Are you tired of your A/C unit taking up precious patio space? Are you done with its huffing and puffing, all within earshot of your Adirondack chair? Has your repairman ever scratched his head wondering where your broken system’s malfunction is, in the unit or somewhere inside the house?
If you answered yes to any of these questions, you may want to upgrade your central air system to a rooftop unit. Yes, these units are not cheap to install. But, their efficiency and secluded location may balance out the higher cost. That makes it worthwhile to check if it is right for you.
Commercial and industrial facilities have favored rooftop HVAC systems for decades, as their buildings’ flat roofs allow for easy installation and maintenance and save precious ground-level space. In single-family residential homes, however, rooftop systems are lagging in popularity. Most homeowners are discouraged by the higher costs and opt for central air systems instead.
If you have a central A/C, chances are it’s sitting on a concrete pad outside your home. While the location is ideal for installation and service, it also comes with several challenges. Being on the ground, the units are debris-magnets. Their easy access may encourage thieves and vandals, while children and animals playing nearby can cause inadvertent damage. Because of their reliance on the home’s forced-air furnace system to function, maintenance and repairs of central air systems may be tedious and expensive.
On the other hand, rooftop systems are largely out of harm's way thanks to their location. Unauthorized access to a rooftop unit is usually difficult, easily detectable, and there’s less risk of debris accumulating and clogging the mechanism.
Rooftop A/C systems bypass the furnace and circulate air via their ducts, so repairs and maintenance are limited to the unit itself. Because cold air descends, an HVAC system placed above the space it serves can operate with reduced fan speeds and consumes less power.
If you decide to move your system to the roof, you have some homework to do. You should find out an optimal size for your system, determine whether your roof can handle the added weight, and make sure that the roof will still protect you from the elements after your system is installed.
1. Determine the system capacity
Before you purchase a new HVAC system, take the time to figure out what size it needs to be. Systems that are too small or too large are not energy efficient, and will not condition your home the way you want them to. A properly sized unit, however, will create a comfortable indoor environment and help reduce your energy bill.
To get a rough idea of your home’s conditioning needs, you can multiply your total floor area by the base heating factor of 25 British Thermal Units (BTUs). The product of this calculation will point you in the right direction but lacks accuracy as it excludes factors like the number of occupants, the local climate, the home’s insulation, and the orientation of windows. For a precise calculation that accounts for all these variables, call your HVAC contractor. They will work out the optimal system size based on “Manual J” - a standard calculation manual for HVAC systems.
2. Can your roof support the system?
Once you’ve established your system’s capacity, you’ll know its weight and dimensions. With this knowledge in hand, you can plan to install your unit. The first question on your mind should be, “Is my roof strong enough to support the added weight?”
Don’t rely on an HVAC contractor to answer this question for you; it’s not their job. Instead, hire a licensed structural engineer to perform a structural analysis of your roof. An engineer will examine your roof’s structure, perform load calculations, factor in the roof’s age and material composition to see if it needs any touch-ups before installation. Depending on your roof’s design and condition, you may need to reinforce, repair, or replace it.
After confirming your roof’s ability to support the HVAC system, you can figure out where to mount it. An ideal location will offer sufficient clearance around the unit to satisfy safety requirements and allow for easy maintenance access.
To ensure proper condensate flow during operation, the HVAC unit must be level. If your roof is pitched, the supporting curb, or the frame, must be designed to create a level surface. Your engineer can collaborate with the HVAC contractor to establish the optimal location for the unit and design its support structure.
3. Ensure envelope integrity
Installing a rooftop HVAC unit will call for penetrations through the roofing membrane and assembly. These should only be made for ductwork and reinforcement as needed, not the footprint of the entire unit. Keeping penetrations to a minimum will enhance the unit’s acoustic isolation. Likewise, isolators may be needed beneath the unit to reduce the transmission of vibrations into the building.
After the unit is installed, the roofing contractor will have to ensure that the roof can still keep out the weather. This means providing adequate flashing around all openings made during installation.
Once the penetrations are flashed, your HVAC contractor can test the unit and walk you through the operation manual. And you’re all set to enjoy your new A/C!
How We Can Help
If you’d like to take advantage of the many benefits a rooftop HVAC system offers, Design Everest can help. Our licensed, professional engineers will perform a full structural assessment of your roof, and recommend modifications to support your new unit. If your roof needs some work before the system can go up, we will strive to minimize your project costs and help you sail through the permitting process. Contact us now and we will provide you a FREE Consultation with a quote. We also provide virtual on-sites and virtual consultations.
*Note: The content published above was made in collaboration with members of Design Everest.