Specialty Terminal Units

Specialty hydronic terminal units provide space heating in architecturally constrained applications where conventional baseboard, radiators, or convectors cannot be installed due to dimensional limitations, aesthetic requirements, or functional constraints. These compact forced-convection or natural-convection devices integrate into building elements (under cabinets, in floor trenches, behind architectural features) while maintaining adequate heat output (3,000-15,000 Btu/h per unit) through enhanced fin surface, optimized airflow patterns, or strategic placement to address local heat loss effectively.

Kickspace Heaters (Under-Cabinet)

Design and Construction

Configuration:

Compact fan-forced heater: 3-6 in high, 9-15 in deep, 10-30 in wide
Finned-tube hot water coil with tangential or centrifugal blower
Toe-space installation: Beneath kitchen cabinets, vanities, built-ins
Front grille discharge: Horizontal air delivery into room

Heat exchanger:

Copper tube, aluminum fins
Fin density: 10-14 fins per inch
1-2 coil rows deep
Compact coil face area (60-200 in²)

Fan:

Tangential (cross-flow) blower most common
Low profile to fit 3-4 in height
Airflow: 80-200 CFM
Quiet operation (critical for residential applications)

Enclosure:

Sheet metal cabinet with mounting flanges
Removable front grille
Integrated thermostat or remote sensor
Optional built-in control valve

Performance Characteristics

Heat output:

Small (12-18 in wide): 3,000-6,000 Btu/h @ 180°F water
Medium (18-24 in wide): 6,000-9,000 Btu/h
Large (24-30 in wide): 9,000-12,000 Btu/h

Water temperature dependency:

$$Q \propto (T_w - T_a)$$

Approximately linear for forced convection over typical range.

Water Temp	Entering Air 65°F	Output (% of 180°F rating)
200°F	65°F	115%
180°F	65°F	100% (rated)
160°F	65°F	85%
140°F	65°F	70%

Airflow pattern:

Horizontal discharge parallel to floor
Throw: 6-12 ft depending on fan speed
Spreads along floor, rises via natural convection
Effective for localized heat delivery

Installation and Application

Mounting:

Rough-in during cabinet installation (new construction) or retrofit
Support from floor or wall mounting brackets
Align with toe-kick recess (typically 3-4 in high, 3 in deep)
Electrical and plumbing concealed behind cabinet

Piping connections:

1/2 in supply and return (typical)
Isolation valves accessible for service
Air vent at coil high point
Flexible connectors accommodate cabinet removal

Electrical:

120V or 24V fan motor
Integral thermostat or remote wall sensor
Line voltage or low voltage control

Applications:

Kitchens: Primary or supplemental heat where wall space occupied by cabinets
Bathrooms: Vanity toe-kick heating, fast warm-up
Entryways: Built-in benches or cabinetry with integrated heating
Offices: Under desk/credenza heating without visible terminals

Advantages:

Invisible (no wall space consumption)
Fast response (forced convection, low thermal mass)
Effective local heating (at floor level where people stand)

Limitations:

Requires cabinet installation (not suitable for open walls)
Fan noise (though quieter models available)
Filter maintenance access (remove grille)
Electrical power required

Trench Heaters (In-Floor Perimeter)

Design and Construction

Configuration:

Linear finned-tube convector recessed in floor trench
Installed along perimeter (typically beneath curtain walls, glazing)
Grated top cover: Flush with finished floor
Natural convection or fan-assisted

Trench construction:

Depth: 6-12 in below finished floor
Width: 4-10 in
Length: Continuous runs 10-50+ ft
Waterproof liner (prevent groundwater intrusion)
Insulation beneath and sides (minimize heat loss to ground)

Heat exchanger element:

Copper tube, aluminum fins
Fin density: 8-12 fpi (natural convection), 10-14 fpi (fan-assisted)
1-3 coil rows depending on output requirements

Cover grating:

Aluminum, steel, or bronze finish
Slot or perforated pattern (40-60% free area)
Removable sections for maintenance access
Load rating: Adequate for foot traffic (light-duty) or vehicle traffic (heavy-duty)

Performance Characteristics

Heat output (natural convection):

Low output: 150-300 Btu/h per linear foot @ 180°F water
Medium output: 300-500 Btu/h per linear foot
High output: 500-800 Btu/h per linear foot

Heat output (fan-assisted):

Medium output: 500-800 Btu/h per linear foot
High output: 800-1,200 Btu/h per linear foot
Very high output: 1,200-2,000 Btu/h per linear foot

Fan-assisted units achieve 1.5-3× output of natural convection in same physical size.

Temperature correction:

$$Q_{actual} = Q_{rated} \left(\frac{T_{w,actual} - T_a}{T_{w,rated} - T_a}\right)^{1.3}$$

Natural convection follows similar correction as baseboard/radiators.

Installation and Application

Installation procedure:

Form or install trench liner during floor construction
Install insulation (prevent downward heat loss)
Install element with piping connections
Test for leaks before closing floor
Install grating after floor finish complete

Piping:

Supply and return connections at trench ends or intervals
Isolation valves accessible in adjacent spaces
Air vents at high points (typically at connections)
Pitch for drainage if required

Applications:

Curtain wall perimeters: Counteract cold downdraft from glazing
Atriums and lobbies: Invisible heating for architectural spaces
Swimming pool decks: Perimeter heating, condensation control
Retail storefronts: Floor-level heating without obstructing displays

Advantages:

Completely invisible (floor-flush installation)
Excellent coverage along perimeter
No wall space required
Effective at addressing curtain wall heat loss

Limitations:

Expensive installation (floor integration)
Limited to new construction or major renovation
Maintenance access requires grating removal
Potential for moisture intrusion if not properly sealed

Toe-Kick Heaters (Low-Profile)

Design Variants

Toe-kick heaters encompass both kickspace heaters (fan-forced, under-cabinet) and low-profile natural convection units installed at floor level.

Low-profile natural convection toe-kick:

Height: 2-4 in
Finned-tube element with front grille
No fan (natural convection only)
Quiet operation, no electrical
Lower output than fan-forced: 200-500 Btu/h per linear foot

Installation:

Along wall base (replace baseboard)
Beneath low sills (window seats)
Recessed in wall cavity at floor level

Output comparison:

Unit Type	Output per Linear Foot	Fan Required	Electrical
Natural convection toe-kick	200-500 Btu/h	No	No
Fan-forced kickspace (per unit)	3,000-12,000 Btu/h	Yes	Yes
Standard baseboard	400-800 Btu/h	No	No

Applications

Natural convection toe-kick:

Residential bathrooms (supplement)
Low-sill applications (8-12 in sill height)
Quiet environments (bedrooms)
Locations without electrical access

Fan-forced kickspace:

Higher output requirements
Fast warm-up needed
Acceptable noise levels (kitchens, baths)
Electrical available

Valance Heating Units

Design and Construction

Configuration:

Linear convector mounted in architectural valance
Concealed behind fascia, grille, or architectural feature
Horizontal or angled installation above windows, along ceilings
Natural convection airflow pattern

Element construction:

Finned-tube similar to baseboard or convector
Exposed on room side (radiant and convective output)
Concealed by architectural element
Continuous runs: 6-40 ft typical

Valance integration:

Wood, metal, or gypsum board fascia
Air inlet: Gap at bottom or rear of valance
Air outlet: Top of valance or through fascia perforations
Minimum clearance to element: 2-4 in (adequate airflow)

Performance and Output

Heat output:

Similar to baseboard: 400-800 Btu/h per linear foot @ 180°F
Enhanced output with optimized valance design (engineered airflow path)
Reduced output if valance restricts airflow (20-40% penalty possible)

Optimization:

Adequate inlet and outlet free area (minimize restriction)
Vertical clearance for chimney effect (6-12 in valance depth)
Reflective backing (enhance radiant component toward room)

Installation and Application

Installation:

Support element from wall or ceiling structure
Conceal piping within valance or ceiling space
Coordinate with architectural/interior design

Applications:

Perimeter window heating: Valance above windows integrates heating with daylighting control (curtains, shades)
Lobbies and public spaces: Architectural heating without visible terminals
Historical buildings: Concealed modern heating in preserved interiors
Commercial offices: Perimeter heating integrated with window treatments

Advantages:

Aesthetically integrated (architecturally designed)
Effective perimeter heating location
No floor or wall space consumption
Radiant and convective output

Limitations:

Custom design/fabrication (higher cost)
Output reduced if valance poorly designed (airflow restriction)
Service access requires valance removal or access panels
Not suitable for retrofit without major interior work

Specialty Linear and Panel Units

Slim-Profile Panel Convectors

Description:

Ultra-thin panel convectors: 1-2 in deep
Surface-mounted or recessed
Fan-forced or natural convection
Applications: Tight spaces, modern minimalist aesthetics

Output:

30-60 Btu/h per square foot of panel area (natural convection)
60-120 Btu/h per square foot (fan-forced)

Under-Window Convectors

Description:

Compact cabinet convectors specifically designed for under-window installation
Height: 6-10 in (fit below typical sill)
Length: 24-72 in
Natural convection or low-speed fan

Output:

800-2,000 Btu/h per linear foot
Optimized for window heat loss offset

Radiant Ceiling Panels (Hydronic)

Description:

Metal ceiling panels with integrated hydronic tubing
Suspended or recessed in ceiling grid
Primary radiant heat transfer (50-70%)
Low-temperature operation (100-140°F)

Output:

15-40 Btu/h per square foot of panel area
Requires large surface area due to low-temperature operation
Applications: Office buildings, schools, healthcare

Sizing and Selection Considerations

Load Matching

Determine heat loss:

Calculate local heat loss where specialty unit installed
Consider specific loss mechanisms (window transmission, infiltration at doors)

Select unit type:

Based on installation constraints (space, aesthetics, noise tolerance)
Match output capability to load

Verify capacity:

$$Q_{required} \leq Q_{unit} \times CF_{temp} \times F_{safety}$$

Where $F_{safety}$ = 1.1-1.15.

Example Selection

Kitchen perimeter wall:

Heat loss: 4,500 Btu/h
Cabinet run: 10 ft with 3 cabinet sections
Water temperature: 160°F average

Options:

Three kickspace heaters (one per cabinet): 3 × 5,000 Btu/h @ 180°F = 15,000 Btu/h rated
- @ 160°F: 15,000 × 0.85 = 12,750 Btu/h ✓ (adequate, though oversized)
- Advantage: Distributed heating, fast response
- Disadvantage: Three fan units (cost, maintenance, noise)
Trench heater: 10 ft × 500 Btu/h·ft @ 180°F = 5,000 Btu/h rated
- @ 160°F: 5,000 × 0.75 = 3,750 Btu/h (inadequate)
- Would require high-output trench (800 Btu/h·ft rated)
- Advantage: Invisible, quiet (natural convection)
- Disadvantage: Expensive installation, floor integration
Two kickspace heaters: 2 × 6,000 Btu/h @ 180°F = 12,000 Btu/h
- @ 160°F: 12,000 × 0.85 = 10,200 Btu/h ✓
- Compromise: Adequate output, fewer units, lower cost than option 1

Control and Integration

Individual Unit Control

Kickspace heaters:

Integral thermostat (mechanical or electronic)
Remote wall sensor with relay
Line voltage or low voltage control
Fan cycles with heating demand

Trench heaters:

Zone valve control (multiple trenches on single valve)
Thermostatic valve for individual control
Fan control (if fan-assisted): Thermostat or manual switch

System Integration

Series or parallel piping:

Small units: Series acceptable (limited water volume)
Multiple units: Parallel preferred (independent control, balanced flow)

Mixed system:

Specialty units on same zones as conventional terminals
Size piping for combined flow
Balance for adequate flow to all terminals

Maintenance Considerations

Access:

Ensure removable grilles, panels, or covers for service
Plan access during design phase
Document hidden unit locations

Filter maintenance (fan units):

Monthly inspection during heavy use
Replace or clean per schedule
Restricted airflow reduces output 20-40%

Coil cleaning:

Annual vacuuming of fins
Chemical cleaning if heavily soiled
Access via grille or panel removal

Piping:

Verify isolation valves accessible
Bleed air from coils annually
Drain if freeze risk

Specialty hydronic terminal units address heating requirements in architecturally constrained applications through compact forced-convection and natural-convection designs integrated into building elements. Kickspace heaters, trench heaters, toe-kick units, and valance heaters provide effective space heating where conventional terminals cannot be installed, with proper selection matching output capability to local heat loss while respecting dimensional, aesthetic, and functional constraints.