Do's and Dont's for the Manual J 8th Edition Procedures
There is often debate on how to handle and interpret certain
aspects of the home that affect the sizing calculations of the
HVAC system. Listed here are some of these items that provide
great tips for those creating Manual J calculations:
Numerous studies have shown that HVAC systems are consistently over-sized. This has led to many comfort complaints. Comfort problems are seldom caused by under-sizing equipment. Over-sized equipment undermines potential energy savings and makes the home vulnerable to comfort and moisture problems. The best way to ensure proper sizing is to have an independent third party verifier perform the load calculations. Builders and home owners often question why a third party verifier should perform this task when the mechanical contractor is suppose to be doing this? Here are several points to consider:
- The load calculation affects every aspect of the system design procedure. Thus, it must be as accurate as possible.
- Equipment capacity that matches the size of the heating and cooling loads will deliver comfort, efficiency and reliability over the entire range of the operating conditions.
- Heating and cooling loads determine the total air delivery requirement (blower CFM) and the air flow requirements for each room (room CFM).
- Excessively oversized equipment causes short-cycles, marginalizes part-load temperature control, creates pockets of stagnate air, degrades humidity control during the cooling season, requires larger duct runs, and increases the installed and operating costs.
- Manual J calculations should be aggressive, meaning the
designer should take full advantage of legitimate
opportunities to minimize the size of the estimated loads.
This would include using the outdoor and indoor design
conditions recommended by the Manual J tables and taking
full credit for:
- All internal shading devices and external overhangs
- All certifiable efforts to insulate and seal duct runs located in unconditioned space
- Load factors and diversity when estimating internal loads
- Diversity when estimating the cooling load on central equipment
- The total capacity of the cooling equipment not exceeding the total load by more than 15% for cooling-only applications
- For new construction, these additional comments are
given:
- Take full credit for all external overhangs
- Assume that all glass doors and conventional windows will be equipped with an internal shading device that is compatible with the room type
- Assume that a window will not have internal shading if it is specifically used for day lighting and has no effect on privacy
- Custom homes that feature a large amount of architectural glass that provides a view of sets the theme of the architecture may not have internal shades (may need special zone design for these rooms)
- Note on safety factors:
- Manual J calculations should be aggressive, which means that the designer should take full advantage of legitimate opportunities to minimize the size of the estimated loads. Research studies and the experience of knowledgeable system designers indicate that aggressive use of Manual J procedures provides and adequate factor of safety. No additional safety factors are required when load estimates are based on accurate information pertaining to envelope construction and duct system design. When an HVAC system is selected, the designer's first obligation is to deliver comfort, which means that the system must be able to control temperature and humidty during any reasonable operating condition
Numerous studies have shown that HVAC systems are consistently over-sized. This has led to many comfort complaints. Comfort problems are seldom caused by under-sizing equipment. Over-sized equipment undermines potential energy savings and makes the home vulnerable to comfort and moisture problems. The best way to ensure proper sizing is to have an independent third party verifier perform the load calculations. Builders and home owners often question why a third party verifier should perform this task when the mechanical contractor is suppose to be doing this? Here are several points to consider:
- Fear of Under-Sizing.
- Use of Outdated Rules of Thumb - Ones developed when homes were much less efficient
- Can't Afford to Do It Right - Rather than gathering the information on windows, insulation, orientation, etc., many HVAC estimators use worst-case assumptions that result in over-sized equipment
- Compensation for Poor Installation Quality - The major reason for comfort complaints is usually tied to poorly designed and constructed ducts (on top of an over-sized system). Many HVAC contractors tend to compensate for substandard workmanship by upsizing the equipment.
- Lower Return for Quality Work - The HVAC contractor who has taken on the added overhead to generate accurate equipment sizing and provide quality installations is competing with contractors performing at a level to just meet code. The higher cost for the increased value might not be recognized by the builder.
- Low Level of Verification - Over-sizing HVAC equipment also persists due to the lack of enforcement. Code officials rarely get into the details of the calculations. They typically verify that a Manual J report was generated.
- Low Incentives to Correctly Size the Systems - A larger HVAC system means a larger contract and larger profit for the HVAC contractor
- Economics - Over-Sized equipment has a higher first-cost and costs more to operate. Higher operating costs include lower system efficiencies due to short cycling and higher power draws. The short cycling also reduces the life of the compressor.
- Reduced System Efficiency - The highest efficiency rate HVAC equipment is the multi-stage equipment. These systems are designed to run in the lower output stage most of the time, which allows longer run times. When a multi-stage system is over-sized, the efficiency that was to be gained is lost as the over-sized system will cycle on and off in lieu of running continuously.
- Reduced Comfort - Over-sized HVAC equipment reduces the system's ability to remove moisture during the cooling season, a very important function of the AC system. The relative humidity in the home plays an important role in comfort. Homes with lower humidity levels (50% RH to 60% RH) feel more comfortable at higher thermostat settings. This saves energy.
- Higher Noise Levels - Over-sized HVAC equipment produces more noise than properly sized equipment. They can also create annoying high air speed levels that interfere with paper and book use.
- Reduce Indoor Air Quality - If moisture is not adequately removed during the cooling season, the indoor humidity may reach levels that can lead to dust mites, mildew and mold growth in extreme cases. Also, short-cycling results in less air filtration. It can also lead to pockets of stale air in the home. Builders of high performance homes need to be aware of these potential side effects of over-sized HVAC equipment.
