Air Filters 101: MERV Ratings, Thickness, and Bay Area Recommendations

Air filters are the simplest, cheapest HVAC maintenance task—yet most homeowners get them wrong. Wrong MERV rating restricts airflow and damages your system. Wrong thickness doesn't fit. Wrong replacement frequency wastes money or harms air quality.

This guide explains everything you need to know about furnace and heat pump air filters, with specific recommendations for Bay Area homes.

Filter Thickness: 1" vs 2" vs 4" vs 5"

1-Inch Filters (Most Common)

Dimensions: 16x20x1, 16x25x1, 20x25x1, etc.
Replacement frequency: Every 30-60 days
Best for: Standard residential furnaces, tight spaces
Cost: $5-$15 per filter

Pros:

• Fits almost all systems
• Cheap and widely available
• Easy to store multiple filters

Cons:

• Least surface area (clogs faster)
• Must replace monthly in wildfire season
• Limited to MERV 11-13 (higher ratings restrict airflow)

2-Inch Filters (Best Value)

Dimensions: 16x25x2, 20x20x2, 20x25x2, etc.
Replacement frequency: Every 60-90 days
Best for: Most homes, best balance of cost and performance
Cost: $12-$25 per filter

Pros:

• 2x surface area of 1" filters (lasts longer)
• Can use MERV 11-13 without airflow restriction
• Less frequent replacement

Cons:

• Not all systems accommodate 2" filters
• Slightly more expensive upfront

Recommendation: If your system accepts 2" filters, use them. They're the sweet spot for Bay Area homes.

4-Inch Filters (Premium)

Dimensions: 16x25x4, 20x20x4, 20x25x4, etc.
Replacement frequency: Every 6-12 months
Best for: High-end systems, allergy sufferers
Cost: $25-$50 per filter

Pros:

• 4x surface area (exceptional longevity)
• Can use MERV 13-16 without airflow restriction
• Best for wildfire smoke season
• Replace 1-2 times per year

Cons:

• Requires special filter cabinet or media filter housing
• Higher upfront cost ($25-$50 per filter)
• May require HVAC modification to install

5-Inch Filters (Whole-House Systems)

Use case: Specialty whole-house filtration systems only
Not common: Most Bay Area homes use 1" or 2" filters

MERV Ratings Explained

MERV = Minimum Efficiency Reporting Value. Higher number = finer filtration.

MERV Rating	Captures	Best For	Airflow Restriction
MERV 1-4	Dust, lint, carpet fibers	Not recommended	Minimal
MERV 5-8	Mold spores, dust mites, pollen	Basic protection	Low
MERV 9-10	Auto emissions, Legionella	Better filtration	Moderate
MERV 11-13	Smoke, pet dander, bacteria	Recommended for Bay Area	Moderate
MERV 14-16	Viruses, ultrafine particles	Allergy sufferers, clean rooms	High
MERV 17-20	Hospital-grade HEPA	Not for residential HVAC	Very High

Bay Area recommendation: MERV 11 for most homes, MERV 13 for allergy sufferers or wildfire zones.

What Size Filter Do I Need?

Step 1: Check Existing Filter

Remove your current filter and look for size printed on cardboard frame:

• Example: "16 x 25 x 1"
• Format: Width x Height x Thickness (all in inches)

Step 2: Measure If No Label

Use tape measure to measure:

1. Width (left to right)
2. Height (top to bottom)
3. Thickness (depth)

Important: Actual size vs nominal size

• Nominal: 16x25x1 (what you order)
• Actual: 15.75 x 24.75 x 0.75 (physical dimensions)

Always order by nominal size (rounded numbers).

Step 3: Check Filter Direction

Look for arrow on filter frame indicating airflow direction. Arrow should point TOWARD furnace/air handler (away from return vent).

Bay Area Specific Filter Recommendations

For Wildfire Smoke Season (June-October)

Challenge: Fine smoke particles (0.3-1 microns) require MERV 13+
Solution: Upgrade to MERV 13 during fire season, return to MERV 11 off-season
Alternative: Use portable HEPA air purifiers in bedrooms during active fires

Why not always use MERV 13?

• Restricts airflow (makes HVAC work harder)
• Costs 2-3x more than MERV 11
• Not necessary when air quality is good

For Coastal Homes (SF, Pacifica, Half Moon Bay)

Challenge: Salt air accelerates filter degradation
Solution: Check filter monthly, replace every 45-60 days instead of 60-90
Recommendation: MERV 11 sufficient (salt particles are larger)

For Allergy Sufferers

Recommendation: MERV 13 year-round, plus portable HEPA purifiers in bedrooms
Bonus: Consider electronic air cleaner for better fine particle capture

For Pet Owners

Challenge: Pet dander and hair clog filters faster
Solution: MERV 11, replace every 30-45 days instead of 60-90
Pro tip: Brush pets outdoors to reduce indoor dander

Filter Materials: Fiberglass vs Pleated

Fiberglass Filters (Blue/Pink Spun Fiberglass)

MERV Rating: 1-4
Cost: $1-$3
Lifespan: 30 days

Verdict: NOT RECOMMENDED. These protect your HVAC equipment but do nothing for air quality. Only use if your system absolutely can't handle pleated filters.

Pleated Filters (Accordion-Folded Paper/Synthetic)

MERV Rating: 7-16
Cost: $8-$50
Lifespan: 60-90 days (varies by thickness)

Verdict: ALWAYS USE PLEATED FILTERS. Much better filtration for minimal cost difference.

Top Filter Brands for Bay Area

Budget-Friendly (Best Value)

Nordic Pure MERV 11

• $10-$15 per 1" filter
• Excellent performance
• Buy in bulk (6-12 packs) for discounts

Filtrete MPR 1000-1500 (MERV 11-13 equivalent)

• $12-$18 per filter
• Widely available (Home Depot, Lowe's, Amazon)
• Good quality, consistent performance

Premium Options

Honeywell Elite MERV 13

• $18-$25 per filter
• Superior fine particle capture
• Great for wildfire season

AprilAire MERV 13

• $20-$30 per filter
• Medical-grade filtration
• Best for severe allergies

Replacement Frequency Guide

Filter Type	Normal Conditions	Wildfire Season	Pets	Allergies
1" MERV 11	60 days	30 days	45 days	30 days
2" MERV 11	90 days	45 days	60 days	45 days
4" MERV 13	12 months	6 months	9 months	6 months

Bay Area advantage: Our mild climate means HVAC runs less = filters last longer than hot/humid climates.

Common Filter Mistakes

Mistake #1: Using MERV 16+ in standard residential systems
Result: Restricted airflow damages blower motor, freezes evaporator coil
Solution: Stick to MERV 11-13 unless you have special filtration cabinet

Mistake #2: Installing filter backwards
Result: Filter fails prematurely, debris bypasses filter
Solution: Arrow on filter points TOWARD furnace

Mistake #3: Forgetting to change filter
Result: 50-70% airflow restriction, frozen coil, $500+ repair bill
Solution: Set phone reminder for every 60 days

Mistake #4: Buying cheapest fiberglass filters
Result: Poor air quality, no filtration of pollen/smoke/dust
Solution: Spend $10-$15 for MERV 11 pleated filter

The Bottom Line

For most Bay Area homes:

• Use 2-inch MERV 11 pleated filters if your system accepts them
• If only 1-inch filters fit, use 1-inch MERV 11
• Replace every 60-90 days (or monthly during wildfire season)
• Buy 6-12 filters in bulk from Amazon/Home Depot for convenience

Expected costs:

• MERV 11 filters: $60-$120/year
• Filter upgrades pay for themselves through energy savings and prevented repairs

Quick shopping list:

1. Measure your filter size (e.g., 16x25x1)
2. Buy MERV 11 pleated filters in 6-pack
3. Set calendar reminder for replacement every 60 days
4. Keep spare filters near furnace

Need help finding the right filter for your system? Galaxy Heating & Air technicians can recommend the optimal filter type, size, and MERV rating during your annual tune-up. Call (925) 578-3293.