Limits & Interlocks Flashcards
What type of controller is used for a hot water boiler?
Temperature control - operator aquatstat
What is limit control actuated by? (3)
- Temperature
- Pressure
- Liquid level
How are safety limits typically wired in an electrical circuit?
In series
What type of control switch is the best example of a running interlock?
combustion air switch
Describe what a recycling interlock is?
When broken, the cycle resets and allows the flame safeguard control to recycle once conditions return to normal
Describe what a running interlock is?
Can be opened at the beginning of start up cycle but must be
closed prior to the pre-purge cycle, and remain closed throughout the burner’s ignition and
run cycle.
Describe what a non-recycling interlock is?
An Interlock which, when broken during the burner’s running cycle, does not reset and allow
the flame safeguard control to recycle once conditions return to normal
Describe what a pre-ignition interlock is.
An Interlock that must be made prior to the burner’s ignition cycle, and once the burner is
successfully started the interlocks can be opened/bypassed
A flue gas spill switch does not require manual reset. True or False?
False
What is the purpose of the end switch in a flue vent damper?
Proves the damper is open and allows normal start-up of the burner
What is the purpose of an appliance draft inducer air proving switch?
Proves the fan is running and allows normal start-up of the burner
Where is a low gas pressure switch located on a fuel train?
Upstream of the two safety shutoff valves
______ controls can be either high or low limit type and are normally set beyond the normal operating range of the heating unit or appliance
Limit
Limits are normally wired in series. True or False?
True
Insure a safe burner start up and run
sequence by interlocking the various
burner equipment functions with the
_______ __________ control system
flame safeguard
________ provide additional
safety and reliability to the operating of the heating unit
interlocks
A burner motor with an air switch is an example of an _______ in series with the flame safeguard control system
interlock
what is a pressuretrol?
Steam pressure switch that cuts out burner once satisfied - operates on a pressure differential
Flow switches can be of the water or air type. True or False?
True
_______ flow witches are generally a
proving switch proving that the pump is
operating.
water
If the pump is running it will push water past the flow switch, moving the _______ and completing the circuit in the housing. Electricity is then sent to the _______ and the burner can now fire
paddle, burner
High limit aquastat senses water
temperature and opens contacts if
temperature reaches _____f
200f
High limit on a furnace senses air
temperature and opens contacts if air temperature reaches _____f
200f
Aquastat is a normally ______ switch
closed
Aquastat remains closed until water reaches a temperature around _____f
180f
Once the water cools a number of
degrees (usually ___) the aqua stat contacts close the burner fires up again.
20
Name the two types of aquastats
- direct-mounted aqaustat with horizontal probe
- remote bulb aquastat
Where are spill switches located?
Entrance to the draft hood
What is the purpose of a spill switch?
To detect flue gases that may spill from the draft vent or hood
Most flue gas spill switches must be
reset manually although some are
automatic reset. True or False
True
End switches are ______ switches. Proves that another piece of equipment has done its job.
proving
Name two examples where you may find an end switch?
Zone valve on a boiler
Vent damper on a furnace
Both instances won’t allow the boiler/furnace to operate
Gas pressure controls should be set at __% above the highest measured pressure under flow
25%
B149.3 says to not more than 125% of the normal operating pressure at the maximum firing rate (This doesn’t work when the control is installed before the ______ rate valve
firing
Gas pressure switches can be used for? (3) and are usually _______ reset
Gas
Air
Propane
manual reset
Gas pressure switches can also be used for proving __________ air and are automatic reset
combustion
_________ are conditions that need
to be satisfied before you start the
appliance.
Permissives
Example of permissives are (4)
- Thermostats
- Aquastats
- DDC contacts
- timer
Permissive switches are wired in ______ and will open the entire circuit if one condition is not met
series
An interlock can be a permissive
but a permissive cannot be an
interlock. True or False?
True
Difference between a permissive and interlock?
Permissive need to be met before a machine starts and interlocks activate during a process that has already started
A pump that is shut down due to a low level of liquid is a __________
interlock
Low suction pressure that does not allow a compressor to start is an example of a __________
permissive
