NOTE This standard covers factory-assembled terminal heating units that deliver heat to a single zone or localized area using a hydronic coil, a steam coil, or electric resistance elements. Terminal heating units are the workhorses of perimeter and spot heating: they hang in a stairwell, tuck under a storefront, or blow across a loading dock door, each serving its own small load rather than feeding a distribution system. The unit types in scope are propeller unit heaters, blower (cabinet) unit heaters, cabinet unit heaters, fin-tube radiation, convectors, electric baseboard heaters, and electric force-flow heaters, each pairing a heat source (hot water, steam, or electric resistance) with a means of air movement (forced by a fan or natural convection). Units in this standard serve a single thermostatic zone and are controlled by a zone thermostat, an integral aquastat, or a signal from the building automation system. (1.1)

NOTE This standard does not cover the following, which are specified elsewhere: (1.4)

NOTE The following terms are used in this standard: (2.1)

NOTE The following standards are referenced in this document. The edition in force on the project's date of issue for permit governs unless a specific edition is cited. (3.1)

NOTE There is no single AHRI certification program that exclusively rates hydronic propeller and blower unit heaters; capacity for these units is published by the manufacturer on a Hydronics Institute or proprietary test basis, and the rating basis shall be stated on the submittal. (3.2)

NOTE Capacity ratings shall be submitted at the scheduled entering and leaving water temperatures, not at a generic catalog condition, so that the reviewer can confirm the unit performs at the actual system supply temperature. (4.2)

NOTE The manufacturer shall have produced terminal heating units of the specified type for not less than five years. (5.5)

NOTE A listing label demonstrates that the unit's electrical construction, element guarding, and over-temperature protection were evaluated by an independent laboratory; an unlisted electric heater is not acceptable for a code-compliant installation. (5.6)

NOTE The single most common capacity error is rating a hydronic unit at 180 °F EWT when the boiler plant is a condensing system delivering 120 °F to 140 °F; capacity falls sharply at low EWT, and a unit selected at the catalog condition will be undersized. (5.8)

NOTE The heat source determines coil material, working pressure, control type, and the energy code requirements that apply; it is the first selection to make. (6.1)

NOTE The default entering water temperature of 140 °F reflects the prevalence of condensing boiler plants in current practice; specify a higher EWT only where the unit is served by a non-condensing plant confirmed to deliver that temperature. (6.4)

NOTE The unit type matches the air-movement method to the space: propeller and blower units for high-bay and industrial spaces, cabinet units and convectors for occupied rooms, fin-tube and baseboard for perimeter heating. (7.1)

NOTE Match the unit type to the space as follows: (7.3)

NOTE The fan motor type sets both the noise of the unit and its compliance with the energy code's part-load fan-control requirements; it is not a free choice on fan-powered units. (8.1)

NOTE A PSC motor cannot follow a modulating fan-speed signal; installing one on a BAS-controlled unit that the energy code expects to modulate defeats the compliance path and wastes fan energy at part load. ECM motors also enable continuous low-speed operation for freeze protection and quiet night setback, which a single-speed PSC motor cannot provide. (8.4)

NOTE Propeller unit heaters commonly produce NC 45 to NC 55; specifying a propeller unit in an open office or a vestibule adjacent to a conference room without an NC limit invites occupant complaints, and a blower or cabinet unit should be selected instead. (8.6)

NOTE A standard steam coil floods and freezes when cold entering air condenses steam unevenly across the tubes; a steam-distributing coil delivers steam along the full tube length and keeps the entire coil warm. (9.5)

NOTE The element thermal cutout protects against fire if airflow is blocked or the fan fails; it is a code-required safety device, not an optional accessory. (9.7)

NOTE The enclosure protects the working parts, sets the unit's appearance in occupied space, and determines how well the unit survives a corrosive environment; it is selected for the installed location, not by default. (10.1)

NOTE A standard baked-enamel steel cabinet corrodes within a season in a wash-down or chlorinated environment; the corrosion-resistant enclosure and coil coating must be specified explicitly because it is not the catalog default. (10.4)

NOTE Architectural enclosures shall align with adjacent wall finishes and reveals where the unit is exposed in a finished space. (10.6)

NOTE A voltage mismatch — an electric heater ordered for 240 V on a 277 V lighting panel, or the reverse — is among the most common field problems on this equipment and produces a non-functional unit and a change order; the voltage must be coordinated with the serving panel before ordering. (11.2)

NOTE The control strategy ties the unit to its zone and to the energy code's thermostat requirements; a simple line-voltage thermostat is no longer universally acceptable. (12.1)

NOTE A line-voltage single-setpoint thermostat cannot provide the scheduled setback and programmability that current energy-code editions require; in jurisdictions enforcing ASHRAE 90.1-2019 or later it fails the control requirement and a programmable or BAS-integrated thermostat is required instead. (12.4)

NOTE A fan that runs while the control valve is closed blows unheated air into the space, which feels like a draft and wastes fan energy; the end-switch interlock prevents cold-blow by holding the fan off until the valve opens. (12.7)

NOTE Modulating units served by a variable-speed plant shall be provided with a 0-10 V modulating valve actuator compatible with the BAS, as coordinated with Hvac Variable Frequency Drives where applicable. (12.8)

NOTE Any hydronic or steam unit serving a vestibule, entry, loading dock, or other space exposed to freezing air shall be provided with a freeze-protection strategy; omitting it is a leading cause of coil failure. (13.1)

NOTE Specifying a vestibule or loading dock unit with no aquastat, no heat trace, and no continuous-flow provision leaves the coil to freeze the first time the entry door is propped open in cold weather; the freeze-protection method must be an explicit part of the specification, not an afterthought. (13.4)

NOTE Steam terminal units carry their own hazards — pressure class, condensate removal, and trap selection — that do not apply to hydronic units and must be addressed in the specification. (14.1)

NOTE Leaving trap selection entirely to the field without defining the condensate load, back-pressure, and trap type produces chronic trap failures and waterlogged coils; the trap basis shall be defined in the specification. (14.5)

NOTE Cabinet units installed too close to a wall or recessed into a tight pocket cannot be serviced; the access clearances must be coordinated with the architecture before rough-in, because a unit that cannot be opened cannot have its filter or motor changed. (15.3)

NOTE Each hydronic unit shall be provided with a means of isolation, balancing, and air venting at the coil so the coil can be served and balanced independently. (15.7)

NOTE The freeze-protection aquastat or low-limit shall be tested by simulating a low coil temperature and confirming the fan de-energizes. (16.3)

NOTE Test results shall be recorded and submitted in the field quality-control report. (16.5)

NOTE Units used for temporary construction heating shall have their coils, filters, and motors inspected and filters replaced before final acceptance, and shall not be accepted with construction debris in the coil. (17.3)

NOTE Replacement filters shall match the size and rating recorded in the closeout filter schedule so the owner can reorder without re-measuring. (19.3)