Extend HVAC System
Your new addition needs heating and cooling, but simply adding a duct run rarely works well. You have four main options: extend your existing system, add a zone, install a mini-split, or upgrade to a new system. The right choice depends on your current equipment's capacity and the size of your addition.
Quick Summary
Typical cost
$2,000-$15,000+
Time needed
1-3 days
Professional help
HVAC contractor
Why This Step Matters
An addition without proper HVAC is uncomfortable at best and unusable at worst. Undersized heating means the room is cold in winter; undersized cooling means it's hot and humid in summer. But oversizing is also a problem—it leads to short-cycling, humidity issues, and wasted energy.
Common Mistake
Many contractors just run a duct to the addition without checking if the existing system has spare capacity. This often results in the whole house being less comfortable, not just the addition.
Start with a Manual J Calculation
Before choosing an HVAC approach, you need to know the heating and cooling load of your addition. A Manual J calculation considers:
Factors Considered
- • Square footage and ceiling height
- • Window size, type, and orientation
- • Insulation R-values (walls, ceiling, floor)
- • Local climate data
- • Number of occupants
- • Heat-generating equipment
Typical Results
- • Heating load: BTU/hr needed
- • Cooling load: BTU/hr (or tons) needed
- • CFM (airflow) requirements per room
- • Equipment sizing recommendations
- • Duct sizing specifications
Rule of thumb: In moderate climates, expect 20-30 BTU per square foot for cooling and 30-50 BTU per square foot for heating. A 500 sq ft addition might need 10,000-15,000 BTU of cooling (about 1 ton) and 15,000-25,000 BTU of heating.
Four Options for HVAC in Your Addition
Extend Existing System
Add duct runs from your current furnace/AC to the new space. This is the simplest approach but only works if your existing system has spare capacity.
Pros
- • Lowest cost ($500-$2,000)
- • Single thermostat
- • No new equipment to maintain
Cons
- • Only if system has 20%+ spare capacity
- • May reduce comfort in existing rooms
- • Long duct runs lose efficiency
Best for: Small additions (<300 sq ft) close to existing ductwork, with an oversized or recently upgraded system.
Add a Zone
Install a zoning system that uses dampers to direct conditioned air where it's needed. The addition becomes its own zone with its own thermostat.
Pros
- • Independent temperature control
- • Uses existing equipment
- • Better comfort than simple extension
Cons
- • Still needs spare system capacity
- • Adds complexity ($2,000-$5,000)
- • Requires compatible equipment
Best for: Medium additions with some system capacity, especially if the addition will be used at different times than the main house.
Ductless Mini-Split
Install an independent heating/cooling system just for the addition. A mini-split has an outdoor compressor and one or more indoor air handlers, connected by refrigerant lines through a small wall penetration.
Pros
- • Completely independent—no ductwork
- • Very efficient (heat pump technology)
- • Individual room control
- • Works regardless of existing system
Cons
- • Higher upfront cost ($3,000-$8,000)
- • Wall-mounted units visible inside
- • Outdoor unit needs space/clearance
Best for: Most additions, especially when existing system is at capacity. Excellent for bonus rooms, sunrooms, and ADUs.
New/Upgraded System for Entire House
If your existing system is old (15+ years), undersized, or inefficient, the addition is a good opportunity to replace it with a larger, more efficient system that serves the whole house including the addition.
Pros
- • Solves everything at once
- • Better efficiency (lower bills)
- • New warranty on equipment
- • Properly sized for entire home
Cons
- • Highest cost ($8,000-$20,000+)
- • Larger project scope
- • May require new ductwork throughout
Best for: Homes with aging equipment (15+ years) or systems already struggling to keep up. Often makes financial sense when you factor in efficiency gains and avoided repair costs.
Quick Decision Guide
| Your Situation | Recommended Approach |
|---|---|
| Small addition, new system (<5 yrs) | Extend existing or add zone |
| Any size addition, old system (15+ yrs) | Replace entire system |
| Medium/large addition, mid-age system | Mini-split for addition |
| ADU or detached addition | Mini-split (independent system) |
| Sunroom/bonus room (occasional use) | Mini-split |
Frequently Asked Questions
How do I know if my current system has spare capacity?
An HVAC contractor can compare your system's rated capacity (in BTUs or tons) against your home's calculated load. If your system is running less than 70-80% of the time on the hottest/coldest days, you may have spare capacity. If it runs constantly and struggles to maintain temperature, you're already at capacity.
What size mini-split do I need?
Mini-splits are sized in BTUs. For a rough estimate: 20-25 BTU per square foot of well-insulated space. A 500 sq ft addition typically needs a 12,000 BTU (1-ton) mini-split. Have a Manual J calculation done for accurate sizing.
Do I need to run ductwork before insulation?
Yes. Ductwork should be installed during the rough-in phase, before insulation and drywall. This allows ducts to run through walls and ceilings easily. Installing ducts after finish work is significantly more expensive and limits placement options.
Can I use a portable AC or space heater instead?
Technically yes, but for permitted additions, most codes require permanent HVAC. Beyond code, portable units are inefficient, noisy, and inadequate for year-round comfort. They're acceptable for temporary use during construction but not as a permanent solution.
Ready for the Next Step?
With HVAC rough-in complete, the next step is installing insulation. This locks in your thermal envelope and protects your investment in heating and cooling equipment.