Furnace Filters

[Robert Gammon]

I got to reading up on this topic as research fora new house that I want to start building soon.

I read that most furnace fans are only capable of driving about 0.5 in water of pressure. AC coils take 0.2 to 0.3 in of water pressure drop, ducts take another 0.1 to 0.2 in of pressure and then nothing is left for the furnace filter. That's why the cheapest of all filters are generally included on furnaces.

To top it off, furnace installers often do not read the specs for the units that they are installing.

We had our HVAC replaced about 3 years ago with a 12 SEER,80% gas combo. At the time, I insisted that the existing 20x20x1 furnace filter be replaced with a 20x25x1 model ($50 part,$50 filter(20x25x5 Honeywell), $200 labor).

As I have done this research, I have looked at the face of the $6 filter I have installed in the unit presently. It is heavily bowed when the furnace is operating (but makes less noise than the old furnace with a

20x20x1 filter in it). So much bow is there that I worry about the filter collapsing. So far, so good, none have collapsed.

However, reading the specs NOW, its clear that the HVAC installers were not reading the manual. In the mfg's manual, it clearly says for this size fan, operating at roughly 1200 CFM (can operate as high as 1328 with no load),the unit needs not 400sq inches that I had, not 480 sq inches that I now have, but 576 inches (24x24). This extra back pressure on the fan is ROBBING performance, and costing me money. It is at least a 10% drop in air flow.

Cheap solution is to revert to 99 cent fiberglass and change them every few weeks. I want to think of something better than this. BTW increasing the size of the filter to 20x30 is not do able as the structural supports for the furnace are in the way. Any ideas are welcome.

[Eigenvector]

Mass in equal mass out always (that doesn't mean air speed equals air speed). If your furnace is sucking in air at a certain amount per time it is blowing it out at a certain amount per time - otherwise a leak would be indicated. A long duct would increase frictional forces on the air and reduce that amount per time.

Go get an electronic filter, if air efficiency is what you are after. Honestly you are losing more in the ducting than you are in the filter. The ducting of a typical house is not airtight, has very high friction surfaces (dust, mice, insects, water, melted crayons, etc..), and the layout of the ducting is less than optimal for airflow. In the distances you are talking about only the friction really matters all that much.

So if you want to maximize your airflow, get a good air filter and keep your ducts clean. Those cheapo filters I'm sure minimize air flow resistance, but they are putting lots of dust into your furnace and wearing it out faster, as well as putting lots of dust in your lungs.

[professorpaul]

I went to my local HVAC guy and had him make me a washable filter for $10. Some sort of rubber covered hair like stuff. I also got a spray bottle of soluable oil to help pick up the dust. Every month I simply give it a hose down in the shower, let drip dry, and spray again. This is what I did many moons ago with the filter in an old Tektronic oscilloscope.

The friction loss in the pipe/ducts is the main culprit. I have had an electrostatic air filter and mechanical filter backup in one house I owned. Had to turn a hose on it every 3-4 months, and also used the filter oil on the screen, which became FILTHY!

[wayne]

[Don Young]

If you have room to do so, you can install a larger filter slanted instead of flat or install two filters in a V configuration to get more filter area. A little work with pieces of metal angle (the suspended ceiling type is cheap and light weight), a drill, and screws or pop rivets will make a suitable holder. You can put some heavy welding rods behind the filters to keep them from bowing, if needed.

Don Young

[Robert Gammon]

There were some good suggestions here. Thanks for the ideas.

Electrostatic filters do a good job of cleaning, but they are lossy filters. A significant amount of dirt escapes and must pass thru the filter a second or third time before it gets captured. By that time, it probably been trapped on the AC coil, in a duct somewhere, settled onto a surface (ANY surface) in the home, or in an occupant's lungs. Electrostatic filters get a bad rap in the HVAC industry when they are used as the primary filter. Interior surfaces of the furnace get plated up with dust. They work better as a stand alone filter like the one that Sharper Image sells (Richard Thalheimer on TV with a baby in the most recent commercial)

The V-bank is a good idea too. Used in commercial air handlers, it doubles the air filter area, and halves the air flow rate thru the filters. We may be able to buy such a rack and install it, or build it as suggested.

Most of the data I have seen on the 2" filters indicates that the filter mfgs are using more efficient filter media in them so the friction loss is higher. 0.20 clean pressure drop are the figures I have seen for MERV 8 - MERV 11 filters.

As static pressure increases (duct resistance plus, AC coil resistance plus, filter resistance) velocity falls. With zero external resistance, my furnace can deliver 1328 CFM. With 0.5 in wg of external resistance, air flow drops to about 1150 CFM.

In this house, the furnace is centrally located. roughly 90% of the airflow is thru ducts that are less than 10 feet in length. Unfortunately, most of these ideas fail to address an issue important to my health. I am mold spore sensitive. To keep my chest clear, I need a filtration system that captures most of the particles 1 micron in size and larger. The low efficiency filters that the furnace mfgs want us to use are only effective on the 10 micron and larger particles, the stuff we can see on surfaces, the junk that covers the surface of the air filter.

One solution is a separately powered HEPA filter. Has its own fan so it does not load the air handler,and it can be set to run continuously, circulating air thru the entire house. Unfortunately, I can't install it in this very compact house. It attaches to the return air plenum, processes some (or all in some configurations) of the air and returns it to the plenum.

Another possible solution is another commercial/industrial filter design. Synthetic media bag filters are available in sizes to fit my filter frame. As per ASHRAE 52.2 ratings MERV 12 is what they get spec'd at. These filters capture about 70% of the 1 micron and larger particles, over 90% of the 3micron and larger particles and near 99% of the 10 micron and larger particles.

Ok what are the issues with this filter?

1. Price - not too bad, about to each
2. Lifetime of the filter - excellent, change once or twice a year
3. Pressure drop -Excellent, I can pick models that approach the pressure drop of MERV3-4 filters,i.e. 0.07-0.08 in wg at my furnace's flow rate
4. Available sizes - BIG problem in some cases 12x24, 20x20, 20x24, and

24x24 are the ONLY sizes available. You can pair these up to create larger filters if you provide gaskets between adjacent filters, or you give each one its own frame.

1. Mounting - Good, The headers on the filter are compatible with 1" filter frames.
2. Strength - bending. Excellent, these filters have Steel frames. They are designed for use with systems that can achieve 500fpm across the filter face vs the 300 fpm that disposable filters can tolerate.

7.Availability - any HVAC company that services commercial buildings knows where to get these and there are several suppliers on Internet that will accept orders by phone too.

There are still other mechanical options for these filters. Some come in steel or wood boxes. Some come in rigid plastic frames (several V pattern filter faces). Most of these other options have at least one or two problems.

1.Pressure drop - the unsupported bag filters have the lowest pressure drop of any available MERV 11 or MERV 12 filter

1. Mounting - many of these other options do not include header we need to mount the filter in a residential filter frame.

[wayne]

[Robert Gammon]

I have ONE return duct. It sits right under the furnace. I put the filter here, there is no other place for it.

2" and 4" filters, even MERV 8 versions, put a serious burden on the air handler

I have seen the specs for Filtrete, PrecisionAire, Purolator, Accumulair and several others in a 2" deep size. They all put too much pressure on the fan.

I change the filters on a regular basis, usually about every two months with a pleated MERV 8 filter.

The message from the furnace manufacturers is CLEAR, we care about the equipment, we don't give a damn about you. Except for some ground source heat pump manufacturers,who now include very large 28x30 (and larger) MERV11 2 inch filters with the unit, AND they have increased the fan HP from 1/3 to 1/2 HP AND they offer a fan upgrade to 1 HP on the most popular 3 ton and 4 ton models. These units come with a 2 inch filter rack installed and there is an option to install a 4 inch filter rack.

My fan is a 1/3 HP fan like almost everyone has in a 3 ton unit these days.

[wayne]

[Robert Gammon]

Built in 1982. 1200 sq ft.

Return air comes thru ONE single point source, a plenum of plywood and timber underneath the furnace (up flow model).

Extremely common building practice in the late 70s and early 80s in most parts of the USA that use forced air heating and cooling.

Houses in the 90s started the return duct per room idea ( at least for rooms with size)

Supply ducts to laundry room, master bedroom, master dressing area and master bath are under 6 feet long. Kitchen and living room ducts are under 10 feet long