Rheem 90 Plus Furnace problem


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Old 01-13-10, 11:51 AM
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Rheem 90 Plus Furnace problem

I've got an 11 year-old furnace that has started to shut itself off at about the same time each day. When the unit shuts off, raising or lowering the temperature on the thermostat becomes inactive. The only way to get it back on is to reset either from the thermostat or from the heater reset button.

The furnace runs perfectly except for that once a day when it shuts itself down.

After checking everything out, and installing a new thermostat, my heating service people can't seem to come up with an answer, saying they have never heard of such a problem as a daily timed shut down on the unit.

My question: is there an internal timer on these units and if so, where should I look to reset or fix the problem?
 
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Old 01-13-10, 12:18 PM
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There is no internal timer.

Your furnace probably has an intermittent problem ---- it's right on the edge of not working, and sometimes it falls over that edge.

Such problems usually get worse over time until they are diagnosed and repaired.

Unfortunately, if your repair service replaced the thermostat they are probably marginally competent and/or lazy.

A lot of intermittent problems can be diagnosed the first time out, given sufficient skill in testing a patience in repeating tests until the condition manifests itself.

Does your furnace have a hot surface ignitor that glows whit hot to light the gas?

And what is the model number of the furnace, which can be obtained off the rating plate in the burner compartment of the furnace?

Do you have a multimeter to measure AC voltage and understand how to use it?
 
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Old 01-13-10, 12:55 PM
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Thanks Seattle,

If I'm picking up the model number correctly, this is what i see on the unit: VR8205H 8016

The ignitor is fine (new), burners are fine (two years old), etc., etc.

The repairman was here for over an hour, running cycles and found nothing because he was not here at around 4 pm when the shut off usually takes place.

Since it's almost accurately on time, it really isn't an intermittent problem but one that happens with regularity. In fact, it should shut down any minute now and I am keeping close tabs on it.

You bring up voltage. I'm wondering if maybe we experience a voltage drop here at that time of day. In any case, I haven't a meter to measure voltage.
 
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Old 01-13-10, 01:08 PM
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What you want to do is to remove the cover of the furnace so you can observe what is happening when it fails to light.


When the thermostat calls for heat, the inducer motor should start up and continue to run. When it comes up to speed the pressure switch should close and then the HSI should light up. After that the gas should turn on and the burners should light.


How far into that ignition sequence does your furnace get?
 
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Old 01-13-10, 02:02 PM
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Seattle,

I'm not being clear.

The furnace works for 23 hours without incident and then nothing happens between the last time it fires and when the temp in the house has dropped below the bottom thermostat setting. It stops responding to the thermostat completely. No attempt is made for it to turn on and light. Dead.

I reset and it works fine until the next day and the same process occurs.

Earlier this morning, I opened the control box, looked at the schematic and pulled a few plugs from the wiring that seemed near the area that worked on message from thermostat to motor. I waited a few seconds and then plugged them all back in. I think there were three of them.

The furnace did not do its usual shut down today, but I am unsure if that means anything.
 
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Old 01-13-10, 02:24 PM
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Well, you need a multimeter if you want to try to identify the problem yourself.


Lots of odd associations can be made with a problem, but they can be just additional proof that "correlation is not causation"


You need to catch it when it's failed to turn on and then determine why, and you need a multimeter to do that.
 
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Old 01-13-10, 04:59 PM
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If the inducer does not even come on, which I guess does not, based on your 'nothing happens" - that means you have to check:

..to see if you have 24 volts at transformer, so you know you have furnace line voltage and 24 volts available.

And to see if you have 24 volts between low voltage terminal block connection W to C.

And to see if the blower door switch is working and blower door is tightly up against the switch.

And then you need to know if you have a 3-wire pressure switch that may have gotten stuck in the previous run cycle's closed position (which closes the NO to C contacts on the pressure switch), which then makes the start of the new cycle "open", because the plunger in the diaphram is stuck the wrong direction. On these types of pressure switches, nothing will happen unless out of the 3 terminals on the switch that say NO -NC and C, that you have to have continuity from NC to C, for the furnace to start up.

If these issues are not it, then there also is a relay that converts 24 volt signal of the thermostat to 120 volts to get the inducer to run. Sometimes that relay goes to pot. If that relay is not working, then nothing works. Unfortunately with newer furnaces, that relay is usually on the control board and not replaceable without replacing the board. On older furnaces, some had the relay as a separate item not on any board.

And to unplug and replug all electrical connections (both spade and jacks containing pin connectors).

These are some places to start looking.
 
 

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