Forensic Engineers and Consultants

Archive: Mechanical Systems

Hail Isn’t Cool

Hail property damage is frequently reported after an HVAC service call.  Building owners are often unaware there is damage until the power bill starts trending higher and the HVAC system is simply not cooling effectively. Take look at the fins! The National Weather Service reported over $722 million in property damage from hail in 2018.  Based on NWS data, hail caused more property damage than tornadoes or thunderstorms.  Only Tropical Storms/Hurricanes at $12 billion, coastal storms at $1 billion and flooding at $1 billion were more costly than hail to property.

One of the items very susceptible to damage is the HVAC system.  The heat that is removed from the interior of a building must be rejected to the environment.   As such the HVAC system is placed where it can have unhindered access to outdoor air in order to function properly.  This often results in the unit being placed in a large open space such as a rooftop, making it susceptible to damage such as hail.

An air conditioning or HVAC system must reject the heat it removes from a building to the environment.  This rejection of heat to the environment is often accomplished by an air-cooled condensing unit which contains copper or aluminum tubes (coils) bonded to aluminum fins.  The coils and especially the fins are very thin to promote heat transfer between the refrigerant inside the coils and the air outside the coils rushing through the fins.  These fins measure around 0.006 of an inch in thickness, with a sheet of standard printer paper measuring 0.0035 of an inch.  So, the fins are roughly the thickness of two sheets of paper!

BEFORE COMBING                    |         AFTER COMBINGHVAC hail damage coils fins                                         

HVAC Condensing Unit Coil and Fins, both before and after combing. Fins are typically made of thin aluminum roughly the size of two sheets of paper.

Consequently, fins can be easily damaged by impact forces such as those generated in a hailstorm.  The image below shows damage to a condensing unit caused by hail.

hail-damage-central-air AE Mechanical Services, Inc.-Recovered englarged

Condensing Unit with Hail Damage

In this case the damage was minor, and the actual performance of the condensing unit was likely not diminished.  This damage can be repaired by “combing” the fins.   The tool, available commercially from many sources, is just as the name implies; a comb to straighten bent fins.  The tool must match the fins per inch for the respective coil and the technician must use care in the process, but the fins can be returned to a near factory state.Fin comb in use - Source EbayFin Comb in use

Fin Straightening Comb for multiple coils

Fin Straightening Comb for multiple coils

Often combing the fins will result in a return to full factory performance without requiring the replacement of the condenser coil.  Even examples where the fins have been almost completely blinded due to vandalism have responded well to repair by this combing method.  While the task of combing the fins may result in 1 – 2 hours of a technician’s time depending on unit size and the extent of the damage, the labor and parts savings over coil replacement can justify this expense.  If the actual coils themselves have sustained impact damage and not just the fins, this might not be the repair for you.  In the case of damage to the coil, replacement may be the best option to return the unit to factory performance. Property damage claims may or may not be inflated due to a quick decision to replace the coil or the entire unit rather than the tedious process of combing the coil fins, driving up the cost.

With condensing unit coils known to be susceptible to damage and hail being a very real threat, what options are out there to protect an HVAC investment?  One simple, but often missed item is to specify coil guards, often called “hail guards” on all condensing units.  Higher end residential condensing units often have them installed to protect the coil not as much from hail but routine household threats such as string trimmers, debris thrown by lawnmowers, and vandalism.

Rooftop HVAC units, however, aren’t likely to be damaged by routine activities such as mowing the lawn.   Access to the building roof area is often limited to maintenance personnel so vandalism is often reduced compared to a ground mounted unit.   Coil guards are not factory installed on many commercial units but are optional.  They should be specified in the bid package if the job is being competitively bid or they will likely not be included in the job.   See the figure below for factory installed hail guards.

hail-guard 2

Rooftop HVAC unit with suggested “hail guards”

With hail a heavy hitter for property damage, a little prevention in the form of hail guards for a building’s HVAC units will go a long way and provide some protection from vandalism.  If hail damage or even vandalism does occur but only impacts the fins and not the coil; combing the fins can be a cost effective and satisfactory repair to get the HVAC system running efficiently again.

Chad Jones, PE has a Bachelor of Science in Mechanical Engineering from Clemson University. Chad has over 23 years of engineering experience including mechanical, process, and manufacturing engineering. He participated in and led industrial incident investigations and participated in in-depth process safety audits. This work has included equipment design, machine safeguarding, cost estimating and safety compliance.  Chad also has over 10 years of commercial, industrial, and residential HVAC and plumbing design experience.


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  3. Scaffolding, general requirements, construction (29 CFR 1926.451)
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  7. Powered industrial trucks, general industry (29 CFR 1910.178)
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  10. Eye and Face Protection (29 CFR 1926.102)


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