How many HERS points is a heat pump water heater worth in a multifamily unit?

On the 296-unit Massachusetts project we modeled, switching from an HPWH to an electric resistance tank created a 6 to 8 HERS point penalty per unit. The exact number depends on unit size, hot water demand, and the HERS reference DHW system. In practice, assume you are giving up at least 6 points per unit by moving from an HPWH to electric resistance on a modern all-electric multifamily project.

Does the Specialized Stretch Code require heat pump water heaters?

Not explicitly. The Specialized Stretch Code is performance-based through HERS or ERI targets, and in theory any equipment mix that hits the target is compliant. In practice, the efficiency gap between HPWHs and electric resistance is large enough that most multifamily projects cannot meet the target without HPWHs, unless they use central gas DHW or pursue Passive House.

Is it cheaper to install heat pump water heaters or central gas DHW in large multifamily buildings?

It depends on unit count, mechanical room space, gas service availability, and the project's electrification commitments. Central gas DHW can be cost-competitive above roughly 100 units, but stretch code path selection and local gas policy often close that door before the pure cost comparison matters. Model both against your actual compliance path before the design is locked in.

When Heat Pump Water Heaters Are Not Optional

On a 296-unit Massachusetts project, we ran 25+ model variations testing whether envelope and HVAC upgrades could replace heat pump water heaters. None of them cleared code. Here is why the DHW gap is not recoverable through insulation.

April 22, 2026

The default assumption on a high-performance multifamily project is that envelope and HVAC upgrades can always compensate for a less efficient piece of equipment. On a 296-unit Massachusetts project, we ran 25+ variations testing that exact trade against a water heater swap, and none of them cleared code. If you are building multifamily in Massachusetts under the Specialized Stretch Code, heat pump water heaters are often not a choice you get to make.

The Real Question

A 296-unit developer came to us with a straightforward ask. Could they skip heat pump water heaters (HPWHs), install electric resistance tanks instead, and make up the performance loss with envelope or HVAC upgrades? We modeled the scenario 25+ different ways to find out.

The variations included triple-pane windows, continuous exterior insulation, premium variable-speed heat pump HVAC, tighter air sealing targets, and stacked combinations of all of the above. Each scenario was evaluated against the HERS target the project needed to hit for compliance.

None of them passed.

The HERS Penalty

Switching from HPWHs to electric resistance tanks created a 6 to 8 HERS point penalty per unit. In compliance terms, that is a large number. A single HERS point can decide whether a tight project passes or fails.

Six to eight points is not something you recover through spec upgrades. It requires structural changes to how the building performs as a whole, not tweaks to windows, insulation R-values, or HVAC SEER ratings.

On a 296-unit project, the penalty compounds across every dwelling. Each unit has to clear its target individually in most workflows, and a unit with typical geometry does not have 6 to 8 points of headroom sitting idle, waiting to be spent on a weaker DHW system.

Where Envelope Upgrades Fall Short

Domestic hot water carries significant weight in the HERS score because it is one of the largest end-uses in a modern electric multifamily unit. Heat pumps deliver hot water at a COP of roughly 3.0 or higher. Electric resistance tanks run at an effective COP of about 1.0. That is a 3x efficiency delta on a major load, and it lands directly in the energy index.

Envelope upgrades reduce heating and cooling loads, not DHW loads. You can insulate and glaze a building until the heating load approaches zero, and the HERS index still carries the full DHW penalty. The gap lives in a different part of the energy model than the part your envelope dollars can reach.

When Avoiding HPWHs Still Works

There are project types where the electric-resistance path still clears compliance. These are the exceptions, not the rule.

Small or prescriptive-path projects. Some smaller multifamily projects can qualify for a prescriptive compliance route where performance is not driven by HERS scoring the same way, and the DHW choice carries less weight in the approval.

Buildings with central gas DHW. A central gas DHW plant is a different conversation from in-unit electric tanks, and it is still allowed on some compliance paths depending on the project's electrification commitments and the local jurisdiction.

Projects targeting Passive House. Passive House uses its own energy metrics rather than the HERS index. The compliance math is different, though the underlying DHW efficiency question does not disappear, it just shows up in a different column.

Units with unusually favorable geometry. Small, well-shaded, interior-zoned units with minimal exterior exposure sometimes have enough headroom in their HERS target to absorb a weaker DHW. This is the exception we modeled 25+ times on the 296-unit project and did not find.

The Decision Framework

If you are building multifamily in Massachusetts and the compliance path uses HERS, start from the assumption that HPWHs are the baseline and design forward. Do not begin the project with a plan to engineer around them and then hope the model cooperates. The scenarios where avoidance works are narrow enough that modeling confirms them, not assumption.

If avoiding HPWHs is a hard requirement for the project, the real conversation happens at the compliance path level, not the equipment level. Ask whether the project should pursue a different path or a different water system architecture, not whether another inch of insulation can close a 6 to 8 point gap.