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Rheem/Ruud gas furnace design and troubleshoot
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When troubleshooting, understand you are working with potentially lethal voltages and a highly flammable gas. If you do not have the ability to do these operations safely, do not attempt them.
Rheem/Ruud Model RGDA gas furnace
This furnace was installed in the late 1980s. It was one of the last draft hood furnaces to be made.
The burner was atmospheric (meaning the heat of combustion moved the vent gasses out of the vent).
It did not have a standing pilot. Instead it used an HSI to ignite the burners.
The HSI also sensed flame for the flame safety control.
Other than the ignition system, it was made similar to how gas furnaces were made in earlier years.
The below is an overview of how the furnace works.
The burner was atmospheric (meaning the heat of combustion moved the vent gasses out of the vent).
It did not have a standing pilot. Instead it used an HSI to ignite the burners.
The HSI also sensed flame for the flame safety control.
Other than the ignition system, it was made similar to how gas furnaces were made in earlier years.
The below is an overview of how the furnace works.
This furnace uses an ignition control to start the burners and provide flame safety control.
The sequence of operation of the ignition control is as follows:
On a call for heat, 24 volt power travels from the thermostat through the limit switches to energize the ignition control. When power is applied to the control, the ignition sequence begins. A prepurge time begins. After a short time, the HSI is energized. After the warmup time, the gas valve is energized for a trial for ignition. Gas flows to the burners and is lit. The HSI is used to sense if the flame is present. If the flame is sensed, the burners will continue to burn. If flame is not sensed on the first trial, 2 more trials for ignition are done. If flame is not sensed after 3 tries, the control locks out and will not try again until the power is shut off and turned back on again. Below is a video on the ignition control.
The sequence of operation of the ignition control is as follows:
On a call for heat, 24 volt power travels from the thermostat through the limit switches to energize the ignition control. When power is applied to the control, the ignition sequence begins. A prepurge time begins. After a short time, the HSI is energized. After the warmup time, the gas valve is energized for a trial for ignition. Gas flows to the burners and is lit. The HSI is used to sense if the flame is present. If the flame is sensed, the burners will continue to burn. If flame is not sensed on the first trial, 2 more trials for ignition are done. If flame is not sensed after 3 tries, the control locks out and will not try again until the power is shut off and turned back on again. Below is a video on the ignition control.
Service of the Rheem RGDA gas furnace
Annual service of this furnace is similar to the older conventional gas furnace. The biggest difference with this furnace is the HSI ignition control.
Burners should be cleaned with all annual services. The video below will show how to clean the burners and check the heat exchanger for cracks or rust.
Burners should be cleaned with all annual services. The video below will show how to clean the burners and check the heat exchanger for cracks or rust.
Blower service
The blower should be serviced when annual service is done.
The bearings should be checked to see if they are stiff or too loose.
The motor should be oiled if there are oilers on the motor.
The video below shows how the motor should be oiled.
The bearings should be checked to see if they are stiff or too loose.
The motor should be oiled if there are oilers on the motor.
The video below shows how the motor should be oiled.
The blower wheel should be checked/cleaned with the annual service.
If filters have not been changed regularly or poor quality ones used, the blower wheel may be dirty. The video below will show how it is done.
If filters have not been changed regularly or poor quality ones used, the blower wheel may be dirty. The video below will show how it is done.
Flame safety and high temperature shut down should be checked.
As with any furnace, conditions should be created that will allow the safeties to be checked. The gas supply should be shut off and the unit started. The ignition control should be allowed to attempt to start the unit and safety out until it goes into hard lockout.
The power to the blower should be disconnected and the unit should be started and allowed to operate until the high temperature limit shuts down the burners.
The power to the blower should be disconnected and the unit should be started and allowed to operate until the high temperature limit shuts down the burners.
Troubleshoot The Rheem/Ruud gas furnace
The first step in troubleshoot is: what is the the furnace doing or not doing.
Does the furnace do nothing at all? If so, is there power?
On this furnace, there is a low voltage (24v) terminal board on the left side. If power is available between "R" and "C", the unit has power. To eliminate the thermostat, measure power between "C" and "W". If so power is available to start the unit. If you find no 24 volt power, you may have no 120v power. Is the breaker on? is the power switch on the furnace on?
For our purposes, lets assume that we have 24v power but nothing is happening. On this furnace there is a special switch that is not on some other furnaces. It is called a spill switch. It is located in the draft hood. If the hot gasses are not traveling out the vent, they are coming out the draft hood. So a blockage in the vent will cause the gasses to come out the draft hood opening. If the spill switch has sensed high temperature gasses, it will shut off and must be manually reset. If this switch has opened, you need to find out why. This a dangerous condition. The furnace could be producing poisonous CO gas.
The video below starts the troubleshoot of this furnace through the draft hood switch.
Does the furnace do nothing at all? If so, is there power?
On this furnace, there is a low voltage (24v) terminal board on the left side. If power is available between "R" and "C", the unit has power. To eliminate the thermostat, measure power between "C" and "W". If so power is available to start the unit. If you find no 24 volt power, you may have no 120v power. Is the breaker on? is the power switch on the furnace on?
For our purposes, lets assume that we have 24v power but nothing is happening. On this furnace there is a special switch that is not on some other furnaces. It is called a spill switch. It is located in the draft hood. If the hot gasses are not traveling out the vent, they are coming out the draft hood. So a blockage in the vent will cause the gasses to come out the draft hood opening. If the spill switch has sensed high temperature gasses, it will shut off and must be manually reset. If this switch has opened, you need to find out why. This a dangerous condition. The furnace could be producing poisonous CO gas.
The video below starts the troubleshoot of this furnace through the draft hood switch.
HSI ignitor problems
One of the most common problems with the HSI equipped furnaces is failure of the HSI.
The HSI (commonly called the hot stick), is a 120volt heater that warms to 2300 degrees F to light the gas. On a call for heat, the ignition control turns on the HSI and allows 11 to 45 seconds for warmup. At that point, the control allows gas to the burners. The gas flows for 4 to 7 seconds. If the ignition control senses flame, the burner stays on. If the flame is not sensed the unit shuts off the gas. The HSI can break and have no connection (they are very fragile). They are easy to check. If the probes of an ohmmeter are placed across the terminals of the HSI. If it is open it has failed. If it shows a resistance and the resistance is over 150 ohms it has not failed but will soon fail. These parts have a limited number of lights or cycles in them. If your thermostat is set to cycle too often, they fail early. Also, if the air filter is plugged or fan dirty, the limit will shut off the burners during the cycle and the HSI will fail early.
The video below covers the troubleshoot of the HSI.
The HSI (commonly called the hot stick), is a 120volt heater that warms to 2300 degrees F to light the gas. On a call for heat, the ignition control turns on the HSI and allows 11 to 45 seconds for warmup. At that point, the control allows gas to the burners. The gas flows for 4 to 7 seconds. If the ignition control senses flame, the burner stays on. If the flame is not sensed the unit shuts off the gas. The HSI can break and have no connection (they are very fragile). They are easy to check. If the probes of an ohmmeter are placed across the terminals of the HSI. If it is open it has failed. If it shows a resistance and the resistance is over 150 ohms it has not failed but will soon fail. These parts have a limited number of lights or cycles in them. If your thermostat is set to cycle too often, they fail early. Also, if the air filter is plugged or fan dirty, the limit will shut off the burners during the cycle and the HSI will fail early.
The video below covers the troubleshoot of the HSI.
The ignition control problems
The video below demonstrates a problem with the grounding of the ignition control.
Troubleshoot the control.
This control has no indicator lights for troubleshoot. If the furnace does not work and you suspect a control board problem, kill the power to the control. Either shut off the thermostat or turn off the power to the furnace. Turn the power back on to see if the unit will try again. If the HSI does not come on, check the HSI with an ohmmeter as above. If no luck, be sure the grounding screw on the control is tight.
Next, with the power on, place an voltmeter between "TH" and "GND". You should read 24v. If 24v is read and nothing happens, the control has failed.
If 24v is not read, the problem is not in the control.
Next, with the power on, place an voltmeter between "TH" and "GND". You should read 24v. If 24v is read and nothing happens, the control has failed.
If 24v is not read, the problem is not in the control.
If power is not available at the control, you will have trace the power back through the limit switch, the flame rollout switch or the spillage switch.
Troubleshoot the gas valve
Symptoms of a failed gas valve are somewhat similar to an ignition control failure. The HSI will come on but the gas will not flow. The way to tell the difference is to place voltmeter leads on the terminals of the gas valve. The gas valve will only receive 24 volt power for 4 to 7 seconds after the HSI has warmed up. Clip your leads on the terminals and start the unit. If the HSI warms then after the warmup shuts down with no power shown on the voltmeter, the ignition control has failed. If power is shown on the meter and gas does not flow, either the gas valve has failed or no gas is available.
The following videos cover the RGDA model that used the mercury pilot system.
The video below gives the sequence of operation of this furnace. This video below will be available 2-20-15.
The video below disassembles the mercury driver of the mercury pilot safety. This video below will be available 2-22-15.