Contrary to the higher level of understanding amongst the builders for building envelope, framing, etc, the industry falls short on heating and cooling systems in a house. Heating Ventilation and Air Conditioning (HVAC) and water heating is by far the largest energy user in the home.
High efficiency equipment and efficient system are two separate things. The distinction is often times looked over in this highly competitive market. Market pressures, increased turnover in property are among many reasons which continue to drive the skills even lower. There is little incentive to make things better.
The evident void in the industry for mechanical system understanding must be addressed. The builder, who is gaining competence in key trades such as building envelope, framing, insulation, etc., often lacks basic understanding of mechanical systems. With little to no training in duct design or equipment sizing and selection, most builders are ill-prepared to take on HVAC, which is quite demanding on technical level.
Oversizing of equipment is the most common industry ailment. There is a general tendency to size heating and cooling equipment by educated guess and not on actual calculations. It is imperative equipment sizing and selection be based on Calculations, not estimates. Systems should be designed with guides from the TECA (Thermal Environmental Comfort Association) or ACCA (Air Conditioning Contractors of America). It is Some jurisdictions in North America make it mandatory to have engineering calculations and design for residential systems.
Oversizing is a huge economic disadvantage. Initial higher cost, and higher operational cost to operate a bigger equipment. To top that the equipment life is compromised due to short cycling. It is like driving a large truck in stop and go city traffic.
Often oversizing issue is camouflaged with high efficiency equipment. It is seen that contractors oversize to get the reserve capacity for extreme weather and rely on first stage for most of the time. As an engineer, I would contest to size the equipment to meet 95% of the times. The last 5% extremes are spread out over a longer period of time.. In other words the 15 to 60 minutes extremes would be spread over several days – mostly a non-issue as the system catches up for those times without any noticeable change.
Oversized equipment and underperforming ductwork can result in wider temperature swings, especially in perimeter areas. On the coldest day, a furnace only operates 30 minutes an hour. As soon as the furnace cycles off, the house begins to cool from the outside in. The thermostat is purposely located away from exterior walls and windows, often in a hallway. While the furnace is off, air doesn’t circulate. By the time the thermostat senses the lower temperature, perimeter areas may have dropped by several degrees. During mild weather when loads are tiny, minimum runtime logic enforced by the furnace controls assures significant overshoot. Either scenario can cause discomfort – the greater the oversize ratio, the larger the temperature swings. The same result occurs in cooling mode.
Not to mention, larger equipment generates more noise. Short run runtimes also means less filtration which could be detrimental for an allergic person who is relying on clean filtered air.
Oversizing runs the risk for inadequate moisture control – on other words unit’s compromised latent capacity. When a unit starts, it takes 10-15 minutes for the coil to get cold enough to condense moisture and provide the required dehumidification. Shorter runtimes, result in little or no moisture removal which will be more evident in shoulder seasons. The humidity could accumulate over time and lead to increased mildew, and mold issues. Destructive fungi thrive in high moisture, which can cause damage to paint, drywall, flooring etc.
It begs a question, despite so many disadvantages, why do we still see oversizing and installations without proper design.
Educating the client is an inhibitor for most. Not designing for the worst-case scenario is hard to put across. It is important the client be educated that the system will work better if it is optimally designed to meet 90-95% of your needs. The next size up is usually not a huge jump in price so people just opt for the larger system, even though the jump can result in not so optimum oversizing.
Another common reason we get is – we have always done so. Experience is not a bad thing to rely on, but building technology has changed over the past years and houses have become tighter and better designed. Having done it in a certain way for decades does not make it right.
Designers and engineers are an added cost, a cost which is not factored in when one quotes a price. So technically the contractors or installers cannot afford to hire an engineer. Engineers are brought in only when the local jurisdiction needs one. The builders are hesitant to even bring it up with the home owners, the reluctance is primarily attributed to lack of understanding of the implications of an improper design.
Ideally the local jurisdictions should realize the inefficiencies and protect the industry by raising the bar. If proper calculations and design are mandated, we will have more sustainable homes and this will also promote more education. In commercial buildings, the design is done by the engineers and designers and installation can be done by the contractors. The contractor is relieved of the onus of defending the selection and design. A similar methodology in residential sector will elevate the industry to the next level.
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