intermittent gas furnace

I have a Rheem 80 Plus gas furnace that has been operating intermittently for over a month. When the thermostat calls for heat, the inducer motor runs, but that is the only thing that happens. There is no gas, the electronic ignition doesn't ignite the pilot light, and, of course, there is no fire in the box.

This began last December and has been happening off and on ever since. Today is the longest it has gone without the furnace resuming normal operation.

I'm a single parent with a teen, and I was laid off just before Christmas, so hiring an HVAC tech to come out and troubleshoot/repair is out of the question right now. Living without heat is also out of the question. Any advice would be greatly appreciated.

 

You said a spark lights a pilot. Do you hear the rhythmic sparking sound? If not, you need to see if you are getting 24 volts at the ignition control. If so, and the pilot is not lighting, you may need to 1) clean the pilot orifice to make sure that gas is reaching the pilot hood, and/or 2) make sure that the pilot section of the gas valve is getting 24 volts. To do this, while the unit is sparking, check for 24 VAC across the PV and MV/PV terminals at the gas valve. If you are getting 24VAC there, you may have a bad gas valve. If not, check for 24 VAC across the PV and PV/MV terminals at the ignition control. If there is 24 VAC at those terminals on the ignition control but not at their counterparts on the gas valve, the wiring between the ignition control and gas valve is bad. If there is not 24 VAC at the MV/PV and PV terminals at the ignition control (while the sparking is occurring), the ignition conntrol is bad.

Which ignition control do you have?

 

Thanks for replying, Eclipse. My ignition control is a RobertShaw SP715A. There is no sparking sound at the pilot, and I don't hear or smell any gas. After considerable research on the internet, I found that I should try cleaning my flame sensor. It was simple to do, I discovered mine was pretty dirty, and cleaning it has brought my furnace back to life, at least for now. I'm also pricing ignition controls on the internet (avg $200), just in case. I will test the voltage on it if it goes out again. I've only read that the 24v should be across TH/TR, which you don't mention, only testing voltage across PV/MV. Can you elaborate on this?

#2 Today, 01:40 PM
eclipse
Members Join Date: Apr 2001
Location: Cleveland, Ohio
Posts: 194

You said a spark lights a pilot. Do you hear the rhythmic sparking sound? If not, you need to see if you are getting 24 volts at the ignition control. If so, and the pilot is not lighting, you may need to 1) clean the pilot orifice to make sure that gas is reaching the pilot hood, and/or 2) make sure that the pilot section of the gas valve is getting 24 volts. To do this, while the unit is sparking, check for 24 VAC across the PV and MV/PV terminals at the gas valve. If you are getting 24VAC there, you may have a bad gas valve. If not, check for 24 VAC across the PV and PV/MV terminals at the ignition control. If there is 24 VAC at those terminals on the ignition control but not at their counterparts on the gas valve, the wiring between the ignition control and gas valve is bad. If there is not 24 VAC at the MV/PV and PV terminals at the ignition control (while the sparking is occurring), the ignition conntrol is bad.

Which ignition control do you have?




shermn8r
I have a Rheem 80 Plus gas furnace that has been operating intermittently for over a month. When the thermostat calls for heat, the inducer motor runs, but that is the only thing that happens. There is no gas, the electronic ignition doesn't ignite the pilot light, and, of course, there is no fire in the box.

This began last December and has been happening off and on ever since. Today is the longest it has gone without the furnace resuming normal operation.

I'm a single parent with a teen, and I was laid off just before Christmas, so hiring an HVAC tech to come out and troubleshoot/repair is out of the question right now. Living without heat is also out of the question. Any advice would be greatly appreciated.

 

I also have a Rheem 80 Plus with the SP715A control. The sole purpose of the flame sensor is to detect the presence of a lit pilot (assuming you have a separate flame sensor and spark electrode). When it does, flame rectification allows the ignition control to energize and open the main valve, sending gas to the burners. While it is a good idea to keep the sensor clean, the symptoms you describe would be caused by something other than a dirty sensor. In other words, I think that this is a coincidence.

 

I forgot -- if you're testing for 24V at the TH and TR terminals on the gas valve itself (a Robertshaw 7000, perhaps?), there will only be 24V when the main (burner) valve (as opposed to the pilot valve) is energized. (The gas valve has two valves -- a main and a pilot).

 

So, in other words, I'm not out of the woods just yet. Does it sound like my ignition control might be bad? Can it perform intermittently like I'm describing? Or should I be looking at something else? I'd like to look at something else since the SP715A is around $200 and more, on the internet.


I also have a Rheem 80 Plus with the SP715A control. The sole purpose of the flame sensor is to detect the presence of a lit pilot (assuming you have a separate flame sensor and spark electrode). When it does, flame rectification allows the ignition control to energize and open the main valve, sending gas to the burners. While it is a good idea to keep the sensor clean, the symptoms you describe would be caused by something other than a dirty sensor. In other words, I think that this is a coincidence.

I forgot -- if you're testing for 24V at the TH and TR terminals on the gas valve itself (a Robertshaw 7000, perhaps?), there will only be 24V when the main (burner) valve (as opposed to the pilot valve) is energized. (The gas valve has two valves -- a main and a pilot).

 

It could be a number of things, not just the ignition control. To assist in narrowing it down, here's the sequence of operation:
The thermostat calls for heat. This starts the inducer motor, which creates negative air pressure in the small tube that runs from the inducer motor to the pressure switch. Asssuming there is sufficient pressure (i.e., no blockage in the chimney), this activates the pressure switch, which sends 24 v to the ignition control. (So, if there is either not enough draft (e.g., a blocked chimney) or if the pressure switch is bad or if the tube is blocked or otherwise compromised), the inducer motor will just run and run and nothing else will happen.) The ignition control then begins the ignition sequence, simultaneously opening the pilot portion of the gas valve and providing a spark to light the pilot. Once the pilot is lit, the flame sensor senses this and allows the ingition control to energize the main gas valve, lighting the burners.

 

Eclipse, thank you for all the good information. Here's some addtl information I have been able to figure out. When the inducer motor runs with no subsequent ignition sequence, I have been turning off the heat (switch to "off" position, thermostat control to lowest temp setting) and waiting for 30 minutes or so. When I turn the thermostat back on, the furnace seems to run fine for several days. I have successfully done this 3 consecutive times now. Does this indicate to you anything more specific as to a cause of my furnace's intermittent behavior? Or is it just coincidence?


It could be a number of things, not just the ignition control. To assist in narrowing it down, here's the sequence of operation:
The thermostat calls for heat. This starts the inducer motor, which creates negative air pressure in the small tube that runs from the inducer motor to the pressure switch. Asssuming there is sufficient pressure (i.e., no blockage in the chimney), this activates the pressure switch, which sends 24 v to the ignition control. (So, if there is either not enough draft (e.g., a blocked chimney) or if the pressure switch is bad or if the tube is blocked or otherwise compromised), the inducer motor will just run and run and nothing else will happen.) The ignition control then begins the ignition sequence, simultaneously opening the pilot portion of the gas valve and providing a spark to light the pilot. Once the pilot is lit, the flame sensor senses this and allows the ingition control to energize the main gas valve, lighting the burners

 

Intermittent failures are hard to diagnose. Might be coincidence.

When the failure is present, a simple test lamp or voltmeter can show how far the operational signal goes. We need to know whether the draft vacuum sensor is giving the go ahead to fire up. You might have a marginal draft. Did anyone stick anything in the pipe outside? Wasp nest?

 

Since, during failure, the furnace does not even begin the ignition sequence, it is likely to be a pressure switch issue -- either the pressure switch is failing (not unlikely in an old furnace -- I had to replace mine after 20 years because it would intermittently momentarily drop out) or, as bolide noted, a draft issue (could also be the draft hose clogged or deteriorated, but this would not likelyt be an intermittent problem). When it fails, check for voltage at the ignition control (while the inducer fan is running). If you have voltage, it is a draft or pressure switch issue. It could also be that the relay that is energized once a draft is established could be failing, but again this is not likely to be intermittent.

 

Thanks to eclipse and bolide for all your help. I have a lot to check out the next time my furnace acts up. It quit again about a week ago, but my son did the resetting of the thermostat trick to make it come back on, before I could take my multimeter to it. It's been behaving itself ever since. No wasp nests in the chimney that I can see, or any other obvious signs of distress in my furnace. It looks very clean over all (filter is changed religiously every month, on the 1st). Right now I guess all I can do is wait for the next time.

Does anyone know any good websites where I can find manuals and/or diagrams of the Rheem 80 Plus? I'm not sure where some of the individual components are located that have been discussed. I've found some generic sites, but they raise more questions than they answer.

Thanks again for all the help.
Bob

 

My Rheem Imperial 80+ is from 1984. I have all of the original literature from the furnace. Originally, mine was equipped with a Robertshaw SP710 ignition control, which used a mercury bulb as the flame sensor. When that sensor died, the entire ignition control was replaced with the SP715A, which utilizes a flame sensor rod to provide flame rectification (a totally different method of verifying the existence of the pilot flame). I also have the manual for the SP715. If you like, send me a PM with your e-mail address and I'll send you pdf files of these materials.

 

eclipse.

about 2 years ago you wrote:

My Rheem Imperial 80+ is from 1984. I have all of the original literature from the furnace. Originally, mine was equipped with a Robertshaw SP710 ignition control, which used a mercury bulb as the flame sensor. When that sensor died, the entire ignition control was replaced with the SP715A, which utilizes a flame sensor rod to provide flame rectification (a totally different method of verifying the existence of the pilot flame). I also have the manual for the SP715. If you like, send me a PM with your e-mail address and I'll send you pdf files of these materials.


I have the same Rheem imperial 80plus with the SP715 (not a) ignition module.

my problem is that the flame sensor is not sensing flame...halting the main valve from opening.

I know this because when it is stuck i take the cover off to find the pilot lit...but the igniter clicking away trying to light the pilot.

All I need do is wiggle the sensor wire (close to where in connects to the SP715)and i get main flame. Or sometimes that does not work and i disconnect the sparker wire (the sparking stops) but then the main flame comes on in a second)

The flame sensor is a single wire going to a ceramic rod with a metal rod coming out of that into the flame stream of the pilot flame) Where can I get on of these?

Thanks,

spyros

 

Based on what you said, before replacing the sensor you should try a few other things. Clean the sensor using some fine steel wool (not sandpaper). You might also want to check and, if necessary, replace the wire and spade connector that runs from the the sensor to the ignition control. Also make sure you have a decent pilot flame and that the flame impinges on the sensor. If you need to replace the sensor, you can get them at supply houses.

 

Well,

I took your advice. I removed the flame sensor and the wire connecting it.

I did a resistance check of the wire. ANd i tightened the crimp. Also cleaned the flame sensor.

Replaced. Furnace fired right up. But after a few cycles it started its old tricks again. I want to replace the flame sensor first.

There were no markings on the flame sensor at all.

How can I tell what the part number is to get a spare?

It was white ceramic fatter at the spade....then thinner and has a single metal rod going into the pilot flame stream (which was strong and fine by the way)

 

Frankly, replacing parts in hopes that it will solve problems is the technique of an incompetent repairman.

Replacing a flame sensor rod is very likely to be a waste of time and money, since you have cleaned it.


You need to use the proper procedures to diagnose the problem, which involves measuring the DC microamps flowing through the flame sensor wire. A typical good read would be 5-10 uamps, and a bad read is typically 1-2 uamps.

You should Google up and read some articles on "flame rectification ignition systems" to try to get a little understanding of what you are trying to do.

If you don't have the interest and equipment to do such things, you are better off hiring a competent repairman to diagnose and repair your equipment.

 

I do have an amp meter and I will check it out.

But I know enough to know it is either the sensor or the ignition module.

Seemed prudent to get a spare sensor for 10 bucks before I call in the couple hundred dollar repair guy.


I was just looking from some advice from people who know… as is the point of this forum. Not looking for a lecture pal.

Perhaps you may find a competent psychologist that will help you with your anger issues.

But then that is a total forum.

 

After reading a bit about flame rectification. I have discovered that grounding issues may play large role.

So tonight I will go home and check the entire ignition system for corrosion and resistance to ground.

I will post results.

Thanks,

 

Well after checking all the grounds....(which were fine)

It was an easy fix.

While I was checking the grounds I noticed an oil film.

So I took the old SP715 ignition module out to take a look (after marking the leads)

I am surprised it was working at all.

There is a transformer and an inductor in there that blew up.

I am talking black marks and components physically blown apart.

The oil was from inside the old components.


I replaced it with the Robert Shaw 780-715 module (which is a wire for wire replacement) Furnace now working as it should!

Wife happy (and warm)

 

I think that carefully detaching a control board off a furnace or AHU, and looking at the back of it for burned spots, is worthwhile somerimes.