27.4 Defrost Components, Starters, Solenoids & Limit Switches

27.4 Defrost Components, Starters, Solenoids & Limit Switches

The last numbered controls cluster on Trade D is the hardware that starts, protects, defrosts, and proves the machine: defrost timers, heaters, and thermostats; air-pressure switches; electric humidistats (operating RH — Section 27.2); electronic air cleaners; fan control switches; hermetic overload protectors; limit switches; solenoid coils; magnetic starters; starting relays; duty-motor protection; and motor controllers. Chapter 20.2 already used heat-pump defrost as an energy-code strip exception. Chapter 22 used freezer defrost as a refrigeration install. This section is putting the parts in so the exam’s component list is not a pile of catalog names.

Quick Answer: Defrost is a timer or demand board, a heater (electric or hot gas), and a termination thermostat plus fail-safe. Magnetic starters = contactor + overloads (all three legs on three-phase). Solenoid coils match voltage and can often be changed without cutting pipe. Limits and hermetic overloads are safeties — do not jumper. Electronic air cleaners need a door interlock. Class B still stops at 25 tons / 500,000 Btu.

Defrost timers, heaters, thermostats, and air-pressure switches

When an outdoor heat-pump coil or a freezer evaporator is the evaporator, moisture freezes on it. Time-temperature defrost runs a clock plus a coil sensor: after a set run time, if the coil is cold enough, the control shifts into defrost. Demand defrost looks at coil temperature versus outdoor air, or at pressure / delta-T, and defrosts only when the coil is actually frosted. During heat-pump defrost the reversing valve moves toward cooling, the outdoor fan often stops, and auxiliary heat strips usually come on to temper the supply air. That strip operation during defrost is allowed by FBC Energy R403.1.3. Running strips whenever the owner bumps the setpoint 2°F on a 65°F Tampa afternoon is not.

Commercial freezer / cooler defrost (walk-ins from Chapter 22) uses electric heaters in the coil, hot-gas solenoids, or off-cycle (cooler, not freezer) defrost. The defrost thermostat (termination) opens the heater when the coil is clear — commonly in a 50–70°F coil-temperature band on electric defrost (use the listing). A fail-safe time on the clock ends defrost even if the thermostat never opens, so a heater cannot bake a box all weekend. Drain heaters keep the condensate pan from re-freezing. Wire the termination stat in series with the heaters, not around them.

Air-pressure switches prove fan flow before electric heaters or a burner may run, or they open on a dirty-filter pressure drop. A heat-pump or furnace that runs strips with the blower dead is a fire. Some defrost boards also look at a pressure switch as a frost proxy. These are air bellows or diaphragms, not refrigerant HP/LP.

Fan control switches, electronic air cleaners, electric humidistats

A fan control on a warm-air furnace is often combined with the high-limit in one fan-limit control. Typical mechanical numbers, always overridden by the listing, bring the blower on around 125–135°F plenum temperature and off around 90–100°F so the heat exchanger dumps before the fan stops. Electronic fan controls use a timer or a board. G on the thermostat still commands the blower in cooling and in ‘fan on.’ A sail switch or pressure-proving switch is the commercial cousin. Do not set a fan-on so high that the high-limit trips first; that is a limit used as a fan switch.

Electronic air cleaners (EACs) ionize particles on a high-voltage grid (often on the order of thousands of volts inside the cell) and collect them on plates. Install upstream of the coil per the listing, leave service clearance, and land the door interlock so the high-voltage supply dies when the access door opens. The 24-volt or 120-volt power pack is still HVAC control / dedicated HVAC circuit work inside 489.105. An EAC does not replace a listed filter, does not waive FBC Mechanical filtration, and does not make a Class B contractor into an electrician on a new lighting homerun. Wash cells per the IOM; wet cells with the power on are a shock and a warped-plate complaint.

Electric humidistats in this bullet list are the same RH switches as Section 27.2, often staged with reheat or an EAC/IAQ package. They remain operating controls.

Hermetic overload protectors, limit switches, solenoid coils

A hermetic overload protector lives inside the compressor (buried in the windings) or on the shell (a Klixon-style disc). Line-break overloads open the run circuit; pilot-duty overloads open the contactor coil only. Internal overloads can take minutes to hours to reset after a high-pressure or loss-of-charge event — ‘the compressor is open’ on an ohm check between common and run/start, then closed after it cools, is a classic exam read. External overloads must sit where the listing put them, with the correct current stamp. They protect the motor, not the refrigeration circuit; you still need HP/LP and oil failure on machines that use those safeties.

Limit switches are temperature or position safeties: furnace high-limit, rollout, auxiliary limit in the blower, heat-strip high-limit / firestat (Section 20.2), door interlocks, and end-of-travel limits on dampers. Automatic-reset primary limits recycle; manual-reset secondaries (or fusible links) require a person. Never jumper a limit ‘until the part comes in.’ That is how attics burn and how CILB complaints start.

A solenoid coil is the electromagnetic half of a solenoid valve: liquid-line pumpdown, hot-gas defrost, reheat, reversing-valve pilot, humidifier water. Match the voltage stamped on the coil (24, 120, or 240 V are the usual three). Inrush is higher than holding; a undersized transformer (Section 27.3) will drop a 24-volt liquid-line solenoid every time the contactor pulls in. Many coils slide off the stem without cutting the valve out of the refrigerant circuit — replace the coil, not the valve, when the winding is open and the body still seats. A 24-volt coil on a 120-volt hot-gas valve is a puff of smoke, not a ‘stronger coil.’

Magnetic starters, starting relays, duty-motor protection, motor controllers

A magnetic starter is a contactor plus overload protection in one listed assembly, often with an HOA (hand-off-auto) and a holding circuit. Three-phase motors need overload sensing in each phase so a single-phasing event still trips. Overload heaters (melting-alloy or bimetal) are selected from the motor FLA and the starter table — not from breaker size, not from ‘the next size up so it stops nuisance tripping.’ Electronic overload modules use the same FLA dial. The starter coil is still downstream of the Section 27.1 safeties.

Starting relays (current and potential) from Section 27.3 belong here when the outline says starting relays as a component: they take the start winding out at speed. Hard-start kits (potential relay + start capacitor) on a hermetic that already has a run capacitor are a listed accessory, not a substitute for a failed dual run capacitor and not a substitute for a low-voltage problem.

Duty-motor protection is the match between motor duty (continuous, intermittent, air-over) and the protector. A continuous-duty condenser fan in a Florida sun needs a motor rated for that duty and ambient, plus overloads or an integral protector. An air-over motor that you box in without airflow will take out the protector and then the winding. Motor controllers are the listed means of starting and stopping: across-the-line (full voltage — the usual HVAC starter), part-winding, star-delta, soft start, or a VFD. VFDs on condenser fans and blowers are still HVAC equipment for control wiring; the line-side power stays inside the 489.105 fence. Class B candidates still need this theory on a 30-ton RTU starter even though Class B cannot install that 30-ton system.

Florida HVAC scenario

A certified Class B firm in Miami-Dade (HVHZ, Climate Zone 1A) services a walk-in freezer (legal as a refrigeration system under the 25-ton cap) and a 3-ton heat pump next door. On the freezer the helper wires a new defrost clock but parallels the termination thermostat so the heaters can ‘always finish.’ Fail-safe time is the only thing that will ever end defrost; the coil cooks, and the product thaws from the heater, not from a failed compressor. On the heat pump the helper jumpers the high-limit on a 5 kW strip kit because the fan-limit was set too high and the limit opened, then tapes the EAC door interlock so the cells can be ‘inspected while on.’ Four independent failures: defrost termination bypassed, limit jumpered, fan control mis-set, EAC interlock defeated. The same campus wants a 30-ton RTU with a magnetic starter and a VFD on the supply fan. Class B still has to size overload heaters from FLA and explain HOA and across-the-line starting on the exam; Class B does not contract that 30-ton system (F.S. 489.105(3)(g)). A new 460-volt feeder to that starter is still an electrical contractor.

Traps: (1) Jumping defrost termination, limits, or EAC door switches. (2) Wrong solenoid coil voltage. (3) Oversized starter heaters. (4) Treating a hot internal overload as a dead compressor. (5) Strips on every thermostat bump instead of during defrost per R403.1.3. (6) Class B installing a 30-ton starter as if the controller size were the license limit.