so ever. I do however agree on some of the design's just do not get it right and this includes LENNOX.
The Duststop Premium Filter are the best of the throw away as they can be cleaned on the surface and I believe they even set up a monitor service to change them out.
I am not sure if they are available in the US.
Supreme Filters

Not a problem. A little bubble busting now and then is healthy .

Of course, we are from different areas of the world (err, continent), but the blue throw away fiberglass filters here are as free flowing as it gets. The pleated filters (again, in our area...) are most always comprised of a much denser material, as is the case with the Supreme Filters you link to. No filter with a merv rating 10 can be more free flowing than the blue fiberglass.

The trade off is filtration performance, but this is often an acceptable trade when the cost of restricted airflow is recognized. I'm a proponent of better air filtration, but system performance and operational (and repair) costs must also be a factor. The health benefits of well filtered air are important, but those benefits evaporate when the air quality issues that arise from restricted air conditions are considered.

If both the return air chase and opening are properly sized, denser filters are a positive. Unfortunately (as also previously mentioned), in this region, R/As are frequently not large enough to make the denser filters practical. Chases and openings must be designed (or changed) to accommodate denser filtration unless the system has been designed with denser filters in mind.

Since denser filters are a recent event and marketing frenzy, this leaves many owners of older homes holding the bag for what could be some fairly expensive "repair" bills for enlarging chases and openings. And yes, they could use the denser filters anyway, but then the aforementioned disadvantages come into play.

Measure with a manometer and guessing goes away. Most mfrs want total External Static Pressure under .5 inches of water column or you burn up lots of things, including a lot of energy inefficiently (and yes, fans and heat exchangers too.)

If you don't measure, the cheap Fiberglass is probably the best suggestion, though AC coils will probably require more frequent cleaning.

Modern furnaces with "communicating features" actually measure static, which is nice because it will tattle on bad design (too much furnace attached to too little duct), and also let you know as filters get dangerously restrictive.

The best filter systems are 4" wide media type. They tend to be least restrictive AND most effective at filtering air. If you pull them apart they have a LOT of surface area for air to flow through, and for dirt collection.