Steps to Redesign an Existing HVAC System

March 21, 2012
Redesigning an existing HVAC system is completely different than designing a new HVAC system from scratch. The primary difference is that you redesign an existing system to remove the defects that prevent it from operating at maximum capacity.

Redesigning an existing HVAC system is completely different than designing a new HVAC system from scratch. The primary difference is that you redesign an existing system to remove the defects that prevent it from operating at maximum capacity. Redesigning system modifications requires a completely different skill set and perspective, than new construction design.

Here at NCI, we are completing a new training course in response to contractors, technicians and salespeople that have become proficient at diagnostic testing and are seeking a fast, simple method of HVAC system redesign. One of the requirements for the course was that it could be easily performed in the field and away from the office during the sales process.

Find Out What Your Customers Really Want

The first step in redesigning a system is to interview the occupants and get first hand information about how well the system is performing. They have specific knowledge about issues concerning energy consumption, levels of comfort, safety and noise that they would very much like solved. They also often have undeveloped impressions about what they may want by way of upgrades that you can redesign into the system. So take time to find out what they are looking for and teach them what’s available by involving them in the redesign process.

Assess System Performance Before Redesign

Nearly every existing system has inherent defects that can be redesigned out of a system if you know what these defects are. The problem is that many performance reducing defects are often invisible and can only be discovered through testing.

Testing includes the measurement of system static pressures, temperatures, airflow, fan speed and electrical properties. These measurements are fed into a series of efficiency calculations as a step in the performance assessment process.

Topping the list of energy and comfort thieves are duct temperature loss, duct leakage, excessive restrictions that increase static pressure, decreased system capacity, poor airflow distribution and refrigerant and combustion issues.

Detailed performance testing takes an hour or more on a typical residential system, but can produce amazing results for your customer and additional high margin work for your company. Without it, redesign is ineffective and would be just another design assumption at best.

Unfortunately most of your competitors overlook these issues and swap the box promising the efficiency stated in the manufacturer’s sales literature. It’s a pure delight to watch a customer become upset when they see the results of your system performance assessment and realize your competitors are only interested in replacing the box leaving the customer with a poorly operating system and unfulfilled promises of efficiency.

Analyze the Test Results

Once the field performance data has been collected, compare it to manufacturer’s engineering data and industry standards. Steps in the redesign process include identifying the discrepancies that exist, engineering a solution and prescribing repairs that are needed. The repairs pour into a scope of work which defines the content of the system renovation proposal.

Static Pressure Issues

Typical findings include a number of usual suspects in residential systems. We find the average operating total external static pressure between .80-in. w.c and 1.0-in. w.c. This produces the average cooling airflow often below 280 CFM per ton.

The redesign process identifies each defect and then provides a solution which specifies how to renovate or replace system components that are causing the excessive static pressure. These include excessive filter pressure drop, (in well over 50% of systems) restrictive remote cooling coils, (in over 40% of systems) and low fan speed (in nearly 30% of systems). Other defects may require the redesign of restrictive registers and grilles, replacement of duct fittings, or removal of unnecessary balancing dampers.

BTU Loss or Gain Through the Ducts

Live system temperature profiles are taken across the supply and return duct system to reveal excessive percentages of BTU losses. The redesign process identifies these issues and if NCI CommonCents™ software is used, these effects can be annualized to see their actual impact.

Annualized losses exceeding 5% or more normally require additional duct insulation far above code or may include a relocation of the duct into conditioned space where the BTU loss can be drastically reduced.

Undersized Ducting

System assessment often reveals undersized ducting found in nearly 80% of residential HVAC systems. System redesign includes a simplified field fan selection process coupled with NCI’s field duct sizing method. This enables the redesigner to easily evaluate and specify replacement or addition of duct system components to a system within minutes with no office engineering time.

Often increasing or adding duct capacity is a universal solution. On the return side and the supply side adding or increasing duct capacity will increase system performance and enhances individual room comfort.

Redesigning a duct system in the field using live system measurements will help you avoid the hazards of just duct sealing systems without adding badly needed additional duct capacity. Static pressure testing must be mandatory with every duct sealing job.

Replacement Equipment

In creating the redesign process we found the industry needed a quick method to evaluate the heat loss or heat gain of a home in the field. This step had to be shortened and followed up with the essential engineering using ACCA Manual J, after the sale is made. No more load calculations for free! Precise room by room airflows can also be verified using Manual J after the sale is made when the cost of required documentation is included in the price of the job.

Contractors also told us they needed a quicker version of Manual S that they could do before the sale is made to help select equipment to develop their proposals, so we have provided a new tool called EQSelect. Final calculations to select equipment can be completed after the sale using Manual S or the valuable selection tools found on manufacturer’s websites.

Grilles and Registers

As room by room airflow measurements are taken in an existing system, the redesigner can check airflow volumes and patterns in each room to verify adequate ventilation and conditioned space air velocities. This is the most accurate method to select and specify replacement registers available today and emphasizes the value of live system testing coupled with the redesign process.

Pressure drops can be taken over existing grilles so they can be redesigned on the spot, if needed. Include your customers in the process and teach them about some of the redesign decisions you make and get their preapproval of each component of the final proposal during the redesign process. One advantage of onsite redesign is that you can craft the proposal in real time with your customer present.

Custom Accessories

By involving your customers in onsite redesign you will also discover other needs that your customer may previously have not considered as you involve them in the redesign of their system. Add-on components like new filter systems, humidification, dehumidification, UV lights, and many other needs will arise out of conversations springing from their new understanding of their system and what features are available today.

It has been a delightful mission to create a redesign process that’s fast, effective and accurate and will be such an asset to the new role of redesigners in progressive HVAC contracting companies.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free sneak peek at NCI’s redesign training, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.