An HVAC system will perform its functions of space conditioning only to the integrity of the envelope which contains the conditioned air. It's true – an air conditioning unit or furnace can be properly sized and installed according to exacting specifications, but without proper insulation and air sealing properties in the home, the system will run inefficiently. The same can also be said in reverse – if a well-insulated home does not have the proper mechanical systems in place, the functions of both, working in tandem, become ineffective.
A few simple oversights can prove costly for HVAC contractors who are forced to spend money and man hours answering call-backs in response to homeowners' and builders' complaints. In my job as building science manager with natural fiber cellulose insulation manufacturer GreenFiber, I have the opportunity to work closely with HVAC professionals and other trades in the above mentioned high performance programs and often share these steps to help save time and money.
Make Your Manual J Calculations
Every HVAC contractor should make their Manual J Calculations for every home they outfit, regardless of how long they have been in the business or how much they know a system is a correct fit. In fact, many jurisdictions and states now require Manual J Calculations for every structure as part of a series of high-performance building programming measures.
Manual J Calculations evaluate a building footprint, square footage, R-values of walls, window U-values and geographic climate to determine what size heating and cooling unit is appropriate. These should be an essential part of the job bidding process, but some contractors take a short cut and come up with a quick and dirty Manual J Calculation; some don't do it at all.
Another bad practice many contractors take when sizing a home for an HVAC unit is to allow for a ''fudge-factor'' and oversize the unit. While contractors may think they are doing themselves and the homeowner a favor by providing more bang for their buck, they are actually doing the opposite. When an AC unit starts up, a quick blast of cool air into the home may immediately satisfy the thermostat, but the unit won't run long enough to properly homogenize and dehumidify the air. This creates alternating pockets of warm and cool air that aren't allowed to mix. Also, when the unit does not run for extended periods of time it cannot achieve its full potential for dehumidification. This creates a cold, clammy house that not only creates an uncomfortable environment but also raises the potential for mold and mildew growth and reduces the air quality inside the home.
Short-cycling the unit also reduces its energy efficiency because most of the energy use from an AC unit comes from a power surge required at system start up. And, not only does an energy bill increase with short-cycling – so does the likelihood that the expensive compressors inside the unit can burn out.
Tight Duct Systems
The importance of a properly sealed duct system is equally as important as having an appropriately sized HVAC system. A leaky duct system not only lets conditioned air outside, it requires an HVAC system to work harder and longer to make up for an exponential loss of heated or cooled air.
When a duct system leaks to the outside, it creates a negative pressure within the envelope. For every cubic feet per minute (CFM) of air that leaks out of the system through faulty duct work, the system then pulls an equal CFM of air back into the system from wherever it can – usually through the largest available hole that offers the least amount of resistance. This means during colder months, warm, expensive air that leaks out is replaced by colder air that must then be reheated. The same is true in reverse, in warmer months – cool, conditioned air that leaks through gaps in the duct system is replaced by warmer outside air that must be cooled. And a sealed return duct also eliminates the possibility of contaminants becoming infused into the conditioned air.
HVAC performance and the Air Envelope
The importance of a properly functioning HVAC system and tight ductwork are lost without a properly insulated home, and vice versa. If the envelope – the insulation and pressure barrier that keep conditioned air inside the home – cannot hold the conditioned air that is circulated by a HVAC system, the efficiency of the system diminishes dramatically. The same is true for a well-insulated home that is not properly heated or cooled by a HVAC system.
Trade professionals in both fields need to understand how interdependent these elements are to each other for success – the success and consequences of each are reliant on each other.
Most houses have mechanical systems in an attic, basement or garage, and often times the responsibility for sealing the system's routing pathway through the pressure barrier that hold the return and distribution ducts can be lost in the shuffle.
Personal comfort is dictated by surrounding surface temperatures more than air temperature or the movement of air around you. An unsealed chase in the center of a living area will create uncomfortable zones that could have been easily avoided by following a few simple steps. Follow a simple rule: if you make the hole, you own the hole. Any hole that is opened up by you or a member of your crew should be sealed up by you or a member of your crew.
Third-Party Testing
HVAC professionals should embrace a third-party evaluation of the systems they install to assure their customers that they provide a quality product and service and stand behind their work. Some states have passed legislation that requires testing of all installed systems and subjects one of every seven systems to third party testing and review.
Quality control measures show builders that the HVAC industry embraces technology that helps to validate the quality of their work and also provides a competitive advantage when bidding jobs. Testing with quantifiable results places you in a different category from your competitors and provides added value for a builder as well.
No step in HVAC system installation is too small or inconsequential, and mechanical system providers should work in tandem with the building community to ensure the work is not for naught. By following these tips you can help reduce the number of costly and time-consuming call backs that you might get – call backs for problems that might not even be your fault.
About Author
Bohdan Boyko is building science manager with GreenFiber, the largest manufacturer of natural fiber insulation, fire and sound products. www.greenfiber.com. Boyko is a 27-year veteran of the building performance industry and started his career gaining hands-on experience in the field as a blow-in insulation installer. In his current role with GreenFiber, his focus is on understanding the heat, air and moisture transfer and its role in a house as a system.
