Convectors

Convectors

Hydronic convectors transfer thermal energy from hot water or steam primarily through natural convection (70-80% of total heat transfer), utilizing finned-tube heat exchanger elements enclosed in cabinets or recessed housings that create controlled airflow paths for enhanced convective heat transfer. Unlike baseboard radiation which distributes heat output along extended linear runs, convectors concentrate heat delivery in compact terminal units with output densities of 1,000-3,000 Btu/h per linear foot, making them suitable for applications requiring high output in limited wall space such as lobbies, corridors, and perimeter zones beneath windows with restricted installation height.

Convector Construction and Types

Cabinet Convectors (Surface-Mounted)

Configuration:

• Sheet metal cabinet: 6-10 in deep, 8-12 in high, 24-96 in long typical
• Finned-tube element(s) inside cabinet
• Inlet grille: Bottom or lower front (cool air entry)
• Outlet grille: Top or upper front (warm air discharge)
• Damper (optional): Adjustable airflow control
• Removable front panel: Maintenance access

Element arrangement:

• Single element: 1 row of finned tube
• Double element: 2 rows stacked vertically
• Multiple elements: Up to 4 rows for high output applications

Natural convection flow path:

Air enters bottom, passes upward through fins, exits top:

$$\dot{m}{air} = C_d A{inlet} \sqrt{2gH(\rho_{cold} - \rho_{hot})}$$

Where:

• $C_d$ = Discharge coefficient (0.6-0.7)
• $A_{inlet}$ = Net free area of inlet
• $H$ = Vertical height between inlet and outlet
• Density difference drives flow

Greater cabinet height increases draft and airflow, enhancing output.

Recessed Convectors (Wall-Mounted)

Installation:

• Recessed into wall cavity between studs
• Flush or near-flush front grille
• Depth: 4-6 in (fits 2×4 or 2×6 wall)
• Length: 16-72 in (fits standard stud spacing)
• Minimal intrusion into room space

Advantages:

• No floor space consumption
• Protected from damage
• Aesthetic (minimal visual impact)
• Suitable for corridors, tight spaces

Disadvantages:

• Installation complexity (requires wall cavity)
• Limited to new construction or major renovation
• Restricted to locations with accessible wall cavities
• Service access requires grille removal

Output characteristics:

• Slightly lower than cabinet units (confined enclosure, reduced airflow)
• Typical: 800-2,000 Btu/h per linear foot

Free-Standing Convectors

Configuration:

• Stand-alone enclosure on legs or pedestal
• Four-sided cabinet (finished on all sides)
• 360° air intake (bottom perimeter)
• Top discharge grille

Applications:

• Open areas without suitable wall space
• Room dividers (heating while defining space)
• Temporary or relocatable heating
• Historical buildings (no wall penetration)

Performance:

• Higher output than wall-mounted (greater air intake area)
• Typical: 1,500-3,500 Btu/h per unit
• More expensive per Btu/h (four-sided enclosure)

Finned-Tube Element Design

Heat Exchanger Configuration

Standard element:

• Copper tube: 3/4 in, 1 in, or 1-1/4 in OD
• Aluminum fins: 0.006-0.012 in thickness
• Fin density: 40-60 fins per foot
• Bonding: Mechanical expansion (tube expanded into fin collar)
• Element length: Factory sections 2-8 ft, field-joined for longer runs

Heat transfer:

$$Q = \eta_o h_c A_{total} (T_w - T_a)$$

Where:

• $\eta_o$ = Overall fin efficiency (0.85-0.92)
• $h_c$ = Natural convection coefficient
• $A_{total}$ = Total surface area (tube + fins)
• $T_w$ = Water temperature
• $T_a$ = Air temperature

Fin Efficiency and Optimization

Fin effectiveness depends on thermal conductivity, geometry, and airflow:

$$\eta_f = \frac{\tanh(mL)}{mL}$$

Where:

$$m = \sqrt{\frac{2h_c}{k_f t_f}}$$

• $k_f$ = Aluminum conductivity (120 Btu/h·ft·°F)
• $t_f$ = Fin thickness
• $L$ = Fin height

Optimization trade-offs:

• More fins: Greater surface area, but lower individual efficiency and restricted airflow
• Fewer fins: Higher individual efficiency, but less total area
• Typical optimal: 45-50 fpi for natural convection applications

Enclosure impact on convection coefficient:

Confined enclosure with controlled air path increases $h_c$ by 20-40% compared to open baseboard:

• Baseboard (open): $h_c$ = 1.5-2.0 Btu/h·ft²·°F
• Convector (enclosed): $h_c$ = 2.0-2.8 Btu/h·ft²·°F
• Enhanced airflow velocity increases heat transfer

Output Ratings and Performance

Standard Rating Conditions

Hot water convectors:

• Average water temperature: 215°F (steam equivalent rating)
• Entering air temperature: 65°F
• Adequate water flow: ≤20°F temperature drop
• Unrestricted enclosure airflow
• Sea level altitude

Typical output ratings:

Temperature Correction Factors

Actual output adjusts for operating water temperature:

$$Q_{actual} = Q_{rated} \left(\frac{T_{w,avg} - T_a}{215 - 65}\right)^{1.3}$$

Correction factor table:

Example: 4 ft cabinet convector, double element, 2,000 Btu/h·ft rated

• Rated total output @ 215°F: 4 × 2,000 = 8,000 Btu/h
• Operating @ 180°F average water temperature
• Actual output = 8,000 × 0.61 = 4,880 Btu/h

Damper Control Effects

Adjustable outlet dampers modulate airflow and output:

Damper position vs. output:

Control characteristics:

• Non-linear response (especially at higher closures)
• Residual output even when “closed” (conduction through cabinet)
• Manual adjustment only (not suitable for automatic control)
• Potential for incorrect user settings compromising performance

Automatic control alternative:

Use thermostatic valve on water supply instead of damper for more precise, automatic control.

Convector Sizing Methodology

Design Procedure

Step 1: Calculate zone heat loss

$$Q_{loss} = \sum (UA\Delta T) + \dot{V}\rho c_p \Delta T_{infiltration}$$

Step 2: Determine water temperature

Based on boiler/system design:

• High temperature: 180-200°F average
• Medium temperature: 160-180°F
• Steam: 215°F (1-2 psig)

Step 3: Select convector type

Based on installation constraints:

• Cabinet: Standard application, ample wall/floor space
• Recessed: Limited projection acceptable, wall cavity available
• Free-standing: No suitable wall mounting location

Step 4: Determine output per foot

From manufacturer catalog at rated conditions, apply temperature correction:

$$Output_{actual} = Output_{rated} \times CF_{temp}$$

Step 5: Calculate required length

$$L_{required} = \frac{Q_{loss}}{Output_{actual} \text{ per foot}}$$

Step 6: Verify available space

Confirm required length fits available wall/floor space. If not, use higher-output configuration (more elements) or add multiple convectors.

Sizing Example

Perimeter zone:

• Heat loss: 18,000 Btu/h
• Available wall space: 12 ft beneath windows
• Water temperature: 180°F average (190°F supply, 170°F return)
• Air temperature: 65°F

Calculation:

1. CF_temp @ 180°F = 0.61
2. Select cabinet convector, double element: Rated 2,000 Btu/h·ft @ 215°F
3. Actual output = 2,000 × 0.61 = 1,220 Btu/h·ft
4. Required length = 18,000 / 1,220 = 14.8 ft
5. Problem: Exceeds 12 ft available space
6. Solution: Use triple element configuration
   • Rated: 2,800 Btu/h·ft
   • Actual: 2,800 × 0.61 = 1,708 Btu/h·ft
   • Required length = 18,000 / 1,708 = 10.5 ft ✓
7. Select: 10.5 ft (round to 11 ft for standard section lengths)

Installation and Application

Mounting and Location

Cabinet convectors:

• Wall-mounted with brackets or floor-supported
• Locate beneath windows (counteract downdraft)
• Along exterior walls (address transmission losses)
• Minimum 1 in clearance to wall (air circulation behind)
• 3-6 in above floor (inlet airflow)

Recessed convectors:

• Rough-in sleeve installed in wall framing
• Element and grille installed after wall finish
• Verify adequate wall depth (2×6 wall for deep units)
• Maintain clearances to insulation (fire safety)

Floor-to-enclosure clearance:

Critical for inlet airflow:

• Carpet: 2-3 in minimum clearance
• Hardwood/tile: 1-2 in acceptable
• Furniture/obstructions: Maintain 6 in clear in front

Piping Connections

Element connections:

• Supply and return at element ends
• 1/2 in or 3/4 in depending on capacity
• Isolation valves for service
• Air vent at high point (automatic or manual)

Steam applications:

• Steam trap at condensate outlet
• Pitch element toward trap (1/4 in per 10 ft)
• Vacuum breaker if vacuum return system

Hot water:

• Two-pipe: Supply one end, return other end
• Thermostatic valve for individual control
• Balancing valve on return

Piping routing:

• Concealed in wall or floor preferred
• Through enclosure end panels if surface-mounted
• Adequate pipe insulation (prevent heat loss upstream)

Institutional and Commercial Applications

Healthcare facilities:

• Recessed convectors avoid damage from carts, equipment
• Smooth grilles for cleaning/disinfection
• Tamper-resistant damper or no damper
• Copper-fin elements (antimicrobial properties)

Schools:

• Cabinet convectors with protective grilles
• Recessed units in corridors (prevent vandalism)
• Lockable or fixed dampers (prevent misadjustment)
• Adequate output for high ventilation rates

Offices:

• Cabinet convectors beneath perimeter windows
• Thermostat or TRV control for each zone
• Quiet operation (low air velocity)
• Professional appearance

Corridors:

• Recessed convectors (no protrusion)
• Distributed along length (match heat loss distribution)
• Fire-rated enclosures where required

Maintenance and Troubleshooting

Routine Maintenance

Annual:

• Remove grilles and vacuum fins (dust reduces output 15-25%)
• Check for air leaks in enclosure (bypass reduces performance)
• Bleed air from elements (hot water systems)
• Verify damper operation
• Check for corrosion on enclosure

Multi-year:

• Repaint enclosure if surface deteriorating
• Inspect element for corrosion
• Water treatment analysis (hot water)
• Verify adequate airflow (no furniture blockage)

Common Problems

Inadequate output:

• Dust/dirt accumulation on fins (clean)
• Damper partially closed (open fully or adjust)
• Air trapped in element (bleed air)
• Low water temperature (check boiler, mixing valve)
• Insufficient flow (balancing, pump capacity)

Uneven heating:

• Air pockets in element (bleed thoroughly)
• Flow imbalance between convectors (adjust balancing valves)
• Partially obstructed airflow (remove furniture, verify clearances)

Noise:

• Water velocity >4 ft/s (reduce pump speed, open valves)
• Steam trap malfunction (inspect and repair/replace)
• Thermal expansion movement (allow for expansion, secure mounting)

Enclosure damage:

• Dents, scratches (cosmetic repair or replace panel)
• Rust/corrosion (repaint, improve ventilation)
• Grille damage (replace grille assembly)

Hydronic convectors provide compact, high-output heating through enhanced natural convection in enclosed finned-tube elements, with cabinet, recessed, and free-standing configurations suited to diverse application requirements. Proper sizing integrates room heat loss with water temperature and enclosure type, while effective installation ensures adequate airflow clearances and piping connections for reliable operation in institutional, commercial, and residential applications.