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Richard
Richard, Technical Manager
Category: HVAC
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Experience:  27 years of Experience, NATE Certified, professional trainer
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Hello I have a Rheem Imperial 90 plus furnace with a problem.

Resolved Question:

Hello I have a Rheem Imperial 90 plus furnace with a problem.
Tstat calls for heat
Inducer comes on
Igniter glows
Gas Valve opens and nothing.......
Sometimes it does lite but a very violent delayed fire
When I remove the screws holding the combustion chamber cover and allow the cover to create a small gap around it the furnace lites just fine. Almost like it isn't getting enough air for the combustion to take place. Where does the air come from and why don't I have enough? The exhaust pipe is free of obstruction. Flame sensor is clean. "watercolumn" at lp regulator on outside of home is 12.5 inches. dont know what it is at gas valve
Submitted: 4 years ago.
Category: HVAC
Expert:  Richard replied 4 years ago.
Hello since you on propane we could have a heat exchanger problem. Propane has a tendency to soot up in the secondary of the HX. However before we go there lets check some simple things. If it has a hot surface style of igniter let it cool down. Take it out and measure the resistance with an ohm meter. It should be between 40 and 90 ohms. often as they get older they increase in resistance and glow but don't get hot enough to light the gas. let me know what you find and we can go from there. Richard.
Richard, Technical Manager
Category: HVAC
Satisfied Customers: 1514
Experience: 27 years of Experience, NATE Certified, professional trainer
Richard and 6 other HVAC Specialists are ready to help you
Customer: replied 4 years ago.
Igniter does not explain why it works just fine with the combustion chamber cover off but wont fire or delayed fire with it in place
Customer: replied 4 years ago.
??
Customer: replied 4 years ago.
Relist: expert didn't read all of the information that I made available. Bogus answers..
expert didn't read all of the information that I made available. Bogus answers.
Expert:  Richard replied 4 years ago.
Hello buy taking the door off you just open the combustion chamber to atmospheric pressure this allows the gas to go up a little more rather than straight into the burner tubes. Either way we are trying to solve to problem, so the first step it to look at the igniter. next if this OHMs out OK we will look toward the draft. As I mentioned if on propane this can cause a restricted draft. This is all a theory at this point. Did you check the igniter? Lets solve the problem and i will give you the explanations as to why when we know what failed. Thank you Richard.
Customer: replied 4 years ago.
igniter reads 99 ohms
Customer: replied 4 years ago.
Igniter reads 99 ohms
Expert:  Richard replied 4 years ago.

Hello well that is not awful but borderline bad. The next step is to take some readings since you probably don't have a guage ( magnehelic) we will have to take the long way. First, look at you outdoor vent termination and look closely for any signs of soot. Next we will have to take the draft motor out and look into it to see if any of the fins are broken. Now you have to look close on this because everything inside the motor housing is black. When you take that motor out you will see a hole going into the heat exchanger this is the secondary coil it has a series of small tubes. Look in there as well for any debris or soot.

I might be a little ahead of my self here because i know this furnace like the back of my hand. So if you can take the burners out and blow air in them if you can to clean anything that could be in there. Next step is we will need to check the gas pressure and the vent pressure and we will need a gauge for this. Now getting the burners out is a little tricky but you don't have to take the gas valve off of the manifold just the 4 screws that hold it in place. If your good you can get the burners out. One last thing since this must be an older unit to have hot surface ignition the burners are most likely small Robert shaw individual burners instead of a solid manifold this may require you to take the burner box out. if it has the Robert shaw burners there is a little field fix for them if we get this far. Can you provide me the model/ serial number? Take the top door off and it will be on a tag to the side let me know Richard.

Customer: replied 4 years ago.
Rgeb-07ec-fs is the model number. Burner is 1 unit(manifold style). I already took it out and thoroughly cleaned it. I have a new igniter on the way. I will remove the draft inducer and inspect it in the morning. Thank you for your time. Oh BTW, the burner was replaced in the mid 90's. House was built in 1990 so I am assuming the furnace is model year 1990 as well.
Expert:  Richard replied 4 years ago.
Hello. if it has a single burner it should be a a RGEC or RGED can you verify? The RBEG has several burners. If it has the single burner this is different from just about any other furnace made. I need to know the 5th digit of the serial number it will either be a 3, 6 or 7 will make a big difference. Either way there was a factory program for these models. Let me know
Customer: replied 4 years ago.
The fifth digit is a 3. The burner looks like it could be 3 seperate ones, except there is a small tube with a bunch of holes across it welded across the burner. I am assuming that this is where the gas goes to light the main burner? It has a smaller tube that runs from there down to the gas valve. Removed the inducer this morning. Looks and turns just fine. BUT, the amount of rust and debris that fell out of the hole behind it was unbelievable! Could that hole have been pluggged not allowing enough "draft" in the combustion chamber?
Expert:  Richard replied 4 years ago.

Hello, since you can see the burner i am going to assume it is an RGEB or EA ( older version) This unit used aluminumized heat exchanger cells. These were very prone to cracking. If your able to remove the burners and with a light look in side and up you may see them. A crack will cause the issue you have. It is possible it is just rust and debris however with this furnace and the age i would strongly recommend you or someone look. Now they did have a program for this furnace because they no longer make the heat cells. I looked it up but it expired in 08. If you look at the serial number there will be a F followed but 4 numbers. F1088 example, made 10th week of 1988. Many of these came with 20 year warranty so if it is less than 20 years you could qualify for something.

Getting back to your issue. Clean it , inspect the heat cells i can send you a picture of what they look like, then try again. Now this was the second generation (RGEB) of 90 plus furnaces so they had limited safeties built into them. Often the pressure switch would not take the furnace out or removing the door would make it run under a not so good condition. Now the heat exchanger is in a negative pressure so the chances of CO escaping is very small but don't take any chances here. Let me know

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Richard
Richard
District Technical Manager
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27 years of Experience, NATE Certified, professional trainer