The use of a gas-fired or oil-fired space heating boiler system, otherwise in accordance with Section L503.0, to provide the total space heating and service water heating for a building is allowed where one of the following conditions is met:
- The single space-heating boiler, or the component
of a modular or multiple boiler system that is heating
the service water, has a standby loss in Btu/h
(kW) not exceeding (13.3 x pmd + 400)/n, where
(pmd) is the probable maximum demand in gallons
per hour, determined in accordance with the procedures
described in generally accepted engineering
standards and handbooks, and (n) is the fraction of
the year where the outdoor daily mean temperature
exceeds 64.9°F (18.28°C).
The standby loss is to be determined for a test period of 24 hours duration while maintaining a boiler water temperature of not less than 90°F (32°C) above ambient, with an ambient temperature between 60°F (16°C) and 90°F (32°C). For a boiler with a modulating burner, this test shall be conducted at the lowest input.
- It is demonstrated to the satisfaction of the Authority Having Jurisdiction that the use of a single heat source will consume less energy than separate units.
- The energy input of the combined boiler and water heater system is less than 150 000 British thermal units per hour (Btu/h) (44 kW). [ASHRAE 90.1:7.5.1]