By Phil Rains
Residential and light-commercial heating, ventilation and air-conditioning (HVAC) equipment distributes air throughout a residential and/or light-commercial structure by using an air distribution system. Designing a delivery system for air to flow in is critical. The proper operation of any heating or cooling system depends upon the duct design and layout. Poor design results in inadequate heating and/or cooling in some or all spaces in a structure. Poor heating and cooling is commonly attributable to insufficient equipment size even though the real problem is the conditioned air “delivery system”. When we say “delivery system,” we are actually talking about the duct work system (air distribution system).
If properly designed, the supply air and return air distribution is even and an approximate uniform temperature is maintained throughout the structure. Field studies have indicated that in some instances, the efficiency of air distribution (duct work) systems is over 50% less than it could be, because of poor design and leaks in the ducts. Many of these mistakes and problems can be corrected for little or no extra cost. However, proper air distribution can alleviate these problems from occurring in the first place, and if corrected after the fact, often increase savings for the occupant(s).
Leaks in air distribution systems can cause energy bills to rise and system capacities to be reduced. Leaking duct work can also cause unbalanced temperatures within a structure. Leaks in the return air duct work can result in dirty coils which can reduce system capacity and increased humidity levels (during summer). Return duct leakage can also allow contaminants to enter the air stream.
Improper design can result in oversizing or undersizing duct work. Undersized duct work can result in low air flow. Also, low air flow can result from too many unnecessary turns rises and drops in the duct layout which all lead to increases in pressure within the system. Additionally, noise can become a problem and the efficiency of the system can also be affected. Oversized ducts can also cause low air flow in some cases, as well as uneven temperatures throughout the structure. Oversized ducts also negatively affect the equipment efficiency.
Good air distribution design leads to occupant comfort, proper air flow, economical cooling and heating, and an economical duct installation. These are established by the duct design process. The final layout is called a duct system. The objectives of the duct system are to:
· Supply conditioned air to each room
· Provide proper pressure drop across the air handler’s coil
· Be sealed to provide proper air flow and stop leakage
· Have a balanced supply and return system
· Reduce duct losses and gain from and to surrounding areas
A thorough knowledge of air flow dynamics and duct work design can enable you to easily uncover and resolve what many would consider to be difficult problems.
The actual purpose of HVAC duct work is to deliver air from the system blower to the outlets which distribute the air to the particular room or space. Air moves through the duct work in response to a pressure difference created by the blower. The necessary pressure difference is a function of the way the duct work is laid out and sized. The objective of duct design is to size the duct work so as to minimize the pressure drop through it, while keeping the size (and cost) to a minimum. Proper duct design requires a thorough knowledge of the factors that affect pressure drop and velocity in the duct.
People typically want economical heating and/or cooling systems, and they also want economical duct work systems. All too often, duct design doesn’t get the respect or attention it deserves.
The 3 most important things to understand about duct work designs are:
1. Furnaces and air conditioners require a certain amount of air flow, measured in CFM (Cubic Feet per Minute), to be passed through the equipment (supply and return ducts) in order for the equipment to function properly and efficiently.
2. All structures have unique requirements and construction that pose obstacles when designing the duct work system to accommodate each room or space with proper air flow.
3. The ideal duct work system achieves both goals by providing enough air flow to and from the heating/cooling equipment as well as the structure. For maximum efficiency, this “ideal” system should also be sealed at all seams and should be properly insulated when exposed to unconditioned areas.
The industry-accepted method for residential and/or light-commercial duct design is the Air Conditioning Contractors of America (ACCA) Residential Duct Systems, Manual D.
Various other methods are available that address duct work design and installation for these applications. These methods are readily available for contractors and technicians. However most are based on the fundamentals identified in Manual D. Large commercial duct work systems must utilize design criteria appropriate for those applications which are beyond those methods identified in Manual D.
The typical residential and light commercial air distribution system consists of supply-air and return-air duct work plus the supply-air outlets and return-air inlets, and any other added accessory. The air distribution system is typically designed simultaneously with new structure layout, unless the structure exists already. New construction requires planning for ductwork locations, framing, plumbing and electrical wiring. Existing structures requires determination of modifications to existing duct work to achieve acceptable results.
You must always design the air distribution system and install one based on its performance characteristics, the climate where the structure exists or will be built, the particular structural features, and the heating and cooling loads determined by methods such as ACCA’s Manual J, or MJ8AE. If the size of the HVAC system is incorrect, you will never achieve the proper heat transfer required for the structure, room or space. Always check to assure the system size is correct for the structure as part of your troubleshooting routine. No single type of air distribution system is ideal for every structure, and often two or more types of air distribution systems may be necessary.
Assuming you will utilize Manual D for proper duct work design, the following steps are recommended:
· The length of the longest circulation path and the available static pressure determine the friction rate used for duct work sizing.
· The length of the circulation path includes the straight runs and the equivalent length of the fittings along the path. One fitting can add from 5 feet to more than 60 feet to the length of the path. Always consider the fittings in the air path.
· External static pressure (ESP) is determined from the equipment manufacturer’s blower performance data, preferably for medium-speed operation.
· The available static pressure (ASP) equals the external static pressure (ESP) minus the pressure drop through all the air-side devices in the circulation path. Always refer to blower table footnotes and manufacturer pressure drop data for devices that were not in place when blower performance was laboratory-tested by the equipment manufacturer.
· Accessory or after-market filters (or any device) that produce a substantial increase in system resistance should not be installed if the blower cannot accommodate the increased resistance by speed change. An arbitrary increase in system resistance may cause low air flow to rooms, a high temperature rise across a furnace heat exchanger (if installed), or low suction pressure at the cooling coil.
· The room heat loss and heat gain estimate (Manual J or equivalent) and the heating and cooling factors (Manual D or equivalent) determine the design value for room airflow.
· Duct work size is determined by sectional flow rate and the design friction rate (FR) value calculated from Manual D procedures, not arbitrarily assigned from “rules of thumb.” as frequently done with duct slide rules.
· The duct slide rule should only be used to size the ducts after the Manual D procedures are complete, not to lay out the ductwork using a “standardized” friction rate that can cause ducts to be the incorrect size and result in hot and cold spots.
· The friction chart or duct slide rule used for duct work sizing must always be technically correct for the type of duct material.
· Duct work velocities should not exceed specified design limits.
· Branch (run out) ducts should be equipped with a hand damper (for balancing).
Air flow and air distribution should always be considered when troubleshooting any HVAC system in the field. As discussed in other articles in this blog, checking superheat and sub-cooling on an operating system is dependent on proper air flow.
Even though superheat and sub-cooling determination can identify air flow issues with a system, other faults assume the air flow is already correct, or has been corrected prior to testing the system.
Correct airflow is of crucial importance to the operation of any HVAC system. Part of the heat transfer rate is determined by the air flow across the indoor and outdoor coils. If the air flow is incorrect, then the heat transfer rate is incorrect and can drastically affect the equipment’s performance.
HVAC equipment manufacturers follow the tenants of the Air Conditioning, Heating, and Refrigeration Institute (AHRI) which has a “Test Stand Value” requiring the measured air volume rate, when divided by the measured indoor air-side total capacity, must not exceed 37.5 SCFM per 1,000 Btu/h [this is a maximum of 450 cubic feet per minute (CFM) of airflow across an indoor coil per 12,000 Btu/h of capacity].
Most manufacturers use an acceptable range of 350 to 450 CFM per 12,000 Btu/h of cooling capacity, and over 750 CFM per 12,000 Btu/h of capacity across outdoor coils (most outdoor fans move approximately 1,000 CFM, up to 1,500 CFM, per 12,000 Btu/h of capacity). In the HVAC/R industry, 12,000 Btu/h of capacity is referred to as a “Ton” of refrigeration. Typically, most manufacturers focus on around 400 CFM per “Ton” of cooling capacity when rating their equipment.
There are various methods that help determine the air flow amount across an indoor coil. The indoor coil is typically checked as the air flow must cross this coil to allow the refrigerant to either absorb (cooling) or reject the heat (in the case of air-source heat pump heating) to the appropriate “sink.” In summer (for all cooling systems and air-source heat pump cooling), the “sink” is the outdoors, and during winter (in the case of air-source heat pump heating), the “sink” is indoors.
When furnaces are used, the system air flow during heating must meet the manufacturer’s specifications to achieve acceptable temperature differences across heat exchangers.
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