bf, in the photo at the top

bf, in the photo at the top of this article, the air is flowing left to right.  The transition piece you see goes from the 20×20 filter cabinet to the 36×8 return plenum on the ducted mini-split air handler in our office.  Ideally, that transition piece would be longer so the change isn’t quite so abrupt, but this works fine.

Yes, the filter cabinet can be larger than the side of the furnace or air handler.  The air would move more slowly through the larger section and faster as it goes into the furnace/air handler.  Here’s an article I wrote on changes in duct size and velocity:

What Happens to Air Flow in Ducts When Size Changes?

Taking into account details in this blog, can you explain which way the air is flowing in the pictures? Is the return “expanding” into the filter cabinet (right to left) or is the return “transitioning” into the furnace (left to right)?

In my case, I have room to spare around it – Can you have a larger filter cabinet than the side of the furnace itself? Does a large return into the media filter that transitions into smaller ductwork work here?

You’re my newest hero 😉
You’re my newest hero 😉

@Marc, 20×25 @ 900 CFM works
@Marc, 20×25 @ 900 CFM works out to 260 feet per minute (FPM), well within Manual D guidelines, so it depends on HOW MUCH over 900. You can estimate system airflow from your A/C capacity – roughly 400 CFM per ton. If you’re not sure about capacity, post the A/C model number here.

In any case, installing a more restrictive filter for temporary duty is unlikely to cause problems. In cooling mode, restricting airflow reduces both sensible capacity and efficiency a bit, but it’s not going to hurt the (furnace) blower. If airflow is reduced too much it could cause the indoor coil to freeze up, but that’s unlikely to happen unless your system is already starved for air. Unfortunately there’s no way to know for sure without having a technician do a static pressure test.

In heat mode, installing a more restrictive filter will increase the supply air temperature, but that’s not a problem unless the airflow is so low that the furnace overheats. If that were to happen, the furnace will shut itself off before anything bad happens. Your particular model has electronic controls and fault monitoring.

As for increasing filter size… the difficulty depends on your particular installation… in particular, if there’s adequate room for the necessary transitions. If there’s space, then yes, this can be a reasonable retrofit project, but always get multiple bids on something like that. If you have fully ducted returns, it may be easier and less expensive to convert the return grilles to filter grilles and lose the filter at the furnace. That could even be a DIY project.

I’m a consumer, and your
I’m a consumer, and your excellent articles led me to the depressing conclusion that our 20×25 filter size is too small to comfortably run a MERV 13, which we’d love to with the current wildfire smoke here in San Francisco (we have a 58TUA Carrier Weathermaker 8000, looks like WELL over 900 cfm). I have two questions:
1) How bad for the furnace is it to run with a low-flow MERV 13 for a couple of weeks during bad smoke periods?
2) Going forward how big a deal is it to put in a bigger filter cabinet or holder, or whatever you call it? Is this a normal retrofit, or something to only consider when redoing the entire furnace installation?

Came across some Lennox
Came across some Lennox promotional stuff that makes clear pineapples, oranges and apples, even if they weigh the same and come in the same box, are not the same. It regarded air flow of its rather spendy 5″ filters and, if not snake oil, indicates informed consumers could run better filters without as much concern about bogging their system and suffering the consequences, all for a “little” fee, of course (about twice, or even more) than we’d pay for other name brand and off brand 5″ thick filters).

Snort, don’t know whether to thank you or hate you for one more rabbit trail. No, seriously, thanks all for your very valuable input.

I wouldn’t mind biting the bullet and spending more for better filtration. To that end, I’m still inclined to think I’d do well to go back to looking at adding an improved filter mounting area at the initial intake. One which would allow me to double the air intake area [side by side (parallel) vs stacking (series)] and to add pre-filters, to extend the life of the spendy ones. Of course, that would include a good quality manometer or mag… by which to monitor filter condition.

It’s important to note that
It’s important to note that MERV ratings are based on a specified face velocity. Some commercial filter manufacturers publish pressure drop and loading specs at a reference flow rate, but good luck getting this data for big box brands. What’s really needed are MERV and pressure drop graphs over a range of face velocities. This would be enormously helpful to designers, but don’t hold your breath for anything like that for products sold through retail channels.

Dwyer’s 2000-series
Dwyer’s 2000-series (differential) Magnehelic is the most economical option. I recommend the Magnehelic 2001, which has a scale of 0-1.0 IWC (inches of water column) with 0.02 minor divisions and 0.02 accuracy (2% full scale). This range is suitable testing total external static pressure (TESP = supply static + return static), yet is sensitive enough for testing filters and refrigerant coils (coils eventually get dirty too!). These gauges typically sell for under $40 on eBay (pre-owned), sometimes with tubing and probe included.

If you buy just the gauge, you’ll need to source a length of 3/16 ID flexible tubing (rubber works best) and a short length of metal tubing (aluminum, brass or ss) to make your probes. You don’t need a pitot-style probe if you’re only testing static pressure. A straight tube must be installed so that it’s perpendicular to the airflow.

@Allison, maybe you should write an article on static pressure testing? Finding suitable locations to drill test ports can be tricky business, especially for furnaces. Jim Bergman discusses this in one of his youtube videos: https://www.youtube.com/watch?reload=9&v=k5GcI6cbL-U (see 1:41-7:12)

I installed an manometer on my home HVAC to better track filter condition. It works, but might be more helpful if it were more sensitive. Following a few rabbit trails led me to magnehelic gauges and they seem like they’d do a far better job, though they are much more expensive.

Now comes the question of which would be best for a home system. I note they read as little as fractions to 30 inches and I’m trying to figure which would give the best function. Have my eye on a 0-15 unit, but still digging.

Figure I’d test drive my vehicle vacuum gauge just to see what it does, then go grab the digital manometer I bought to get a better feel for making the right [expensive] choice.

Do you have an opinion on such?

Figured I could

My LAYMAN’s assumption lead me to that two filters having the same surface area and filtering down to the same micron size would have the same air flow.

NOW, you made me think. For example, taking two 12″x12″ sheet of metal and punching 500 1 micron sized holes in one and a 1000 in the second sheet would give us two entirely different air flows, that latter having twice as much.

All this makes clear the fact MERV ratings ONLY serve to tell us the size of will pass and be filtered, and that filters need a second and associated rating to indicate actual per square inch flow with the filter media uncontaminated.

In short, those claiming expertise in establishing filter efficiencies and such have more work to do to the end of being THE experts on filtration systems.

As you indicate, testing each filter would be a must. Of course, that needs to be at the factory, rather here, in the home with a manometer or differential gauge, after the filters are purchased.