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Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 3089
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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My Rheem "Classic 90 Plus" model gas furnace is close to 10

Resolved Question:

My Rheem "Classic 90 Plus" model gas furnace is close to 10 years old and has been working fine for that long. However, there's one thing about its operation which has bothered me for several years now. The furnace works fine all day long but it has problems starting up in the morning after it's been off for a few hours (I keep the house cooler during the night.) The smaller blower runs for a while (two green lights solidly on) but then it starts making this repeated, loud clanging noise on the gas pipes (clang clang, clang clang,...) for a while and then it gives up trying to fire up (the yellow light doesn't come on when this happens.) It repeats this behavior several times until it finally comes on. If I turn off the power to the furnace when this is happening and then flip the switch back on after a few seconds the furnace fires up almost immediately and works fine until the next morning when the same thing starts over again.
Submitted: 5 years ago.
Category: HVAC
Expert:  Douglas replied 5 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

The XXXXX XXXXXght is a flame sense light that only comes on when the burners are on and if the flame signal is good enough.

Interesting otherwise, so when its making its noise, is just the inducer on? Both green lights are on with one of them not blinking?
Customer: replied 5 years ago.
The XXXXX XXXXXght comes on when the burner starts up and stays solidly on while there are flames. The clanging noise happens only when just the small blower (inducer ??) is on and both green lights are solidly on. I noticed that when the furnace starts normally, there is a single, lower clang noise then the flames start almost immediately.
Expert:  Douglas replied 5 years ago.
Im somewhat baffled, but we'll keep at it.

On a call for heat, the sequence is like this...

  • Thermostat calls for heat
  • Control checks pressure switches to make sure they are open.
  • Inducer motor starts for a prepurge (around 20 seconds) and pressure switches close.
  • Ignitor starts ****(is yours spark igntion or hot surface?)****
  • Gas valve opens and burners light
  • Flame signal is proven via the flame senser
  • 20 seconds warm up then blower comes on
So this noise seems to happen when the inducer is on, prior to the ignition sequence or at the same time the burners should come on?

Expert:  Douglas replied 5 years ago.
90 Plus is a series, a specific model number would help. If you dont have that or cant find it... Is your furnace upflow or downflow (are the burners in the upper door - upflow, or lower door - downflow).
Customer: replied 5 years ago.
Not certain if it is spark or hot surface (how can I check?) but I believe it is spark because when I am next to the furnace as it is starting up I can hear a faint, fast clicking noise (like a spark plug) just prior to the flames coming on.
The loud clanging noise happens when the inducer is on and it is certainly coming from the metal gas pipes. I opened the furnace doors to check inside. The metal gas feed pipe which goes into the furnace threads into a part with electrical wires connected and then, from this part, it goes into the burner. What is this part? and can it generate this loud clanging noise that propagates through the pipes?
Expert:  Douglas replied 5 years ago.
Yes thats spark.

The gas valve has a couple solenoids in it and what you described it sounds like the gas valve is making the noise. And even more so since you feel it in the gas line.

Now whether is a gas valve problem or a low voltage to the solenoid causing it to clang or even an incoming gas pressure issue (incoming gas pressure should be between 4 and 7 inches water column).

Does this do this every morning, just on real cold mornings?

Customer: replied 5 years ago.
It is an up-flow furnace: 92-22204-31-01. It does the clanging noise pretty much every morning, not necessarily very cold ones, and sometimes, although rarely, it does it during the day as well. I don't have an easy way to measure the gas pressure but I have a multimeter and can check the voltage coming into the gas valve if you tell me which wires to measure and what the voltages should be. The flames look really strong through the little window and I don't have an issue reaching the requested temperature in the house (70 degrees F). My gas water boiler is close to the furnace and it works very well so I thought the gas pressure is OK but I can't say for sure what it is.
Expert:  Douglas replied 5 years ago.
Well the voltage should be a nominal 24v AC (give or take a couple).

That number is XXXXX the label part number (the warning label with warnings about carbon monoxide, fire and gas leaks). The model number is XXXXX the furnace vestebule on the right side on a rating label

Is the boiler currently fired when this happens????
Expert:  Douglas replied 5 years ago.
The reason I asked about the boiler is two fold.

If the boiler is firing when this happens, it may be lowering the gas pressure/volume to the furnace enough to cause the gas valve to shutter. If the boiler is not running, the gas pressure may have been increased so it doesnt shutter but may now be too high (rather than having the right volume).

I dont want to condemn the gas valve, that may just be a symptom to a different issue. Yes, you will hear a small clunk when the solenoids open and it runs, but if you are hearing it multiple times and enough to feel it in the gas line, I would want to know the gas pressure coming in (everything off, boiler on, boiler and furnace on). If that remains between 4 and 7 inches water column, then I would look at the voltage to the gas valve. If that's ok too, then I would replace the gas valve but again we wouldnt want to condemn it just shooting from the hip.

Gas pressure is measured with a manometer. I would have a tech look at it, he may not be able to recreate the issue you see in the morning, but can at the least check pressures and voltages. I just dont suspect voltage off the top because voltage shouldnt vary.

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