Indoor Air Quality
HVAC Maintenance
By :
TL;DR
Air Filters
The single most important thing you can do for your HVAC system costs a few dollars and takes about two minutes: changing your air filter. This guide explains what your filter actually does, how to decode MERV ratings and find the right size, the main filter types, and a step by step routine for keeping your system running clean and efficiently.
The single most important thing you can do for your HVAC system costs less than $25 and takes about two minutes. It's also the thing most Denver homeowners forget about entirely until something breaks. We're talking about your air filter — the unsung hero quietly protecting your furnace, air conditioner, and the air your family breathes every day.
If you've ever wondered which filter to buy, what those MERV numbers actually mean, or how often you really need to swap it out here in the Mile High City, this is your complete HVAC air filter guide. Denver's unique conditions — cottonwood season, summer wildfire smoke, and bone-dry winter air — change the rules compared to almost anywhere else in the country.
The short answer to "how often should I change my filter": for most Denver homes, swap a standard 1-inch pleated filter every 60 to 90 days. But during cottonwood season (May–June) or when wildfire smoke rolls in over the Front Range, you should check it monthly and often change it sooner. Read on for the full picture, including a maintenance schedule built specifically for Colorado's climate.
What Your Air Filter Actually Does
Most people assume the air filter exists to clean the air they breathe. That's part of it — but its primary job is actually to protect your equipment. As your furnace blower or AC pulls air through the return ducts, the filter traps dust, pet dander, pollen, and debris before that gunk can coat your blower motor, clog your evaporator coil, and choke airflow.
A clean filter keeps your system running efficiently and extends its lifespan. A clogged one forces your blower to work harder, drives up your energy bills, and can eventually cause the system to overheat or freeze up. In a dry, dusty environment like the Denver Metro Area, filters load up faster than the manufacturer's "90-day" sticker suggests — which is exactly why so many Aurora and Lakewood homeowners are surprised at how filthy theirs looks after just a month.
If your system is short-cycling or struggling to keep up, a dirty filter is one of the first things to check. Our guide on why your AC is not blowing cold air walks through how restricted airflow leads to a frozen coil and warm vents.
MERV Ratings Explained (And Why Higher Isn't Always Better)
MERV stands for Minimum Efficiency Reporting Value, a standardized scale from 1 to 20 developed under the ASHRAE 52.2 test method. The higher the number, the smaller the particles a filter captures. Here's the practical breakdown for residential systems:
MERV 1–4: Cheap fiberglass filters. They protect the equipment but barely touch fine dust, smoke, or allergens. Skip these if air quality matters to you.
MERV 5–8: Solid residential pleated filters. Capture pollen, dust mites, mold spores, and pet dander. A great default for most Denver homes.
MERV 9–12: Higher-efficiency pleated filters that grab fine dust, smog, and finer smoke particles. Excellent during wildfire season — if your system can handle the airflow restriction.
MERV 13–16: Capture bacteria, fine smoke, and many airborne particles. Recommended for households with severe allergies or respiratory concerns.
MERV 17–20: True HEPA territory, used in hospitals and cleanrooms. These are almost never compatible with a standard residential furnace.
Here's the catch most stores won't tell you: a denser, ultra-high-MERV filter is harder to pull air through. Drop a MERV 16 into a system designed for a MERV 8 and you can starve your blower of airflow, spike static pressure, freeze the coil, and stress the motor — the opposite of what you wanted. The U.S. EPA generally recommends targeting the highest MERV your specific system can handle without restricting airflow, which for most homes lands in the MERV 8–13 range. When in doubt, our team can measure your system's static pressure and tell you exactly how high you can safely go.
How to Find the Right Filter Size
Buying the wrong size is the most common filter mistake we see in Wheat Ridge and Arvada homes. A filter that's too small leaves gaps for unfiltered air to slip past, defeating the entire purpose.
Your filter size is almost always printed right on the cardboard frame of the one currently installed — look for dimensions like 16x25x1 or 20x20x1. Those three numbers are length, width, and thickness in inches. If the print has worn off, pull the old filter and measure it yourself with a tape measure, then round to the nearest standard size.
One important note: the size printed on the filter is the nominal size, which is rounded up slightly from the true measured dimensions. That's normal and standard across brands, so don't panic if your tape measure reads 15.5 x 24.5 on a filter labeled 16x25. Thicker media filters (4 to 5 inches) live in a dedicated filter cabinet and last far longer between changes, which is a worthwhile upgrade for many homes.
The Main Types of Air Filters
Walk down the filter aisle and the options can feel overwhelming. Here are the four types that matter for residential HVAC:
Fiberglass (disposable): The cheapest option at $2–$5. Low MERV, minimal filtration. Fine for protecting equipment but poor for air quality. We rarely recommend them for Denver's dusty, smoky conditions.
Pleated (disposable): The sweet spot for most homeowners. The accordion folds add surface area, capturing far more particles while maintaining decent airflow. Typically $10–$25 and rated MERV 8–13.
Washable/reusable: Marketed as eco-friendly and reusable for years. The trade-off is generally lower filtration and the hassle of washing and fully drying them — a damp filter breeds mold, a real concern in Denver's already humidity-stressed homes.
HEPA / high-efficiency: The gold standard for capturing fine particles, but most require a dedicated whole-home filtration setup rather than the standard filter slot. If allergies or smoke are a serious concern, ask us about a properly engineered solution.
For homeowners battling persistent dust, allergies, or dry-air discomfort, the filter is only one piece of the puzzle. Whole-home air quality often calls for a layered approach — see whether a whole-house humidifier is right for your home to pair comfort with cleaner air during our parched winters.
How to Change Your Air Filter, Step by Step
Changing a filter is genuinely one of the easiest pieces of home maintenance there is. Here's how to do it right:
Turn off your system. Switch the thermostat to "off" so the blower isn't running while the slot is open.
Locate the filter. It sits in a slot near the air handler or furnace, or behind a large return-air grille on a wall or ceiling.
Remove the old filter. Slide it out and note how dirty it is — that tells you a lot about your change interval.
Check the airflow arrow. Every filter has an arrow printed on the frame. It must point in the direction air flows — toward the furnace or air handler, away from the return. Installing it backward kills efficiency.
Slide in the new filter with the arrow oriented correctly, making sure it seats fully with no gaps around the edges.
Turn the system back on and write the date on the filter frame or set a phone reminder for the next change.
That's it. If your filter is hidden in an awkward spot, lives in a sealed media cabinet, or you simply want it handled during a professional visit, our technicians swap it as part of routine AC maintenance and tune-up appointments.
How Often to Change Your Filter in Denver Specifically
National "change every 90 days" advice doesn't account for Colorado. Our climate throws several curveballs that load filters faster, so adjust your schedule to the season:
Cottonwood season (May–June): Those fluffy white seeds blanket neighborhoods from Englewood to Thornton and clog filters and outdoor units fast. Check monthly and change as soon as it looks loaded.
Wildfire smoke (summer and fall): When smoke drifts over the Front Range, fine particles overwhelm filters quickly. Run a MERV 11–13 filter if your system allows and change it every 30 days or sooner.
Peak summer cooling (July–August): Your AC runs constantly, so air cycles through the filter far more often. A 60-day interval is wise.
Dry, dusty winters: Furnaces run hard while humidity drops to 10–20%, kicking up fine indoor dust. Check every 60 days; homes with pets should lean toward monthly.
Households with pets, smokers, or family members with allergies should generally cut these intervals in half. The most reliable approach is simple: pull the filter monthly, hold it up to a light, and if you can't see light through it, replace it. A clogged filter not only hurts air quality but also drives up your utility bills — learn more in our guide on how to lower your summer energy bills. A consistent filter routine also pairs naturally with broader Denver HVAC seasonal prep to keep your system ready year-round.
Your Denver Air Filter Maintenance Schedule
Keep this simple seasonal rhythm and your system will reward you with cleaner air, lower bills, and fewer breakdowns:
Spring (Mar–Jun): Start with a fresh filter before allergy and cottonwood season. Check monthly through June.
Summer (Jul–Aug): Change every 30–60 days; go to 30 days whenever wildfire smoke is in the air.
Fall (Sep–Oct): Replace before you fire up the furnace for the heating season; watch for late-season smoke.
Winter (Nov–Feb): Check every 60 days; pet owners and Capitol Hill homeowners in older, dustier homes should check monthly.
For deeper protection against Denver's allergens, smoke, and dry air, a standard filter is just the foundation. Our indoor air quality solutions — from high-efficiency media filters to UV purification — build on that base for whole-home comfort.
Your Local Air Quality Partner in Denver
A well-chosen, regularly changed filter is the cheapest insurance policy your HVAC system will ever have. But choosing the right MERV for your specific equipment, confirming your system can handle a higher-efficiency filter without choking airflow, and building an air quality strategy for Denver's smoke and dust — that's where our expertise pays off.
For 16+ years, our NATE-certified, EPA-certified team has helped homeowners across the Denver Metro Area breathe easier and run more efficient systems. Whether you need a quick filter consultation, a full system tune-up, or a whole-home air quality upgrade, we'll give you honest, specific guidance — no upsells you don't need.
Ready to take the guesswork out of your air quality? Contact MoJo Home Services today to schedule a visit, and let our technicians make sure your home is breathing clean, no matter what season throws at the Front Range.
Sources & References
U.S. Environmental Protection Agency - What is a MERV rating?, 2024
U.S. Environmental Protection Agency - Air Cleaners and Air Filters in the Home, 2024
U.S. Department of Energy - Maintaining Your Air Conditioner, 2024
ENERGY STAR - Heating and Cooling Maintenance Checklist, 2024
ASHRAE, Standard 52.2: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, 2017
Need expert air quality help in Denver? MoJo Home Services provides professional heating, cooling, and indoor air quality services throughout the Denver Metro Area. Contact us at 4000 Newman St, Wheat Ridge, CO 80033 or call (720) 807-4050 for same-day service.
Frequently Asked Questions
How often should I change my HVAC filter in Denver?
For most Denver homes, change a standard 1-inch pleated filter every 60 to 90 days. But Colorado's climate calls for adjustments: during cottonwood season in May and June, and whenever wildfire smoke settles over the Front Range, check it monthly and replace it as soon as it looks loaded. Homes with pets, smokers, or allergy sufferers should cut those intervals in half. The simplest rule is to pull the filter monthly and hold it up to a light — if you can't see light through it, it's time for a new one. Thicker 4-to-5-inch media filters in a dedicated cabinet last much longer between changes.
What MERV rating should I use during Denver wildfire smoke season?
When wildfire smoke drifts over the Denver Metro Area, a MERV 11 to 13 filter captures the fine particles that lower-rated filters miss. That said, you should only go that high if your specific system can handle the added airflow restriction — an ultra-dense filter in an underpowered system can starve the blower and freeze the coil. If smoke is a recurring concern for your household, the safest move is to have us measure your system's static pressure first, then recommend the highest MERV it can run safely. We can also discuss dedicated air purification for serious smoke and allergy protection.
Is a higher MERV filter always better?
No. While a higher MERV captures smaller particles, denser filters are harder to pull air through. If you install a filter rated far above what your system was designed for, you can restrict airflow, raise static pressure, overwork the blower motor, and even freeze the evaporator coil. The EPA recommends targeting the highest MERV your specific system can handle without restricting airflow, which for most homes falls in the MERV 8 to 13 range. The goal is better filtration and healthy airflow — not the biggest number on the box.
How do I find the right size air filter?
The size is printed on the cardboard frame of your current filter — look for three numbers like 16x25x1, which are length, width, and thickness in inches. If the print has worn off, slide the old filter out and measure it with a tape measure, then round to the nearest standard size. Keep in mind the printed "nominal" size is rounded up slightly from the true measured dimensions, so a filter labeled 16x25 may actually measure about 15.5 by 24.5 inches. That's normal across all brands.
Which direction does the air filter arrow point?
Every filter has an arrow printed on its frame indicating airflow direction. The arrow must point in the direction air moves through the system — toward the furnace or air handler and away from the return duct. Installing a filter backward reduces efficiency and can damage the filter. When you slide in a new filter, double-check that the arrow points toward the blower, seat it fully so there are no gaps around the edges, and you're done.
Can a dirty filter really raise my energy bills?
Absolutely. A clogged filter forces your blower motor to work harder to pull air through, which uses more electricity and reduces system efficiency. In severe cases, restricted airflow causes the system to overheat in winter or freeze the coil in summer, leading to costly repairs and even higher bills. Replacing a dirty, clogged filter with a clean one is one of the cheapest, fastest ways to improve efficiency — the U.S. Department of Energy lists routine filter changes among the simplest steps to lower HVAC energy use.
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