Apartments and Multi-Dwellings
When designing a heating / hot water system for an apartment or domitory, providing an adequate supply of hot water is the utmost importance. To prevent the potential of dissatisfied users, the hot water system should be properly sized to provide hot water in adequate supply. Just as an undersized system creates potential problems, an oversized system can create problems such as being an uneconomical system through an increase in standby losses.
Special design considerations:
When hot water is required quickly at the point of use, the contractor may consider using a recirculating loop. When using a recirculating loop, the contractor should insulate the water lines, especially in those conditions in which the recirculating loop is exposed to temperatures substantially lower than the recirculating water.
NOTE: The contractor or designer should understand that studies have shown that recirculating loops are not generally considered energy efficient.
When utilizing a recirculation loop system, the designer should note that the volume of storage will increase by the volume of stored water in the building's hot water mains.
The hot water supplied to the fixtures should not exceed 120ºF as a prevention for potential scalding, therefore the system should be design not to store domestic water above 120ºF.
For indirect water heater applications the designer should ensure that the space heating boiler is compatible with the needs of the hot water system. Oversizing the space heating boiler will increase the standby losses of the hot water system, thus decrease the overall efficiency of the heat/hot water system. This concept also applies to the storage tank, which should be compatible with the hot water demand.
The designer should consult with local building inspector and/or housing authorities on the need to meet specified requirements on recovery capacity and storage tank size. Some areas base the requirements on a per apartment unit or per person basis.
The designer should pay extra attention to those situations which the dwelling units contain either garden tubs or whirlpool tubs. In these applications the hot water system must be able to rapidly fill the tub while maintaining temperature.
General "Rules of Thumb" For Sizing
| Apartments Number of Units |
Max. Hour | Max. Day | Avg. Day |
| 20 unit or less | 12.0 gal/unit | 80.0 gal/unit | 42.0 gal/unit |
| 50 units | 10.0 gal/unit | 73.0 gal/unit | 40.0 gal/unit |
| 75 units | 8.5 gal/unit | 66.0 gal/unit | 38.0 gal/unit |
| 100 units | 7.0 gal/unit | 60.0 gal/unit | 37.0 gal/unit |
| 200 units or more | 5.0 gal/unit | 50.0 gal/unit | 35.0 gal/unit |
| Dormitories | Max. Hour | Max. Day | Avg. Day |
| Men's dormitory | 3.8 gal/student | 22.0 gal/student | 13.1 gal/student |
| Women's dormitory | 5.0 gal/student | 26.5 gal/student | 12.3 ga;/student |
Hot Water Demand per Fixture Detail Sizing
| Fixture | Gallons of Water per Hour per fixture (calculated at a final temperature of 140ºF) |
| Basins, private lavatory | 2 |
| Basins, public lavatory | 4 |
| Bathtubs | 20 |
| Diswasher | 15 |
| Kitchen sink | 10 |
| Laundry, stationary tubs | 20 |
| Pantry sink | 5 |
| Showers | 30 |
| Service Sink | 20 |
Method of Sizing
1. Determine the number of fixtures required for the application and calculate the total hot water demand for those fixture.
Number of Fixtures x Gallons of Water per Hour per Fixture = Possible Maximum Demand
2. Determine the Probable Hot Water Demand for the application.
Probable Hot Water Demand = Possible Maximum Demand x 0.30
3. Compare the Probable Hot Water Demand with the 1st Hour Performance Ratings of the Phase III Indirect Water or with the Heat Master Direct Fired Water Heaters.