Electrical Components up to 400MBH

What are thermally operated controls and what are the two main types?

-Operate mechanically with temperature change, no electrical circuit

-Solid Expansion (Bimetal rods, Rod and Tube)

-Rigid, economical, simple

-Indirect action limits location flexibility

-Fluid Expansion (Liquid or Vapor with capillary tube and bellows)

-Can locate head remotely

-Sensitive to atmospheric pressure and mounting orientation

What should be considered when selecting and wiring room thermostats?

-Match thermostat to correct voltage and amperage to protect internal heat anticipator

-Common types:

Rod & Tube

Liquid Bulb

Bellows

Bimetal (most common)

Bimetal sensor shapes: Coil, Spiral, Leaf, U, Disc

Mercury bulb is most common electrical contact

What are key considerations for thermostat mounting and location?

Mount between rooms, not on outside walls

Avoid heat sources or airflow disruption (curtains, doors, etc.)

Mount level using elongated holes between studs

What are the standard thermostat wiring terminals and what do they control?

R: Power

W: Heating

Y: Cooling

G: Fan

C: Common

Wired in series with transformer (24V system) or with thermopile (mV system)

What is a heat anticipator and how is it configured?

Prevents overshoot by heating thermostat early

Wired in series for heating, parallel for cooling

Types: Fixed, Variable (rheostat), Cycle, Bimetal, Voltage type

Calibration: match to current draw, adjust dial to tune cycle length (If cycle too long, set dial lower setting, if too short, increase dial setting).

What are thermistors and RTDs and how do they differ?

Thermistor: resistance drops with rising temp (NTC) some error expected

RTD: resistance increases with temp (PTC) minimal error

RTDs often made of platinum (high stability, corrosion-resistant)

What is a thermostatic gas valve and when is it used?

Combined modulating valve and temperature control (Modusnap)

Uses liquid bulb to modulate flame height

Must not be installed where burner is not designed for modulating flame

What is an energy cut-off switch and how is it used in hot water tanks?

Replaces temperature relief valve
must be mounted within 6" of tank top

Must be wired in series with 100% safe hookup

May require a power unit with lower draw due to added resistance

What are the two types of safety switches for gas controls?

Safe Hookup: only shuts main burner, not pilot
not for propane

100% Shutoff: shuts both main and pilot gas
manual reset required

How does a diaphragm gas valve operate?

On/off valve for main gas

Mounted after pressure regulator and shutoff

Opens by bleeding pressure from diaphragm

Needs minimum 2" wc pressure to open

Venting required
ambient max ~52°C

What is a gas solenoid valve and what are installation considerations?

Wired in series with thermostat

Inlet closes valve using gas pressure
reversed flow may bypass

Coil types: standard (60°C), high temp (200°C), moisture-proof

Install tips:

Mount vertically

Confirm flow direction

Remove packaging

Bubble test

Match ambient temp and pressure

Ensure correct voltage

What is a redundant gas valve and how does it ensure safety?

Two valves in one unit, using different technologies (e.g. solenoid + diaphragm)

Ensures positive shutoff if one fails

Newer versions separate flame safeguard into ignition module

How do fan and limit controls work in forced air systems?

Fan ON: NO switch closes with heat rise

Limit: NC switch opens with excess heat

Fan delay avoids blowing cold air

Common settings:

Fan on: 50–60°C

Fan off: 38°C

High limit: 93–120°C

Clause 7.8.6: max 120°C (forced air), 175°C (gravity)

What should be considered for electric contact rating in controls?

Use contacts rated for voltage and VA

24V control may be used on 110V-rated contact, but not vice versa

What do rollout and sail switches do?

Rollout switch: senses flame issues at burner throat or blocked ducts

Sail switch: proves airflow/ventilation triggers alarms or shutdowns

What is the function of induced draft vs. forced draft fans?

Induced draft:

Negative pressure in combustion chamber

Works with atmospheric burners

Allows tighter heat exchanger

Type B venting allowed

Forced draft:

Positive pressure, uses power burners

Category III (or IV for high efficiency)

Used in commercial/industrial

What is unique about high-efficiency blower motors in furnaces?

Some use built-in modules to convert AC to DC

Improves power efficiency
found in premium furnaces

What are snap discs and what is a common installation detail?

Can be manual or auto reset

Factory jumper removed in dual-voltage fan/limit controls to isolate voltages

What are the three types of fixed resistors and their materials?

Composition: carbon with binder, general use

Film: thin resistive film on ceramic/glass

Wire Wound: resistance wire (nickel/chromium or copper/nickel) wound on core

What is the difference between rheostats and potentiometers?

Rheostat: 2 terminals, wiper adjusts resistance

Potentiometer: 3 terminals, acts as voltage divider or rheostat (if 2 used)

Another name for a liquid temperature controller?

Aquastat

Common materials in humidity sensors?

nylon, hair, leather, wood, silk, horn

Most important factor when replacing a fan/limit control?

match sensing element length