When installing a new ducted air conditioner or updating the old one, one question that bothers you the most is, “what size ducted air conditioner do I need?” Determining the right size levels up or destroys your air conditioning experience so it is crucial to select the right one. You might think of simply selecting the ducted air conditioner size with respect to the square foot area of your property, but technically this isn’t that simple.
Making a mistake in selecting the right size of ducted air conditioner costs you more than you can think. It disrupts the peace and comfort you seek for during the hot Melbourne days, spikes energy bills and affects air conditioning cycling, causing short cycles or over pressure which result in damaging the air conditioner.
There are many factors that influence ducted air conditioner size and it is advisable to ask a technical expert about what size ducted air conditioner do I need, rather than taking the risk. Being your ducted air conditioner installation experts, Surrey Air combines its 45 years of technical knowledge and experience to guide you regarding ducted air conditioner size selection in this blog.
Factors Affecting The Ducted Air Conditioner Size
Ductwork System Factors
Duct Design And Layout
The duct length, number of bends, transitions, fittings, duct material and friction rate collectively affect the efficiency, airflow and size of the ductwork in the ducted air conditioning system. Ducts must be sized using the Manual D sizing technique and designed professionally to reduce length and bends to deliver the required air flow to each room.
Airflow Requirements
Every room requires enough air flow (CFM) from the ducted system to absorb the warm indoor air and moisture. Ideally, there is 350-450 CFM per ton (12,000 BTU/h) of cooling, but this varies based on climate and latent load needs.
Room-By-Room Requirements
Airflow in the required pressure is mandatory to circulate air and moisture in each room to maintain even temperatures. If using a zoned system, dampers must be correctly sized for their designated airflow.
Static Pressure
Static pressure is the total resistance to airflow from vents, grills, dampers, filters and internal coils. The air handler must be selected to overcome this internal static pressure.
Air Conditioning System And Performance Factors
Thermostat Control And Zoning
Zoning with a single thermostat causes imbalances, so it requires multiple thermostat and careful designing to handle varying airflow and partial load operation.
Equipment Capacity And Performance Data
Equipment capacity doesn’t only rely on tonnage, it is a must to consider the actual capacity defending on outdoor, and indoor conditions. Details about equipment capacity and performance data are available on the manufacturer’s expanded performance tables.
Building Envelope Factors
Climate And Location Factors
Climate of the surrounding area, like extreme outdoor winter and summer temperature at the location, and solar orientation of the building influence the solar heat gain, and increases indoor temperature. The overall effect influences the answer to your question of what size ducted air conditioner do I need for my property.
Building Construction And Materials
Building construction materials, insulation levels in roofs and floors, number and size of windows, glazing type on windows, frame material, shading and the orientation collectively influence the size of the ducted air conditioner.
Heat Load Factors
The internal heat gains due to the number of people in the building, their activity level, number and type of appliance and equipment and lightening types affect the air conditioning. Along these internal heat gain factors, infiltration and ventilation add or subtract total load on the air conditioner.
For example, while calculating what size ducted air conditioner for 3-bedroom house, you can’t simply refer to tonnage to square foot chart, rather you need to consider all the above mentioned factors, along with cooling and heated rating capacity, zoning requirements and energy star ratings.
Why Correct Sizing Is Crucial?
Correct sizing the ducted air conditioner is crucial as it effects the performance efficiency, energy bills, and longevity of the system. Here are details about what sizing issue can cause to the system and what it may lead to.
Oversizing:
The common cause of oversizing is short cycling. Short cycling means frequent on/off of the ducted air conditioning system. This leads to:
- Poor dehumidification (system cools but doesn't run long enough to remove moisture).
- Inconsistent temperatures.
- Increased wear on components.
- Higher energy costs over time.
Undersizing:
Undersizing causes long running cycles. The air conditioning system works hard and longer than usual to meet the required temperature settings. This leads to:
- Inefficient temperature control.
- Increased discomfort.
- Increased wear on components.
- Increased chances of system breakdown.
- Higher energy costs.
How To Avoid The Costly Sizing Mistakes?
As we now know that sizing mistakes can lead towards system wear and tear, high energy bills and system breakdown, we must know how to avoid the mistakes that cause us great monetary loss.
Here are some tried and tested tips to avoid these mistakes and ensure a correctly sized and installed system.
The Golden Rule: Never Use Square Footage Rule Of Thumb
The most common ducted air conditioning sizing mistake is to solely based your air conditioner size on square footage. For example: a 3-ton unit for a 2000 sq foot home. This approach puts all other factors at a side which contribute to performance of the ducted system.
So, when anyone asks what sized ducted air conditioning system for a 4-bedroom house, never ask only square footage to give the final answer.
Professional Assessment:
Before selecting the system size, ask a professional HVAC contractor for a full, room-to-room manual J Load calculation with sensible and latent loads. Also, ask for a proper duct design using Manual D to ensure proper airflow to each room. The Manual D contains details about layout, and ductwork size, length, material that deliver exact CFM required for each area in the property with low static pressure.
Select Equipment On Actual Performance, Not Tonnage:
A 3 ton unit for a 2000 sq foot home isn’t fixed. It varies with temperature and depends on local design conditions. If your area is humid, you need an air conditioning system with lower SHR to remove the moisture effectively. Always check the manufacturer’s expanded performance table to get a complete picture about the system’s performance.
Verify The System Design During Installation:
Ensure the installation matches the specifications in the Manual D design for correct ductwork. The refrigerant must be charged with manufacturer’s specifications using the subcooling/ subheading method, and the thumb rule of pressure test must be avoided.
Commission The System:
Once the system is installed, perform final testing such as static pressure test, temperature drop test and system balancing to ensure even temperatures throughout the zones.
Future Proof Your System With Proper Documentation:
In the long run, protect your investment of installing the ducted air conditioning system with proper documentation. Keep all documentation (Manual J report, Manual D design, equipment model/serial numbers, commissioning report) in your home files safely. This is invaluable for future service, warranty claims, or home sale.
The Final Cost Benefit Insight With Right Ducted System Sizing
Initially, the upfront cost of a proper design by professional HVAC contractor and installation is marginally higher but it pays for itself many times over through:
Lower energy bills:
A right-sized ducted air conditioning system, is a system designed while considering all the factors such as climate, location and property’s indoor factors and works efficiently without over consuming energy.
Greater comfort:
A right ducted system size brings consistent temperature and humidity control, thus bringing even cooling experience and greater comfort.
Fewer repairs and longer equipment life:
When ducted air conditioning system size is right according to the requirements, the systems aren’t strained by short-cycling or high static pressure.
Avoiding the ultimate cost of a full system replacement:
Right system sizing prevents short or long cycling, which saves the system from wear and tear and eventually improves the longevity. You won’t need to replace the system early.
