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HVACR214 – Electrical for Oil. High and Fan Limit Controls. High Limit Control. The high limit control is a thermostat that is used to turn off the heat source when the temperature in the heat exchanger rises too high

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hvacr214 electrical for oil

HVACR214 – Electrical for Oil

High and Fan Limit Controls

high limit control
High Limit Control
  • The high limit control is a thermostat that is used to turn off the heat source when the temperature in the heat exchanger rises too high

A. Adjustable high limit controls normally have a maximum set point around 200 degrees

- The cut out can be lower but not higher

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B. Some units have fixed limits

C. When replacing these limit controls the temperature set point must remain the same as it was from the factory

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2. The high limit will be closed unless the temperature rises above the set point

A. If the temperature rises too high only the heat source will turn off

B. Some limit controls will be wired in series with the transformer primary

C. Blower fan MUST NOT turn off when high limit opens !!!!

fan limit control
Fan Limit Control
  • The fan limit is a thermostat that controls the blower fan motor which circulates the air throughout the building
  • Adjustable Fan Limit

A. These limits have two set points (cut in & cut out)

B. Cut in is when the fan will start to circulate air

C. Cut out is when the switch opens and shuts the blower off

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D. Normal Cut in is 130 degrees

E. Normal cut out is 90-100 degrees

F. Setting the cut out too low may lower the air temperature of the space resulting in nuisance calls with customers complaining of a cold draft

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Fixed Fan Limit Control

A. Some furnaces have fixed limits

B. Just like a high limit when changed these need to match the factory setting

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4. Some fan limit controls incorporate a time delay

A. The manufacturer has decided the proper time for the fan to come on

B. There will be a set time for the fan to cut in and cut out

i. The cut-in on some units may be adjustable

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5. Some new furnaces have integrated circuit board which control blower operation

A. Models with fixed speeds are controlled by time

B. Models with variable speed blowers can adjust speeds to temperature rise

C. Temperature rise is the difference between the return and supply temperature as it passes through the furnace

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PICTORIAL DIAGRAM (SIMPLIFIED)

RED

BK

TRANS

BK

BL

XFMR

S1

S2

L1

L2

COM Lo Hi

Primary

L2

L2

C L M M H

C Y G R W