HCC Flashcards
Closed Loop Control
uses feedback to make the control process automatic
Thermostat
temperature actuated sensor that provides feedback
High limit switch
safety device that shuts gas valve
prevents dangerously high temperature
disc type
located near top of heat exchanger
80 to 120 degrees maximum
Circuit voltage to ground dwelling unit
max 150V
Class 1 extra low voltage power circuit
30V & 1000VA
Class 2 low energy power circuit
150V & 100VA
24V thermostat wiring
LVT cable
Bi-metal temperature sensing device
operates on the principle that different metals expand and contract at different rates as temperature change
Strip type
2 strips of dissimilar metals welded together
Spiral type
wound in sprail or coil
1.5 cm by 20cm
higher sensitivity
ideal for mercury bulb switches
Disc type
2cm by 3 cm
snapping action from mechanical stress
sealed
Bellow type sensing
uses a fluid that vaporizes to move contacts
large snapping action
ideal for line voltage thermostat for unit heaters and electric heating
Liquid bulb type temperature sensing
operates by expanding and contracting volume of liquid
monitoring temperature of water and air in a pipe or tank
Thermocouples
generate voltages proportional to temperature
made of 2 dissimilar wires metals
30mV
hot junction inside metallic capsule
prove existence of open flame
Thermopile
thermocouples in series
800mVh
Thermistor
Solid State Device
resistance of semi-conductor material changes with temperature
small physical size
negative temperature coefficient
accurate
Resistance temperature detector
like thermistor
uses copper, nickel, platinum
positive temperature coeffiecint
platinum most commonly used due to high anti corrision
senses extreme temperature
greater accuracy, used for industrial settings
Pyrometer
lens focused thermocouple
measured at a distance
moving objects
COunter flow furnace
fan at the top
hot air pushed to the bottom
2 high limit switches
121 to 149 degrees maximum
Fan Cycling
fan ON is too low (49 to 60)
fan off is too high (32 to 38)
differential is to narrow (17 to 22)
High Limit cycling
fan failure
Thermostat contacts
mercury contacts
spring activated snap action exposed contacts
glass enclosed magnet and armature snap acting contacts
Mercury Contacts
mercury is tilted towards the 2 wires to complete the circuit
not for high current rating
Spring activated snap action exposed contacts
rubbing action when they close
high current switching
require maintenance
Glass enclosed magnet and armature snap acting contacts
Heating and cooling thermostats
Heating contacts close
cooling contacts open
Thermostat wiring
Red line 1
white Heating
yellow cooling
green fan
Anticipator
generates a small amount of heat to the thermostat
anticipates a rise in temperature
series for heating
parallel for cooling
adjustable for heating
fixed for cooling
Thermocouple power unit
flame proving device generates 26 to 32mV
holds armature in place for gas to flow
10 to 30 secs
Safety pilot switch
in series with main gas valve
stops gas flow
Safety gas valve
stops gas flow to main burner and pilot flame
Soleniod valvees
control flow of gas
in series with operating controllers
Diaphargm valves
turns the supply gas to main burner on or off
Combination gas valve
gas regulator, safety gas valve, power unit, main gas valve, manual has valve
Blower Motor
fractional horsepower
2 speed
Thermocouple test
Open circuit 26 to 32mV
Closed circuit 25 to 50% output
response time
turn down test ensures pilot flame ignites main burner
Cycle of operation
room gets cold, thermostat contacts close
anticipator heat up
solenoid engergizes
gas valve opens
ignition
heat exchanger warms
fan on reached, blower fan turns on
room at desired temp
thermostat contacts open
soleniod drops plunger
gas valve closes
heat exchanger cools
fan off reached
rooms cools
Step open gas valve
2 step gas release
reduce inital inrush of gas
Make up air and exhaust air
interlocked
make up air must be pre-heated
HRV
recovers heat from exhaust air
Efficiency Ratings
standard 55 -72%
mid efficiency 80-83%
high efficiency 90-93%
Combustion air components
combustion air blower
air prove switch
heat exchanger
primary high limit switch
combustion air blower
move products of combustion
purge combustion chamber
air proving switch
pressure controller NO points
creates low pressure
prevents unsafe burner operatiion
burner assembly
burners
gas valve
ignitier
flame sensor
roll out limit switch
hot surface ignition system
no pilot flame
silicon nitride
dont touch with bare hands
intermittent pilot ignition system
ignites pilot flame
flame sensing
use the flame as a conductor
cycle of operation efficient gas heating
thermostat contacts close
combustion air motor purges
air prove switch closes
igntion circuit energizes
gas valve opens
igntion
flame sensor senses flame
deenergize ignition system
blower fan energizes after set temperature
Cartridge air filtration
installed in return air supply
standard fiberglass
mid efficient 3 to 10 micron
high efficiency 1 micron
micro-biocide
electronic filters charges particles with positive charge
electrostatic
connected to HTG ACC COM
humidifiers
standing water absorbent pads
rotating drum humidifier
rotating disc
flow thru
zone valve transformers
40 VA
not loaded beyond 32 VA
Reverse acting aquastat
prevents cold water from circulating the system
cad cell
light sensitive
resistance drop to 300 to 1000 ohm to allow current flow
circulating pumps
1 hp
single stage centrifugal pump can be oil or water lubricated
provides proper pressure
can run continuously with a bypass valve
low water cut out
shuts fuel supply if water level in boiler falls below a point
float switch
combined with make water switch
24V
Aquastats
react to water temperature
fail-safes
immersion aquastat
gas filled bulb switching mechanisim through capilliary tube
surface mounted aquastat
mounted on external pipe or heat exchanger surface
affected by ambient temperature
high limit protection
switch burner mechanism and ignition sequience off
maintain minimum working temperature
compressor
compress refrigerant
in cool low pressure
out hot high pressure
condenser
finned heat radiator
cooled by passing air from convection or cooling fan
cool compressed hot refrigerant vapours
evaporator
finned heat absorber device
located in air stream
expose refrigerant to air stream
refrigerant control valve
released controlled amounts of cooled compressed refrigerant
temperature controlled thermal electric valve
pressure controlled automatic expansion valve
Auxillary devices
filter dryer
pressure control
internal pressure sensor
short cycling protection
crackcase heater
cycle of operation
reciever warm high pressure
refridgerant control valve cold low pressure
evaporator coil warm low pressure
compressor hot high pressure
condensor warm high pressure
disconnecting means
within sight
within 3m
conductor lengths
18 AWG <35m
16AWG > 35m
combined heating and cooling
evaporator coil in warm air plenum
2 speed blower motor low for heat, high for cool
cooling unit
heating/cooling thermostat
indoor fanrelay
transformer
low temp sensor
Heat pumps
reversing refrigerant for both heating and cooling
4 way reversing valve
indoor and outdoor coil
components of HVAC
Heating
cooling
ventiliation
compressor lockout switch
when low pressure, high poressure, low temperature switch is activated
Ventilation
blower (indoor blower motor)
blower motor
supply building with air
evaporator
disconnect on rooftop units
within sight
within 3m
Rooftop heating cycle
thermostat for heating closes
combustion blower motor energize
air pressure switch closes
spark ignition energizes
flame rod proves pilot flame, ignition circuit deenergizes
main gas valve opens
evaporator fan energizes
thermostat contacts opens
gas valve closes
Rooftop cooling cycle
thermostat for cooling close
compressor and condensor start
evaporator fan energizes
thermostat contacts opens
make up air unit
direct fired no heat exchanger
indirect fired with heat exchanger
exhaust fan
2 wire thrmostat
simple homes
R W
3 wire thermostat
R W G
5 wire thrmostat
RC RH W Y G
rc rh jumpered
7 wire thermostat
RH RC W1 W2 Y1 Y2 G
HVAC control systems
DDC
BAS
EMS
ECS
FMS
analog input
varying signal
digital output
on or off
analog output
0-10 V
rooftop enclosure
raintight
MAXIMUM WATTS PER METER
What type of signal is used to communicate between the temperature controller and the electronic controller in a solid state switching arrangement?
MILIAMP
