Recommendations You Can’t Miss Before Expanding Your HVAC System

Published: July 11, 2017 - by Bob Wells

Categories: For Homeowners

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In the typical US household, most of the energy consumed goes to air conditioning and heating. If you’re planning a home addition, it’s important to select an HVAC system configuration that offers high performance and minimizes your energy expenses.

If you currently have oversized HVAC equipment, a ductwork expansion may be enough. But new heating and air conditioning units are often necessary. This article will help you decide the best course of action, based on the specific characteristics of your project. This will help communicate your needs better once you contact an HVAC contractor.

In general, it's only cost effective to expand your current installation when you have oversized units. That is, the units have enough spare capacity to serve the home addition, or your existing equipment is old and would benefit from an upgrade. Otherwise, it's a much better option to install a separate system for the new floor space.

Alternatives to HVAC System Expansion

Packaged terminal air conditioners (PTAC) and window-type units generally have a low upfront cost. But they suffer from poor energy efficiency and end up being the most expensive option once you factor in the lifetime cost of energy. Some window-type units and PTACs come equipped with a resistance heater for the winter. The bad side is that this heating option tends to have the highest energy cost among all available options.

Ductless Systems

Instead of purchasing traditional heating and cooling equipment, you can consider the following energy-efficient alternatives:

  • Ductless air conditioners, also known as mini-split systems, can deliver the same cooling output as a window-type unit while reducing energy consumption by 50% or more. The Seasonal Energy Efficiency Ratio (SEER) indicates the energy efficiency of these units, showing how many BTUs of cooling are delivered for every watt-hour of electricity consumed - it is like the gas mileage of a car, where a higher value translates into reduced fuel expenses.
  • Heat pumps can deliver the same heating output as a resistance-based heater while achieving energy savings of 60% or more. The Heating Seasonal Performance Factor (HSPF) rates the efficiency of heat pumps. The calculation is like the SEER, but using BTUs of heating instead. Some heat pumps are reversible and can operate as air conditioners, allowing you to use the same piece of equipment in the winter and summer. These reversible heat pumps have separate SEER and HSPF values depending on their operating mode.

Equipment with the ENERGY STAR label delivers a minimum SEER of 15 and a minimum HSPF of 8.5. Even higher values are available if you buy a unit that uses solar power or geothermal heating and cooling to improve efficiency. If you use ductless air conditioners or heat pumps, make sure the building addition has proper ventilation. These units do not replenish indoor air.

A key advantage of ductless systems is their modular nature. This makes them a great option in commercial and multifamily residential buildings. Instead of operating a central HVAC system, you only need to activate the units serving occupied areas, optimizing energy consumption. Another advantage of ductless systems is that they don’t spread mold spores, dust and other allergens. This is especially beneficial if someone in your household is sensitive to these pollutants.

Other Options: Gas Furnaces and Hydronic Heating

If you use gas for your heating needs, look for a furnace with a high Annual Fuel Utilization Efficiency (AFUE). Furnaces with the ENERGY STAR label have an AFUE above 95% in northern states, In southern states it's above 90% because yearly heating needs are generally lower.

Given that you are building new rooms from zero, you can also consider the option of hydronic heating. This system configuration uses hot water piping embedded under the floor or going through baseboard heaters. Hydronic heating allows you to use a single high-efficiency boiler or heat pump and then distribute heat throughout indoor spaces using water pipes instead of air ducts. A hydronic heating system allows you to avoid the noise and air drafts associated with air ducts. It's also much easier to service - there are no out-of-sight spaces where dust or mold can accumulate.

Basic Considerations

There are many “rules of thumb” for sizing heating and cooling equipment based on square footage. We strongly recommend that you avoid them. Equipment sized according to these rules tends to be oversized, causing rapid cycling and unstable indoor temperatures. In addition, their blowers end up being oversized as well, which creates a drafty and uncomfortable environment. Oversized HVAC units also tend to suffer from premature failure. They cycle on and off more frequently than a properly-sized unit, which wears down their components.

Ideally, you (or your HVAC tech) should calculate your required HVAC capacity based on window orientation, thermal envelope and expected occupancy. The industry authority on the topic is ACCA, the Air Conditioning Contractors of America Association. They have published a series of manuals that HVAC professionals use to design installations: 

  • Manual J - Load Calculation
  • Manual S - Equipment selection
  • Manual D - Ductwork calculation
  • Manual T - Air distribution design

Once you determine the heating and cooling load of your home addition, you can decide if the existing equipment has enough capacity to serve the new floor area. If this is not the case, installing separate HVAC equipment is generally the best option. Also keep in mind that separate HVAC equipment allows you to condition only occupied spaces, achieving significant energy savings.

Expanding an Existing HVAC Installation A full HVAC expansion is more expensive, but it may be a good decision if your existing installation is very old and inefficient. Central heating and cooling units with ENERGY STAR labels are also available. You can combine them with a control system that allows independent operation for different home areas, which allows you to save energy by running your HVAC system at part-load when needed. If this is your case, you can achieve energy-efficient heating and cooling for your entire home, not just the addition.

Keep in mind that you may need to purchase a larger unit, but the running cost after the upgrade can actually be lower if the new equipment has a superior energy efficiency. For example, if you need 25% more capacity but the new unit is 50% more efficient, your energy bills will be lower even though system capacity has increased.

Expanding the ductwork without upgrading HVAC equipment makes sense if you have spare capacity. However, we recommend an upgrade anyway if current energy efficiency is poor. You should never go for a ductwork expansion for an undersized unit. It will always run at full capacity trying to meet a load larger that what is was designed for. This increases energy consumption drastically, resulting in very high utility bills.

Incentives for HVAC Equipment

Before proceeding with an HVAC upgrade, check the incentives locally available. These are typically offered by utility companies seeking to reduce their environmental footprint. State and federal programs may also offer such incentives. There are two main types of incentives:

  • Rebates are paid in cash when you purchase qualifying HVAC equipment, directly reducing the cost of your project.
  • Tax exemptions and credits reduce your tax burden when you purchase energy-efficient HVAC equipment. For example, a specified percentage of your investment may be deductible from federal taxes. Or there may be equipment that is not subject to the sales tax when purchased.

Incentives vary by location, but qualified HVAC contractors are normally familiar with them because projects become more affordable for clients, while allowing the contractor to make the same profit. In other words, both parties win with the use of incentives.


Once you have determined the heating and cooling needs of your home addition, you can decide on the most suitable HVAC system configuration. You can use existing equipment if it has enough spare capacity, but otherwise you will need either a system upgrade or a separate HVAC installation for the home addition. A separate installation represents a smaller investment, but if you have the budget availability, you can consider a full HVAC upgrade to deliver efficient heating and cooling for existing spaces as well.

Unless you are installing an ordinary window-type unit, you will need to consult a licensed HVAC professional. However, window units are not recommended due to their low efficiency, and even if you are installing one, it’s always better to have help - avoid accidents such as having the unit fall off the window, or getting your back hurt while trying to handle it!


This is a guest post by Bob Wells, a retired HVAC tech who now dedicates himself to sharing knowledge on his website Bob worked over 30 years in the field, 23 of which he ran his own contracting business. He’s dedicated to keeping up with the latest developments in the field and helping others to learn the trade better and advance their own careers. Bob is on Twitter with the handle @hvactraining101 and you can also find his page on Facebook.