I recently fixed a White-Rodgers Model 50A51-242 furnace control module that had a problem with one of the blower control relays. This problem turned out to be so easy to fix that I figured I might as well do a write-up on it.

In my particular case the “HEAT HI” relay was not activating when it should. (For a description of the troubleshooting involved, please see my post on York Deluxe Diamond 80 Furnace repair)

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Removing the bottom cover

I used a 1 inch putty knife and screwdriver to remove the bottom cover. Be careful not to push them in too far which could damage the circuit board.

Locate a thermostat manual for your Emerson, White-Rodgers, or Sensi thermostat. Sensi™ Touch Smart Thermostat (White) 1F95U-42WF. Product Information.

Upon opening I was very happy to see that there were 3 identical blower control relays on the board: one for low heat (“HEAT LO”), one for high heat (“HEAT HI”), and one for a/c (“COOL”). This was good news because it meant I had a spare relay, as we were not using the a/c function. I would be able to use the a/c relay to replace the high heat relay if need be.

Cold solder joint for “HEAT HI” relay

As it turned out, there was no need to replace the high heat relay. A close inspection of the high heat relay’s solder connections showed a very obvious cold solder joint on one of the coil connections. I resoldered this connection, and the problem was fixed. Resoldering the connection did prove to be a little challenging, however. It just would not take the solder. I believe the problem was some kind of coating on the solder. After I carefully scraped the area with a precision screwdriver, and fed in plenty of flux core solder, it finally took the new solder.

Locations of Overheated Area and Cold Solder Joint

I did notice another problem with the board. There are four resistors connected in parallel to provide current to the thermostat. These resistors had obviously become very hot at some point, discoloring the board and melting their solder connections. This probably happened years ago with an older thermostat that drew too much current. Since the furnace is currently hooked up to a digital thermostat, and they draw very little current, I wasn’t too worried. I decided to resolder the resistors just to be on the safe side, but it was probably unnecessary.

Operator: Save these instructions for future use!

FAILURE TO READ AND FOLLOW ALL INSTRUCTIONS CAREFULLY BEFORE INSTALLING OR OPERATING THIS CONTROL COULD CAUSE PERSONAL INJURY AND/OR PROPERTY DAMAGE.

DESCRIPTION

The 50A55-474 and 50A55-571 are automatic gas interrupted ignition controls that employ a microprocessor to continually monitor, analyze, and control the proper operation of the gas burner, inducer, and fan.

Signals interpreted during continual surveillance of the thermostat and flame sensing element initiate automatic ignition of the burner, sensing of the flame, and system shut-off during normal operation.

These controls incorporate system fault analysis for quick gas flow shut-off, coupled with automatic ignition retry upon sensing a fault correction.

PRECAUTIONS

Application of this type of control may cause flame rollout on initial start-up and could cause personal injury and/or property damage.

Replace only with exact model number, including dash number. Failure to use exact replacement control could cause personal injury and/or property damage.

If in doubt about whether your wiring is millivolt, line, or low voltage, have it inspected by a qualified heating and air conditioning contractor or licensed electrician.

Do not exceed the specification ratings.

White Rodgers 50a50 Manual 2017

All wiring must conform to local and national electrical codes and ordinances.

This control is a precision instrument, and should be handled carefully. Rough handling or distorting components could cause the control to malfunction.

! CAUTION

To prevent electrical shock and/or equipment damage, disconnect electric power to system at main fuse or circuit breaker box until installation is complete.

Label all wires prior to disconnection when servicing controls. Wiring errors can cause improper and dangerous operation.

This control is not intended for use in locations where it may come in contact with water. Suitable protection must be provided to shield the control from exposure to water (dripping, spraying, rain, etc.).

!WARNING

Do not use on circuits exceeding specified voltage. Higher voltage will damage control and could cause shock or fire hazard.

Do not short out terminals on gas valve or primary control to test. Short or incorrect wiring will damage thermostat and could cause personal injury and/or property damage.

SPECIFICATIONS

White Rodgers 50a50 230

ELECTRICAL RATINGS [@ 77°F (25°C)]: Input Voltage: 25 VAC 50/60 Hz

Max. Input Current @ 25 VAC: 0.45 amp Relay Load Ratings:

Valve Relay: 1.5 amp @ 25 VAC 50/60 Hz 0.6 pf Ignitor Relay: 6.0 amp @ 120 VAC 50/60 Hz

(resistive)

Flame Current Requirements:

Minimum current to insure flame detection: 1 μa DC* Maximum current for non-detection: 0.1 μa DC* Maximum allowable leakage resistance: 100 M ohms

*Measured with a DC microammeter in the flame probe lead

OPERATING TEMPERATURE RANGE:

-40°to 175°F (-40°to 80°C)

HUMIDITY RANGE:

5% to 93% relative humidity (non-condensing)

MOUNTING:

**At 50 Hz, all timing specifications should be increased by 20%.

Gases Approved: Natural, Manufactured, Mixed, Liquified Petroleum, and LP Gas Air Mixtures are all approved for use.

TIMING SPECIFICATIONS

(All times are in seconds, unless noted otherwise)

*These times will vary depending on option switch position. The control is factory-set at 0 sec. delay to cool off and 100 sec. delay to heat off. See OPERATION section for further information.

†This time will vary depending on the condition of the jumper. With the jumper installed, the delay-to- cool-off is set at 0 seconds. To obtain an 80second delay-to-cool-off, clip the jumper wire and separate the jumper legs.

MOUNTING AND WIRING

!WARNING

Do not use on circuits exceeding specified voltage. Higher voltage will damage control and could cause shock or fire hazard.

! CAUTION

To prevent electrical shock and/or equipment damage, disconnect electric power to system at main fuse or circuit breaker box until installation is complete. Failure to earth ground the appliance or reversing the neutral and hot wire connection to the line can cause shock hazard.

Shut off main gas to heating system until installation is complete.

Route and secure all wiring as far from flame as practical to prevent fire and/or equipment damage.

INSTALLATION

NOTE

All wiring should be installed according to local and national electrical codes and ordinances.

The control must be secured to an area that will experience a minimum of vibration and remain below the maximum ambient temperature rating of 175°F. The control is approved for minimum ambient temperatures of -40°F.

White Rodgers 50a50 405 Manual

Any orientation is acceptable.

Refer to the wiring diagram and wiring table when connecting the 50A55 control to other components of the system.

UL approved, 105°C rated 18 gauge, stranded, 2⁄64” thick insulation wire is recommended for all low voltage safety circuit connections. Refer to 50A55 specification sheet for recommended terminals to mate with those on the control.

UL approved 105°C rated 16 gauge min., stranded, 4⁄64” thick insulation wire is recommended for all line voltage connections. Refer to 50A55 specification sheet for recommended terminals to mate with those on the control.

The 50A55 has only one serviceable part–an automotive type fuse, which protects the low voltage transformer from damage if its output is short-circuited. If the fuse has opened up, remove whatever caused the short circuit and replace the fuse with only a 3 Amp automotive type fuse. If the fuse is not the cause of the control’s problem, replace the entire 50A55 control. There are no other user serviceable parts.