Why Bigger Water Heaters Cost You Twice
Defaulting to 80-gallon HPWHs across every unit type is one of the most expensive multifamily spec mistakes we see in Massachusetts. Right-sizing to unit occupancy cuts equipment cost and improves your HERS score at the same time.
The default assumption on multifamily HPWH specs is that bigger is safer. More gallons, more hot water, fewer complaints. We have modeled enough projects in Massachusetts to know that on a per-unit basis, the oversized 80-gallon default is one of the most expensive mistakes a multifamily team can lock into a spec sheet.
The Real Project Numbers
On a 170-unit development in Worcester, every unit was originally spec'd with an 80-gallon heat pump water heater. Studios, one-bedrooms, two-bedrooms, the same tank in every closet. The logic was simple, and it was wrong on two counts at once.
When we ran the energy model against the Specialized Stretch Code, the oversized tanks were dragging the HERS score in the wrong direction. After we right-sized the HPWHs to actual unit occupancy, the energy performance improved and the per-unit equipment cost dropped at the same time. Across 170 units, the equipment line item alone was a six-figure swing.
This is not a one-off. We see the same default on most multifamily projects that come through our intake.
The HERS Penalty Nobody Flags
A heat pump water heater runs at a higher COP than electric resistance, but standby losses still count against you. An 80-gallon tank holds 60 percent more water at temperature than a 50-gallon tank, and the HERS rater software sees every BTU of that standby loss. In a studio with one or two occupants and minimal draw, you are heating water that nobody is going to use that day.
The penalty compounds in all-electric multifamily, where domestic hot water is already a meaningful share of the modeled energy load. Trim 30 gallons of standby loss across every unit and the building-wide HERS Index moves enough to matter for Mass Save tier qualification.
The Per-Unit Equipment Cost
A 50-gallon HPWH typically lands several hundred dollars cheaper than an 80-gallon model, before you count the tighter mechanical closet footprint and the lighter unit weight on upper floors. On a 170-unit job, that delta scales fast. On a 325-unit job, faster.
The teams who default to 80 gallons are usually doing it to avoid a callback. The data says the callback risk on a properly sized 50-gallon unit in a studio or one-bedroom is effectively zero.
When 80 Gallons Still Wins
Right-sizing is unit-by-unit. There are real scenarios where the larger tank is the correct call.
Three-bedroom and four-bedroom units. Family occupancy with simultaneous morning showers is where 80 gallons earns its spec. Modeling a 50-gallon tank into a four-bedroom is the mirror image of the mistake we are describing.
Units with oversized tubs or multi-head showers. Premium fixture packages change the peak draw math. If the unit has a 75-gallon soaking tub, the recovery rate of a 50-gallon HPWH will not keep up.
Buildings with no central recirculation and long branch runs. When the tank is doing double duty as a buffer for a long hot water line, the extra capacity can be the right trade against scald-protection complaints.
The Decision Framework
Match tank size to unit occupancy and fixture load, not to a single building-wide default. For most studios and one-bedrooms in standard multifamily, 50 gallons is the right answer. For two-bedrooms with two bathrooms, 65 to 80 depending on fixtures. Run the numbers in the energy model before the spec sheet gets locked, because once the procurement order is placed, the HERS penalty is baked in.