Can anyone help with a furnace issue? Older Rheem 90 plus gas furnace. The blower motor was not coming on and the board was not sending a 120 volt signal after startup, flame was good, but no blower motor. But the blower motor worked fine when fed 120 volts. So I got a new board. Now the blower is fine but the gas valve clicks rapidly almost like a machine gun, flame is spotty and goes off and on with each click. Voltage at the valve is not a steady 24 V but rather a wild rapid swing up and down. Do I need another control board? Could anything else be causing this wild voltage swing? I tested another (new) valve and it does the exact same clicking. Any help or suggestions are most welcome!
help with furnace issue
- Thread starter lance8614
- Start date
American Locomotive
Well-known member
Could be a weak transformer supplying 24v (unless the board has the transformer built in), or a defective board. Are you sure the thermostat wires aren't shorted?
Gas valve clicking on and off rapidly? I would check the pressure switches. And just not the switches as they might be fine, but what it the pressure going to them. I have seen a lot of techs toss in a perfectly good pressure switch, and the venting is wrong, or plugged up, or the condensate drain is plugged.
fitter30
Well-known member
Flame rod could be dirty. Needs to be pulled and cleaned along with pilot. Model of furnace? Tag will be in burner compartment on one of the sides
I doubt it is the flame rod. Flame failure would just kil the heating cycle and perhaps restart the burner throught the normal relighting process.
Yesterday my own furnce was doing what the OP mentions. The burner would stay it, but I could hear the gas valve clicking off and instantly back on again. One ot the condensate drains was mostly plugged. I checked the pressure at the pressure switches, one was fine, the other was floating all over, above and below the pressure set of the pressure switch. I cleaned the condensate drains and the pressure was about twice of what the pressure switch was looking for. The furnace ran all day today with zero issues.
thanks for the replies. I checked the pressure switches and they both close with a minimal **** on the hoses. The hoses are clear and it is easy to blow through.....don't think there is any blockage. New circuit board did not help the gas valve problem It is not getting a steady 24 volts, but a rapidly changing up and down current which goes to zero after about 5 min of operation. The thermal overload unit is good, zero ohms across the terminals. Roll out switches have not tripped. Running out of ideas....will check to see if the flue is clogged? any other suggestions? I really want to thank everyone who has responded.
Hi I had a new flame rod and put it in. The furnace is an old Rheem 90 plus it has the hot surface ignitor which is working normally, I put in a new circuit board and had no luck....still rapid clicking off and on, then goes dead after about 5-10 min of operation. I checked the two pressure switches and they operate easily with minimal vacuum and the contacts are good and clean. Zero ohms when they are closed, no continuity when they are open. Do you have any suggestions what or where to look or test? thanks very much
fitter30
Well-known member
If you can't figure it out and call a company want someone out with a flue analyzer to check for a bad hx.
fitter30
Well-known member
Hot surface igniter can glow but if it doesn't pull a certain amperage it will lockout.
MadMechMaster
Well-known member
Had a similar problem, ended up being spider webs in the burner orifices. First one to need cleaning was the last, right at the flame sensor. First 4 burners would light, the last wouldn't due to low gas. The spider must have moved to the first, since the igniter would get hot, but no light. The furnace would try a few times then give up, lots of clicking. Must have been the smallest spider possible to fit through that opening.
I just worked on one a 1/2 hr ago that would occasionally drop the 24v signal to the gas valve. Sometimes it would drop and come back so fast the flame would stay lit.
Ended up being a restricted exhaust. The pressure switch would drop out for a second. It would kill the gas but the control board never gave a code. Just showed a steady red light indicating normal call for heat
Ended up being a restricted exhaust. The pressure switch would drop out for a second. It would kill the gas but the control board never gave a code. Just showed a steady red light indicating normal call for heat
Where'd you get the replacement board? Alot of electronically-controlled furnaces use model specific parts. In other words, from the dealer. You put a generic board in (or one off of Amazon) and it might not work - even if they say it's the right board.
Just guessing like a doctor on the phone!
Just guessing like a doctor on the phone!
fitter30
Well-known member
Look at ebay, appliance parts houses. Search manufacturer with pt #. Might find the same number except the last 3-4 #. Need to look at all the terminals and labeling. Older furnace more than 10 years old might have to go with a aftermarket board possibly is going to take rewiring some controls. The other thing is timing or faults might be a different number which isn't a big deal most cases.
Crazyjake8493
Well-known member
I've had a similar one, but the exhaust was fine. Draft tested good with a manometer. Flame rectification was good. Ended up being failing contacts in the pressure switch, the 24v would flicker off and back on so fast my meter wouldn't pick it up. Put a bunch of cheap test lights across the leads of each pressure switch and saw the faint flicker on the one. Put it on my Fieldpiece draft simulator and the light would flicker around the rated draft. Threw me for a loop but it was only the pressure switch. Still working fine 4 years later.
Crazyjake8493
Well-known member
Have you actually tested the draft on the furnace, or draft tested the pressure switches other than sucking on it? Some can be weak right around their rated draft, could have a failed diaphragm in them.
Have you checked the flue for obstructions? Even a properly functioning pressure switch can flicker if your draft is reduced by an obstruction to where the switch is barely closing.
Dirty circuit boards can have weird issues but since you changed the board I'd rule that out.
Does the equipment have a good ground? Have you tested flame rectification with a meter? Flame rectification circuits are fairly sensitive and a bad equipment ground can mess with that.
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Thanks to everyone who replied. Two HVAC techs looked at the furnace and scratched their heads. I found the problem myself! The draft inducer motor, although spinning good, wasn't firmly attached to the back of the furnace. When I took it off, I saw that the gasket had more or less disintegrated, not surprising after 32 years. So I made a new gasket out of one of the wife's old yoga mats.....nice squishy rubber, cut to fit, a little gasket sealant, and all is good to go. My clue was, when watching the pressure switch, I could see that the little plunger that closes the switch was jumping around, in, out, in, out. So this told me that there wasn't a steady good vacuum pull coming from the inducer motor. Something to think about when faced with a poorly functioning furnace when everything checks out.
Yes, I found that the draft inducer motor wasn't properly sealed to the back of the furnace and needed a new seal (which I made out of an old yoga mat) and reattached it with new sheet metal screws. Two different HVAC pros (?) looked at it and had no answer except to suggest a new furnace for $8000! So much for hiring a trained professional. Turns out I was able to fix it for next to nothing, except for the new board which I needed anyhow because the old one wasn't sending 120 v to the blower motor once there was a steady flame for a minute or two. Thanks very much for your reply and interest.
Wrench97
Well-known member
You got 2 salesmen not 2 repairmen....
Crazyjake8493
Well-known member
A majority of the field (at least in this area) are either parts changers or salesmen. Very few true technicians with actual diagnostic skills.
well, after a lot of head scratching and tracing electrical circuits, I found the issue....the exhaust pipe (PVC for 90% furnace) wasn't properly slanted (towards the furnace) because one of the plastic hangers which are nailed to the joists had broken and the pipe was slanting away from the furnace instead of towards it. So water was building up in one of the elbows and that was cutting off the pressure switch which depends on negative pressure to keep the gas valve open. Duh....it only took me three weeks to figure that out. After a $2.29 plus tax visit to Home Depot all is good. So for you furnace guys and gals, check the drainage issues, like proper slant of the exhaust pipe, also the P trap which can be full of mold, etc when faced with an unusual case of no heat or intermittent heat. Don't be a ******* like me.....
People are so quick to throw parts at it without proving why.
I did HVAC service for years, and ten more years at a HVAC supplier. Service techs would come in all the time looking for pressure switches and then asked if they if they had checked the pressure going to the switch you would get a confused look. 9 times out of 10 the switch is fine, but reacting to some condition in the system that is out of spec. A manometer tee'd into the pressure switch line can clear up a lot of questions.
I did HVAC service for years, and ten more years at a HVAC supplier. Service techs would come in all the time looking for pressure switches and then asked if they if they had checked the pressure going to the switch you would get a confused look. 9 times out of 10 the switch is fine, but reacting to some condition in the system that is out of spec. A manometer tee'd into the pressure switch line can clear up a lot of questions.