We have an Aprilaire 2200 and the standard filters for it are MERV 8. I bought a MERV 11 thinking it would be better, it seems to move the same amount of air and doesn't seem to be plugging up any sooner than the MERV 8 did. Now that I am due for a new filter I see they now have a MERV 13 filter that fits in the same space.

My initial thought was the higher the MERV the better the filtration, but the sooner I would have to change the filter. I do have a Magnehelic (.5) between the room and the suction side of the furnace, after the filter. It usually starts about .15 and I change the filter before it gets to .25

Should I try the MERV 13 or stick with MERV 11?

As you increase the filter efficiency there will be a higher static pressure drop across the filter. Most residential furnaces have only a total of 0.5 esp capability. If you have an evaporator coil along with long duct runs or poorly designed ductwork there will be a loss in airflow and total furnace efficiency.

I'd normally agree with ACwiz, but since you have the Magnehelic go for the MERV 13 and stay with your present changing regimen.

OTOH I'm not sure I'd want a dirty filter to cost me a full tenth INWC, consider changing at 0.05" added INWC

The higher MERV filter may have more pleats, allowing it to go longer before clogging.

All filters collect better as they load up. If you want to start with a higher Merv# (in the same size filter housing) you can look forward to more frequent changes.

All good info. With the MERV8 I believe it was at .13 new and with the MERV 11 it was at .15 new, maybe the MERV 13 would be .17-.18 new? And yes I tend to change them early rather than wait, like right now I am at .19 and looking for a new filter (been 6 months now) but I know once I have the filter sitting there I will change it

But ignoring the technical side of it, what are people actually using or would use given the choice? Would everyone just jump on the MERV 13 or maybe stay with MERV 8 for more air flow? or stick with MERV 11?

Posted By acwizard on 13 Sep 2011 10:35 PM
As you increase the filter efficiency there will be a higher static pressure drop across the filter.

BUT, increasing the surface area will reduce the static pressure drop.

More pleats and/or deeper pleats will increase the surface area.

Fifth Law of Thermodynamics: "All else being equal" ...never happened!

Bear in mind that the basic purpose of an HVAC air filter is to keep the coil clean enough for efficient heat transfer while also keeping delta P down for efficient blower operation. HVAC's primary purpose is to heat and cool the home, so it tends to move a heckuva lot of air. Moving a heckuva lot of air operates at cross purposes to deeply filtering that same air.

HVAC is not HEPA

Rather than sweat MERV values (or whatever scheme / scale that seems to have succeeded MERV) it may be more useful to focus on ensuring that the filter fits securely in its carrier. MERV 1 billion is worthless if return air bypasses filtration owing to a sloppy filter bay. On my air source clients I use Trane's Perfect Fit 5" filter box. It has gaskets and clamps that tightly hold the filter and the 5" depth of the filter allows clients to defer filter changes to our 6 month maintenance visits.

I add foam tape to better seat the filter on geo units that commonly take a 2" filter.

Posted By Looby on 14 Sep 2011 06:18 PM

Posted By acwizard on 13 Sep 2011 10:35 PM
As you increase the filter efficiency there will be a higher static pressure drop across the filter.

BUT, increasing the surface area will reduce the static pressure drop.

More pleats and/or deeper pleats will increase the surface area.

Fifth Law of Thermodynamics: "All else being equal" ...never happened!

All of this information is true but we are talking about a fixed dimension filter. The higher the Merv rating , the higher the filter can trap smaller particles. The media is woven tighter and has a higher arrestance. With the higher arrestance comes an initial higher resistance pressure drop. I do not use these filters because of the pressure drop. By the time one adds up the evap coil, filter and associated ductwork sp drops along with the registers and grilles the available static left on the blower has been consumed. Higher static translates to higher hp and system efficiency losses. The  seer rating of the installed equipment has now been reduced .

The merv 8 had a lower resistance than the merv11 , but not by much. I guess unless I get a MERV 13 and see what it is I won't know how much it will drop.

I should add that initially the furnace had a 20x20x1 filter and I opted for the 20x25x6. I was told not to use both filters but when we first moved in I did. I used a standard filter in the 20x20x1 to catch the big stuff and then the 20x25x6 after that and changed the 20x20x1 monthly. For the heck of it last night I took out the big filter and put in the 20x20x1 which happens to be a MERV8. The pressure on a new 20x20x1 merv 8 was .22, I added the big filter back in and I was at .25 and took the 20x20x1 out and and it was back to .19. So I think even the merv13 big filter would have less pressure than the 20x20x1 merv 8, which makes sense looking at the surface area of both filters.

The numbers stated seem to coincide for the 20 x 20 x 1, but adding the Aprilaire filter in series and getting the reading of .03 addition can not be correct. The magnehelic needs to be at the blower cabinet to see the true negative static pressure. The standard Aprilaire 2200 filter is a Merv 10 @ 1200cfm with an initial pressure drop of .21Inches of static @ 500 feet per minute. Do not be fooled by visually looking at the pleated surface area. What mfg.and model filters are you using and make and model of furnace.

Sorry I know just enough to be dangerous. I have the magnehelic right in front of the fan blower intake at the bottom of the furnace, after the aprilaire2200 box which is after the space for a 20x20x1. During construction they only used the 20x20x1.

I was also surprised by adding the big aprilaire filter if it was .19 on it's own it only went up by .03. My only guess, probably wrong, was that the restriction of the 20x20x1 was doing most of the blocking and the larger filter didn't add much reduction after at that point. I could be totally wrong, but those are what the numbers were. The thing that amazed me was the .22 on a clean 20x20x1 So using the larger aprilaire filters starting at .13 or .15 or whatever the merv13 might be must be better than using a 20x20x1.

I have a bryant plus 95s and the filter I have been using were the merv 8 and merv 11 from
http://www.oxyclean.com/aprilaire-filters-2200-2250-Pre-assembled-s/59.htm

I should add after the furnace I have the air to air heat pump exchanger and then the geo coil after the heat pump exchanger as well. lots of pipes...

What is the model number of the bryant furnace. Do you have a/c or an evaporator coil on top of the furnace

The model is 355-cav and yes I do have a air to air heat pump above it and a geo coil above that.

But what I really want to know is what most people would suggest, merv 8, merv 11 or merv 13?

I would personally use the Merv 8. Post a picture of your setup. What is the range of your magnehelic
.

The mag is a 0-.5, when the furnace kicks in to "high" (very rare) the mag is is running about .4 on this filter, even with a clean 1 inch filter it is pegged above .5

Funny I talked to my dad about it last night and his take was the merv 8 would move more air through the system and in a given amount of time trap more "dust" than the merv11 or merv 13. He agreed the air might have less particles in it after passing through the merv 13, but if you only pass 1/2 the air thought it more air will end up getting filtered with the merv 8. And he said the trapped particle size on the merv 11 or 13 is so small it's not like dust you will see, the merv 8 was more than enough for that.

What is the complete model of the furnace and complete coils model numbers. A picture would be worth a thousand words.Do you own a thermometer you could insert in the supply duct downstream of all coils.I would like to know the delta t across the heat exchangers when only the furnace is operating. I am looking for the temperature rise.I will bet you you are outside the range that the mfg recommends.