How much does the HERS index change when switching from ductless to ducted heat pumps?

In our Massachusetts multifamily models, switching from ductless mini-splits to ducted central heat pumps typically pushes the HERS index up by 4 to 8 points per unit on a standard envelope. The exact number depends on duct location, air handler efficiency, and unit geometry. On units that were already close to the compliance threshold, that shift is enough to fail them outright.

Do ducted heat pumps work for Passive House or Specialized Stretch Code projects in Massachusetts?

Yes, but only when the envelope is tight enough that the heating load is small to begin with. On a Passive House target, the ducted efficiency penalty matters less because total heating demand is a fraction of a code-minimum building. On a Specialized Stretch Code project with a typical envelope, ducted systems often push the model out of compliance, and the fix is usually to go back to ductless or to upgrade the envelope well beyond code.

Is it cheaper to install ductless mini-splits or ducted heat pumps in Massachusetts multifamily?

Installed cost is usually close once you account for all the trades. Ducted systems save on head count and refrigerant line runs but add ductwork, soffits, and mechanical coordination. Ductless systems cost more in equipment but simplify the build and avoid the compliance risk we described above, which is often the deciding factor on a tight energy budget.

Why Ducted Heat Pumps Can Fail Massachusetts Energy Code

Swapping ductless for ducted heat pumps on a Massachusetts multifamily project failed half the units in energy code immediately. Upgrading to triple pane windows still was not enough to recover compliance.

April 22, 2026

Most general contractors treat ducted and ductless heat pumps as interchangeable HVAC options. On a recent Massachusetts multifamily project, that assumption collapsed in the energy model, and no amount of envelope spending could rescue it.

The Real Project

We modeled a Massachusetts multifamily building for energy code compliance. The original design used ductless mini-split heat pumps on a standard code-minimum envelope. Every unit passed.

The client then asked if they could switch to ducted central heat pumps because their GC preferred the install. Same building, same envelope, same orientation. We re-ran the model with one variable changed.

Half of the units in the building failed energy code immediately. We upgraded the windows to triple pane to recover the compliance margin. Still failed.

The Efficiency Penalty

The failure was not an envelope problem. Ducted heat pumps lose 15 to 25 percent of heating efficiency compared to ductless systems in cold climates. That penalty shows up directly in the HERS index and in the prescriptive compliance math for Massachusetts multifamily.

Once the HVAC system is that much less efficient, you are no longer fighting a few kBtu of envelope loss. You are fighting a systemic efficiency gap that envelope upgrades can only partially offset. Triple pane glass moves the needle by a few points. The ducted penalty moves it by much more.

The Duct Loss Factor

Ducted central systems in Massachusetts almost always run ducts through conditioned-ish space: interior soffits, above drop ceilings, through floor assemblies. Even when ducts are technically inside the envelope, static pressure losses, air handler placement, and real-world leakage combine to lower the delivered capacity. Cold-climate performance drops further because defrost cycles and low-ambient derating compound with duct loss.

Ductless mini-splits avoid most of this. Each indoor head sits in the space it serves, heat is delivered without duct losses, and every zone operates independently at its own efficiency curve. In a cold-climate code environment, that architectural advantage is worth a lot of HERS points.

When Ducted Systems Still Win

Ducted central heat pumps are not the wrong answer in every case. We specify them regularly when the building physics support it.

Passive House or very tight envelopes. Once the envelope is aggressive enough, the ducted efficiency penalty shrinks because the heating load itself is small. The HVAC choice stops being the determining compliance variable.

Unit layouts with many rooms. A three-bedroom unit with a dedicated office and closed-door bedrooms can be cleaner to serve with a single ducted system than with five mini-split heads. The HERS hit may be acceptable if the envelope has margin.

Filtration and IAQ requirements. Projects targeting LEED, WELL, or owner-driven air quality specs often need MERV 13 or higher central filtration, which ducted systems handle natively. Mini-splits cannot match that without additional ventilation equipment.

The Decision Framework

HVAC selection is not a late-stage coordination item. It sets the ceiling on what your envelope strategy can achieve, so it has to be modeled before the envelope is priced. If your GC prefers ducted for install reasons, we can usually make it work, but only if we know early enough to adjust the envelope, the window package, or the compliance path.

The projects that get into trouble are the ones where HVAC gets swapped after permit set. By then, the envelope is bought, the windows are ordered, and there is no margin to absorb a 15 to 25 percent efficiency swing.