HVAC realities and experiences: Maintenance, Filters, Brands, etc

In My Humble Opinion
1

I don’t see a thread that’s less than a decade old, so here’s a new one that people can add to as needed.

Current event: My house (built 1982) had central (gas) heat and no A/C when we bought it in 2002. In 2003, we put in a new furnace and whole house A/C. Which, with the usual bumps, hiccups, minor and semi-minor repairs lasted us until, well, right now. It hadn’t died, but we knew that we’d be pushing to make it another year and the low levels of now nigh-unavailable refrigerant pushed it into the “throwing good money after bad”.

[ Note - I’m not complaining on the requirement for more energy efficient and environmentally friendly options, I’m for those, though reaaaaaallly wish they’d treat commercial/industrial companies as aggressively as they do smaller, private entities!]

So onto the little realities where I cannot get a consistent answer. When I did my annual furnace inspection back in early October 2025, I was notified of the issues with that unit, and one of things advised was to reduce the MERV rating of the filter (Had been using 8 which was what the unit was originally rated for) for easier airflow and to replace it more frequently (I was doing 80 days on 90 day filters, so dropped it down to 60-70).

Today we just installed a new AC and Furnace combo - Heat Pump Plus “Cold Weather” secondary gas furnace[1], not to mention going from SEER 10 → SEER 13.4.

But when it comes to filters and maintenance by the owner, they’re suggesting “cheap” filters that are replaced every 30 days. Now, TBH, there don’t seem to be cheaaaap filters much anymore, even the lowest grade, transparent fiberglass in cardboard so weak I wouldn’t wrap a sandwich in it is $2.50, and pretty much not in stock within a 100 mile radius. I’m likely to continue with my MERV 5 units that I get from Lowe’s at $14 for a 3 pack, but take it down to 45-60 days rather than my current 60-70. I get that dirty filters (no matter what they claim to be rated for) put wear and tear on the blower, and that the issue is how dirty they are rather than how long they theoretically last. And of course, there’s the issue of cleaning the ducts as well!

The internet of course let’s you pick from any extreme you like, so not much use other than the honest “check it frequently, change when dirty, forget the rest”. But my filter is rather hard to access in a crawlspace, so doing it less is of course to be preferred.

So I’m reaching out to my fellow Dopers, many of whom have been homeowners far longer than I, or replaced units more frequently than I to get opinions.

And heck, while I’m at it, lets get some opinions on the washable/reusable filters, because dumping these pleated filters every (1-3) months is something that frustrates my desires to create less waste. I’ve heard everything from “Instant mold clusters” to “fine if cleaned frequently and you don’t need HIGH filtration” to “Best thing on earth, endorsed by mother Gaia herself!”.

I’m sure others have similar questions, concerns, or experiences, as well as entirely different ones, so figure having a current thread may be of help to more than just me!

  1. Colorado springs is a LOT warmer than 44 years ago, and a good bit warmer than 23 years ago but the winters can still get below 0 a couple of days a year, and a more than a few days of single digit highs.
    ↩︎

2

Some years ago, I installed a new HVAC system with reusable filters. I may still have these filters somewhere, and if I remember where I put them, I’ll be sure to throw them out, as I now use MERV 11 replaceable filters changed every 3 months. Washable filters tend to have a low MERV rating in the 4-6 range. The air resistance has a tendency to increase with each wash because you will never get it as clean when new. Because of that low rating, over time there was an accumulation of dust on my a/c coil, greatly reducing efficiency. If pollen, mold spores, or pet dander are a concern, washable filters seem to fail in that regard.

Some of these filters are treated with an electrostatic guard, but that tends to lose efficiency with each wash.

You may find it helpful by using an AI inquiry asking for the pros and cons of reusable filters.

Note: I wrote this in another text editor and have no idea why the first part has a large bold font.

3

Had my furnace, A/C replaced 3 years ago. I use MERV 11. Works for me. I have no allergies, no current pets. If MERV 11 is out of stock or not on sale I’ll use MERV 8. Can’t say I notice a difference. I’m slack when it comes to replacing but usually manage a couple times a year. Even then the filter still looks clean but I change it anyway.
A friend uses MERV 16. I point out that she’s probably impeding air flow making her furnace less efficient. But it’s what Costco sells and therefore it’s gold.
I have a humidifier - one that attaches to the furnace duct. I find it’s important to change that filter on schedule.
Lots of online info about filters & such (not AI).

4

Is there no slot on the unit itself to insert the filter? Could one be created — maybe you have a handyman who could cut you a slot for the filter?

In one house we rented, not only was the filter on the floor of the coat closer, all the way in the back, but I had to cut down the filter itself to fit the hole. :enraged_face:

5

Because the paragraph starts with a # sign. That’s the Markdown code for “this paragraph is a headline.” If you’d had more of them you’d get sub-heads like this:

1 leading pound sign

2 leading pound signs

3 leading pound signs

4 leading pound signs



As to the actual OP question, here’s some thought bites not forming an essay:

I have had washable filters. Cannot recommend. Difficult to actually get clean, difficult to dry, just all around difficult. And generally rather high resistance = low-flow. You do you, but I find the idea of worrying about the landfill burden of a couple of air filters per year to be a severe case of flyshit in the pepper.

As to mold, it’s important to condition any discussion with where in the country is the experience. In the humid steamy parts of the country, mold is an actual concern. In the arid parts parts it’s just a baseless phobia. So when reading the “OMG it’s a mold factory!” posts wherever, consider the geographic, well climatological, part of the source.

The last couple of thermostats I have had have a filter replacement reminder feature. Which is just a countdown timer that counts down only while the fan is on. And which you can set to a bunch of reasonable values for runtime hours before the reminder lights up. After all, what makes your filter dirty isn’t calendar days of sitting there, but how much your HVAC system runs. And whether you run the fan non-stop 24/7 or only while heating or cooling. If your new system’s new thermostat(s) don’t have that feature, you might look to get one(s) that do/does.

In my time managing a condo building I saw some truly awfully clogged up filters. In HVAC systems that were a decade or more old and still chugging along just fine. Albeit not with as much CFM airflow as they might have had with a filter that wasn’t burdened with 12 lbs of matted cat hair. IMO / IME the idea that filters must be changed often to prolong fan life is Big Filter trying to sell more product.

This is not a topic that warrants stress. Nor is there a good a prioiri scientific answer to “How long will this filter last in this house with this dust, pollen, and animal debris burden divided by these people’s allergen & “ick” tolerance?” My technique was simply to set my countdown filter change timer to the max value (or my Outlook calender based reminder to 6 months), then check the filter. If I thought it should’ve been changed before it got that grody, tweak the interval down by 1/2 or 1/4 or whatever looks about right. Within a couple replacement cycles you’ll have empirically found your personal answer to your personal “how dirty is dirty enough?” equation.

6

Thanks to all who have posted so far, so, in order:

@Dereknocue67 - those are some of the things I’ve heard, though the issues with cleaning/drying weren’t a big worry, because my intent was to keep my old replaceable options to swap in for a day or two while doing the deep clean and drying them. Our average humidity around here is variable, but the common highs short of actual rain is 40-50%, and single digit (especially in winter) is quite common. Drying things is rarely a problem, and honestly getting worse with the extended droughts across the nation.

But the point on the decreasing efficacy over time is taken as is the lack of permeability.

@Keith1, we were using 8s with no problem until the suggestion to drop to 5s from the prior tech. Again though, part of that was the age of the furnace and our desire to try to put it off for another year (we just replaced an electrical panel, and are in the process of trying to replace the roof but insurance is not cooperating), so we reaaaaaallly didn’t want to do the HVAC this year!). And I am definitely going to be careful about the humidifier filter that’s attached to the new system!

@burpo_the_wonder_mutt - it’s not the placement on the furnace that’s the problem. It’s getting to the furnace itself! To get to it, you open a 3"x3" plywood “door” of sorts (single piece, held in place with a clamp) in the bottom of the downstairs closet. Jump or climb down through the whole about 5" to the earth floor of the crawlspace. Inside, it’s open enough for me (5’9.5") to stand without problems, and the furnace is right there. Replacing filter - nil perspiration. Then pull myself up to the ledge or climb the ladder, and try to get out in the narrow exit area (on solid concrete) without wrecking my knees or hips, or otherwise twisting/pulling something.

So it’s a hassle but not exactly hard to do.

@LSLGuy - as always thanks for a detailed reply. Observation is probably the right answer in the long run. Up until two-three years ago, our filters were noticeably dirty but not clumpy after we switched to 80 days. That was in a house with carpet and two cats, plus the two humans. After we went with vinyl tile flooring in the living area / kitchen / dining room, it got a bit better, and better still after having an alternating day Roomba cleaning program.

I -think- for the moment I’ll stick with the “3 months” MERV 5 or 8 filters (probably go back to 8 once the 5s are used up), but just replace them on the 60 day basis (per my OP) as a compromise between air flow, filtration, and reduction in hassle. If I see them looking dirtier than ideal, I’ll change the rating and/or reduce the time. If they look very clean - maybe I’ll push it back to 70 days. Or maybe I’ll see a pattern.

Climate change has made it harder of course, with needing a lot more A/C in the summer, and noticeably less heat in the winter, but we’ve still got some distinct extremes.

Again, thanks all. And feel free to add more anyone else, or to ask your own HVAC questions.

7

HVAC techs recommend cheap filters because they know people don’t replace them often enough, and a cheap filter takes a lot longer to block up, so they’re less likely to get nuisance calls to poorly performing systems just because the filter hasn’t been changed. The cynical take is that they keep stock of those cheap(er) filters in their trucks and can charge you for a replacement rather than you getting one yourself. Also, the really cheap filters will let enough dust through that it could require an evaporator coil cleaning or other service that makes them more money.

Back to the non-cynical take. A restrictive filter can reduce airflow, but modern furnaces often have blowers that will ramp up to compensate, so it may use more electricity but it will maintain the desired airflow. High MERV filters are very thick because they have a lot of pleats to increase surface area without adding too much restriction. If your filter box is big enough for one of those, then presumably the system was designed to accommodate a high MERV filter with a larger return plenum.

8

All I can offer here, FWIW, is that regarding furnace filters, I’ve long settled on 3M brand Filtrete MPR 1000 MERV 11 which claim to be good for 3 months, but it obviously depends on how much the HVAC system runs. TBH, I often leave it in for 6 months or more, which I don’t recommend as good practice.

Some years ago I tried cheap filters on the basis that filter quality may not matter much. It certainly did in this case, but maybe I went too far with “cheap”. The filter material was quite coarse and I found that after a month or so it had trapped very little dirt at all. So I went back to 3M Filtrete. They’re not cheap but I highly recommend them.

9

The 5 MERV units I’ve been working with for the last 4-5 months have been the Filtrete, replaced every 80ish days. I’m planning on heading back to MERV 8 which seems to be the happy point in price and performance for my household needs.

10

But they don’t always (have an ECM (Electronically Commutated Motor)), and knowing – IMHO – is important in this context.

[Good post, though, in total.]

My add was going to be “static pressure.” More filtration → more restriction. Your system may not care, but it also may, and if it does, you’re putting more strain on your system, possibly decreasing its service life, and possibly spending more on electricity.

An HVAC tech can readily measure static pressure in your system – ideally, before and after your filter – giving you some idea of how your level of filtration is affecting your air flow, and how it compares to what the system wants (in terms of static pressure) – usually specified somewhere on the unit or in the documentation.

To get a relative sense of the relationship between filtration and restriction …

Google Photos

11

My parents decided to go with the “buy cheap, change monthly” plan some time ago. I don’t know where my dad was sourcing his filters. Possibly going to Home Depot every 3 months to buy a 3-pack. When he died in 2023 it became my job to source and change these filters. I set mom up with a subscription from Amazon, and I changed them diligently (every month when her cleaners came). Now I have been less diligent, and keep forgetting, and decided to switch to every 2 months because to be honest the filters still look pretty good after one month. After 2 they do look kinda grimy.

I don’t remember the MERV or the brand of filters. They are not 3M. I’ll try to look this week (your thread reminds me it’s time to change mom’s filters!)

At my house I have one of those big-ass change-every-6-month filters that costs a lot of money. Meh.

12

Test

Test

Test

Test

Well, I’ll be darned! :flushed:

13

Curiously, around here Home Depot only stocks Filtrete in MERV 11, 12, and 13 ratings, the overwhelming majority being MERV 11. MERV 8 is available in HDX and other brands, but not 3M Filtrete.

I just checked my filter out of curiosity since I know it’s been in there a long time and probably hasn’t been changed in 8 to 12 months. I’m definitely changing it for A/C season but it’s not as dirty as I thought it would be. I expected it to be practically clogged with dirt, but it’s a fairly thin layer and you can still see light through it.

14

You also get an entire paragraph converted to large text if there is a hyphen as the first character of the line immediately below it, or preceded by no more than four spaces.

15

Moderator Note

I fixed it for you. As @LSLGuy said there was a # sign at the start of the post.

16

My experience is the same. I also do not recommend.

Have you looked on Amazon to see what’s available?

17

I ended up timing out on my last “Oh, yeah, one more thing” Columbo style of posting.

When I was living in a big house with two HVAC systems in 4-seasons country like you do, and I had the countdown timer thermostats I learned something: The filters lasted about 2x as many days in the shoulder seasons as they did in the depth of winter or summer. Despite running the same number of hours. I also learned that my secondary zone ran about 2/3rds as much as the primary zone did. Which was a consequence of our use pattern of our house, which zone had more windows facing which way, etc.

The key learning is that any filter replacement regimen based on calendar days, or changing both systems at once, is going to be a bunch short of optimal when looked at year round. If 3 vs 4 vs 5 filter changes per year actually matter to you, changing on runtime vs calendar days might be worth doing.

18

Excellent point, which I sorta alluded to earlier. My house in a semi-rural area, combined with my climate preferences, tends to have long “shoulder seasons” (I usually call them “transition seasons”). As fall turns to winter, the furnace doesn’t run very often because I like cool temperatures. As spring turns to summer, the A/C doesn’t usually run that often because the weather around here is usually not that hot until summer really gets a grip, though there are exceptions. So in the end, there are as many as 20 weeks a year when the furnace fan has not much to do or doesn’t run at all.

In a previous house, the previous owner had set up the furnace to run the fan at low speed 24x7. Bad, bad idea – hard on the fan motor, clogs up the filter, and leads to early fan motor failure. When I moved in and renovated, I had a technician disable that setting, but the damage was already done. A couple of years later, the fan motor failed.

19

Oh, of course. It appears that the profit margin on the pleated types that are common these days is so high that the cheapo’s at near/sub $2.00 range aren’t really much of a thing for reputable companies anymore.

I’m mostly taking this into account. During late spring and early fall I get by with using “windows open and let the house cool overnight” as my primary cooling source. The issue with Colorado Springs and other alpine areas, is that there’s often a huge difference between highs and lows, 30+ degrees at times. So it’s… unpredictable. One of the reasons to ditch the old system which as ON/OFF and HEAT/COLD only.

20

I am skeptical.

I think modern systems are built to handle that just fine.

We tend to keep it the fan staying on as it evens out hot cold sections some. Yes the filters are filtering out more crap so should get changed more often and there is marginally more electricity use but killing the fan motor? Not sure I buy that.