Shims To Level An AC Unit: Best Practices, Applications, And Installation Tips

Leveling an air conditioning (AC) unit is crucial for optimal performance, longevity, and safety. Shims offer a simple yet effective solution to achieve precise leveling. This guide offers in-depth insights into using shims for leveling AC units, types of shims available, installation best practices, and tips to avoid common mistakes, ensuring Americans maintain energy efficiency and system reliability.

Shim Material	Best For	Main Benefits	Considerations
Plastic	Outdoor use, composite pads	Weather-resistant, doesn’t rot or corrode	May compress under heavy weight after time
Metal	Heavy-duty, commercial units	Very sturdy, minimal compression	Can corrode if exposed, sharper edges
Composite/Shingle	Residential, all climates	Inexpensive, commonly available	Can degrade over years with moisture
Wood	Temporary fixes, quick jobs	Easy to trim and stack	Prone to rot, pest damage if exposed

Why Leveling An AC Unit Matters

Proper leveling is essential for the efficient and safe operation of an AC unit. An unlevel AC condenser or air handler can produce excessive noise, vibrate more, suffer from oil migration issues in the compressor, and even drain incorrectly, leading to water leakage or system failure. For window units, being slightly tilted to drain condensate is required, but split or packaged systems need careful levelling.

Types Of Shims Used For AC Units

A variety of shim types are available, each with distinct advantages and limitations. Using the right shim material delivers longevity and reduces future maintenance needs.

Plastic Shims

Plastic shims, usually made from polypropylene or high-density polyethylene, are a favorite for exterior AC units. They resist water absorption, don’t rot, and can bear substantial load when chosen in the correct thickness. Commercially-available AC pad leveling shims are often plastic and come in wedge or flat styles. They are easy to cut to size and can be stacked if needed.

Metal Shims

For heavy-duty or rooftop AC units, galvanized steel or stainless steel shims are used for their superior strength and minimal compression under weight. These are ideal for units that are extremely heavy or in commercial installations. Care must be taken to ensure the metal does not rust or corrode over time. Sometimes rubber or plastic is combined for additional stability.

Composite Shims

Composite shims—often made of fiberglass-reinforced plastics or combinations of recycled materials—provide a middle ground, offering resistance to weather, UV rays, and moderate compressibility. These are often suitable for most residential installations on patios or concrete pads.

Wood Shims

Wooden shims, typically cedar or pressure-treated pine, are still used for temporary fixes or where budget is a major concern. While easy to trim and stack, wooden shims are vulnerable to moisture, pests, and eventual decomposition. Use only as a short-term solution outdoors.

Choosing The Right Shims For Your AC Unit

Shim choice depends on unit size, location, pad type, climate, and longevity goals. Here’s a quick reference guide:

• Plastic or composite shims: Best for most residential outdoor condensers.
• Metal shims: Ideal for commercial, heavy units, or rooftop units.
• Wooden shims: Use only indoors, for window units, or temporarily outdoors.

Evaluate the expected weight, site exposure to water, pest risk, and permanency needs before deciding.

When Should HVAC Units Be Shimmed?

Leveling should be checked and corrected:

• During installation: Every new AC should be perfectly level or slightly tilted as required.
• Seasonal inspections: Ground movement, frost heave, and settling can unlevel a previously stable unit.
• After pad replacement or landscaping: Any disturbance near the unit is a cue to check alignment.
• If symptoms arise: Unusual noise, vibration, or visible wobble often signal an unlevel unit.

How To Shim An AC Unit: Step-By-Step Guide

Shimming an AC unit is a straightforward process, but care must be taken to avoid damaging components or stressing refrigerant lines. Here’s a best-practices approach for homeowners and technicians:

1. Turn off the power at the breaker to the AC unit for safety.
2. Inspect the current position of the unit. Use a bubble level or digital inclinometer on all sides and diagonals.
3. Assess which side needs lifting and estimate shim thickness needed.
4. Carefully lift the corner or edge of the AC pad or base using a crowbar with a wood blocking fulcrum, or have a helper pry very slowly. Do not lift the unit itself directly—always lift under the pad or a foundation block.
5. Insert the shim under the pad or base at the target spot. If using flexible, stackable shims, add layers as needed. Use several small shims for stability instead of a single thick wedge where possible.
6. Gently lower the pad and re-check the level.
7. Repeat the process on additional corners/edges as needed until desired level is achieved in both side-to-side and front-to-back planes.
8. Check refrigerant lines and electrical connections for any tension or displacement once leveling is done.
9. Restore power and run the unit for five minutes, watching for vibration or odd noises.

Never force the unit or leave major gaps under the pad. Surface contact must remain broad to prevent tipping or stress.

Considerations When Leveling Window AC Units

Window AC units are unique: they require a slight backward tilt for proper condensate drainage (typically about 1/4 to 1/2 inch from front to back). Shims can be used under the inside sill to ensure:

• No water drips inside

Wedge-style or stackable shims work best for quick adjustments in residential window AC installations.

Potential Risks Of An Unlevel AC Unit

Failing to shim and level an AC system can result in:

• Compressor oil migration away from critical components, leading to damage.
• Inefficient drainage and water leaks indoors or at the base in split systems.
• Vibration noise and accelerated deterioration of fan or mounting brackets.
• Frost or ice buildup from improper refrigerant flow.
• Premature component failure not covered by warranty.

Routine shimming and checks can help avoid costlier issues.

Best Practices For Long-Lasting Shim Installation

Follow these guidelines to ensure shim stability and durability:

• Use shims made specifically for HVAC or outdoor use whenever possible.
• Trim or shape shims for full surface contact beneath the pad or base.
• Seal gaps around the shim with weatherproof sealant to deter pest intrusion and water accumulation.
• For stackable shims, alternate grain direction (wood) or orientation (plastic/metal) for better strength.
• Regularly inspect and replace deteriorated shims during seasonal maintenance.

Professional Versus DIY Leveling

Small AC units or window systems can often be safely shimmed by skilled DIYers. However, large split-system condensers or rooftop units should be professionally evaluated and leveled to protect expensive components and maintain warranties. HVAC technicians use specialized shims, lifts, and measure refrigerant line stress as part of their service.

Signs You Need To Reseat Or Shim An AC Unit

Always check for these warning signs:

• Noticeable tilt or out-of-level pad
• Louder than normal vibration or fan noise
• Water draining under/around unit instead of the designed outlet
• Visible cracks or movement in the mounting pad or surface
• Unit contacts wall, fence, or other nearby objects unevenly

Shim Placement: Flat Or Wedge?

Wedge-shaped shims are ideal for making gradual adjustments and tight-gap corrections. Flat stackable shims are preferable for higher lifts or across larger gaps to spread weight more evenly. For maximum stability, use a combination: a wedge to achieve elevation and flat shims stacked for surface area contact.

Shim Installation On Different Pad Types

Concrete Pads

Insert shims directly under the concrete slab or block for outdoor installations. If the pad is cracked or crumbling, replace it rather than shim extensively. Heavy units may require both shimming and pad rebuilding.

Plastic Or Composite Pads

Plastic pads can be leveled with plastic or composite shims. Always ensure pad and shim materials are compatible to avoid chemical reactions or premature wear.

Roof-Mounted Units

Rooftop HVAC systems often use poured concrete bases or steel frames. Metal shims are most recommended here for long-term load-bearing and weather exposure. Always check local building codes and ensure waterproofing membrane integrity.

Potential Problems With Improper Shimming

Improper shim use can cause:

• Instability or pad tilting over time
• Pest infiltration if gaps are left unsealed
• Reduced airflow from misaligned condenser fan or grill
• Safety hazards from shifting, especially during storms or seismic events

Correct installation and periodic inspection are vital to a trouble-free AC setup.

Cost Comparison: DIY Shimming Vs. Professional Leveling

Project Type	Estimated Cost (DIY)	Estimated Cost (Professional)	Primary Cost Factors
Residential Pad Shim	$10 – $30 Hardware	$100 – $250 Service Call	Labor, warranty, equipment
Rooftop/Commercial Shim	Not Advised	$250 – $500+	Crane, engineer oversight, insurance
Window AC Leveling	$5 – $12	$75 – $200	Minimal tools, single technician

DIY can save money for small units, but the risks of improper installation on heavy systems often outweigh initial savings.

Common Mistakes When Leveling With Shims

• Using the wrong material (e.g., untreated wood outdoors)
• Over-shimming one corner leading to instability
• Leaving significant air gap under pad causing rocking and pest entry
• Failing to check all planes (leveling side-to-side but ignoring front-to-back tilt)
• Ignoring base pad cracks or rot and shimming instead of correcting the foundation issue

Proper planning and periodic checks help prevent these errors, ensuring safe and efficient AC operation.

Recommended Products: Shims For HVAC Applications

The following products are commonly recommended by professionals and widely available in American hardware stores or online:

• Quick-Support Plastic AC Shims: Interlocking and stackable, support up to 1,500 lbs each.
• Galvanized Steel Leveling Plates: For extreme weight, corrosion-resistant, various thicknesses.
• Composite Stack Shims: Made from fiberglass or recycled materials, offer better longevity in outdoor use.
• Heavy-duty Cedar Wood Shims: Used primarily for indoor or temporary outdoor fixes.

Always check load ratings and choose shims suited for prolonged UV and weather exposure when used outside.

FAQs: Leveling AC Units With Shims

Can Any Hardware Shim Be Used For AC Units?

No, always choose shims that are rated for outdoor/weatherproof use and can support the AC’s full weight under all conditions.

Is It Safe To Shim Just One Side?

Shim adjustments must be spread to ensure a stable and level base. Over-raising a single corner risks tilting and damage.

How Often Should AC Level Be Checked?

Annual inspection is recommended, especially in climates with freeze/thaw, settlement, or after major storms or landscaping work.

Can Leveling Solve Vibration Noises?

Yes, in many cases, correcting tilt with shims can eliminate noise caused by a rocking or unstable pad, but persistent noise may indicate other issues with fans or mounts.

Resources And Further Reading

• ENERGY STAR – AC Maintenance
• Air Conditioning Contractors of America (ACCA)
• ASHRAE – HVAC System Standards