How to Size a Commercial Water Heater for an Apartment Building

Commercial water heater sizing for an apartment building comes down to three questions: how much hot water residents use at the busiest time, how much the plant stores versus how fast it reheats, and how much heat the recirculation loop loses.

This guide explains each input. It gives no sizing numbers: those come from a design calculation for your building, which the assessment provides.

Commercial water heater sizing methods

Designers use published demand methods rather than rules of thumb. The ASHRAE Handbook (HVAC Applications, Service Water Heating chapter) is the common reference for gas plants. Central heat pump systems have their own approaches. One example is the Ecosizer, a free educational tool built by Ecotope, with funding from SCE and SMUD, for sizing central heat pump water heater systems in multifamily buildings.

Where possible, measured data beats any method. A few weeks of flow and temperature logging shows how the building actually uses hot water.

Unit count and occupancy

Unit count is a start, not the answer. Designers also look at:

• Bedrooms per unit and expected occupants
• Resident type: family, senior, student or supportive housing use hot water differently
• Fixtures: low-flow showerheads and faucets, in-unit or shared laundry
• When peaks happen, and how long they last

Storage vs recovery

A plant can meet a peak by storing a lot of hot water, by reheating quickly, or by a mix. More storage means smaller heaters but more space and tank cost. Faster recovery means larger heaters and gas or electrical capacity, but less storage.

Gas plants often lean on recovery. Heat pump plants usually lean on storage.

Recirculation losses

The recirculation loop keeps hot water close to every apartment, and it loses heat all day. That loss is part of the load. Pipe length, insulation, return temperature and pump control all change it.

Fixing a poorly insulated or badly balanced loop can reduce the plant size needed, so the survey checks it.

How heat pump sizing differs

• Heat pumps reheat more slowly than gas, so storage carries more of the peak.
• Recirculation losses are often handled by a separate swing tank, to keep the main storage stratified.
• Output changes with outdoor temperature, so the design condition matters.
• Electric backup, if any, has to be included in the electrical calculation.

See central heat pump water heaters for equipment and siting.

Worked example: what gets collected for a 60-unit building

This shows the process, not the result. The numbers depend on the building.

1. Building profile. 60 apartments: the bedroom mix, resident type and laundry arrangement are recorded.
2. Existing plant. Nameplates for heaters and tanks, plus complaint history: when residents run short, and for how long.
3. Peak demand. Calculated with a published method, or from logged data if available.
4. Recirculation. Loop length, insulation and return temperature give the standby load.
5. Plant options. Storage and heater capacity are balanced for each option (gas or heat pump), within the available space and utility capacity.
6. Check. The result is compared against the old plant and the complaint history before equipment is selected.

When to hire an engineer

Bring in a mechanical engineer or design-build contractor when the plant serves many units, when you are converting fuel, when the electrical service may change, or when the permit requires stamped drawings. For a like-for-like swap in a small building, a contractor's sizing check may be enough.

Request a free assessment