Rheem Imperial 80 burner short cycles
I have Model RGDC-07EC-GS. When there is a call for heat, the inducer fan starts, pressure switch is activated, pilot is lit for 1 minute, burner starts, the fan switch rotates and turns the fan on.
The problem is, the burner only stays going for 1-2 minutes.
i have cleaned the thermal couple. I don't believe the burner would start if it was not functioning.
The limit switch is not triggered when the gas valve closes
The voltage between TR and TH is 24v When the burner is on, it drops momentarily, when the burner turns off, then goes back to 24v for fifteen seconds before dropping to zero.
when the gas burner turns off, the inducer fan stays on because the system still sees a call for heat. The blower will run until it cools the fan switch enough. But the pilot restarts after 2 minutes if the call for heat hasn't been met. The whole process then repeats.
i measured the voltage on the blue wire between the control board and gas valve.
With pilot on = 24v
With burner on = 7.1v
With both off (no flames) = 1.2v (this is when inducer is still running)
does anyone have suggestions on a cause or solution?
Thank You,
Michael
ps I've swapped both relays to no avail.
Comments
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I can follow the sequence of operation logic from the original diagram. There is one original relay that is not properly mounted but that should not cause the issue you are talking about. This other white control box is not original and I would like to know what it is. I can't tell if that is part of the problem because I am unfamiliar with its purpose.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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that white box is a PEK power extender kit for my ecobee. It sends power to the thermostat thru the existing 4 wires. I have removed all those wires and jumped the furnace to trigger a heat cycle and the problem still exists. So thst PEK isn't the cause.
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So that PEK allows the 4 wires to act as a 5th wire to the thermostat to produce a common to power the ecobee or something like that. With that eliminated and the R to W jumper the furnace acts the same. That would have been my first step also.
The age of the furnace lends itself to consideration of a new furnace if this issue is expensive to resolve. Now we need to pinpoint the place where the electronic gas control is losing power momentarily which drops out the main flame and needs to go through the relighting cycle or trial for ignition cycle.
The Robertshaw SP710A and the 7000 gas valve work with each other to make that system operate safely. That ignition control is no longer available. From what I understand about this system you would need to convert the ignition control system with a kit that includes a new gas valve and ignition control system. It is somewhat time consuming and should only be completed by a professional tech with the proper test meters and equipment to verify the combustion is within specifications after the parts are installed.
That said, I believe it is time for a new furnace since that furnace is from the 1980s
If you want to keep it operating though, I can recommend a suitable kit for upgrading the gas ignition and valve system. Just know that after you spend all the $$$$ on the new parts and the time to replace them on that old furnace, there is no guarantee that the heat exchanger (HX) will last long enough to realize any benefit from the repair. Once the HX fails it is time for a new furnace.
Do the research and ask ChatGPT how long residential furnace heat exchangers are expected to last.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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edit: looking at the schematic, the 24vac dropping to the gas valve which in this case feeds the ignition control, means it isn't a problem with proving flame if i follow the schematic right.
it may just be failing to prove flame periodically and leaning up the pilot assembly and grounds or replacing a chaffed or burned wire might fix it0 -
I realized it is an old furnace and I will certainly consider the value proposition to fix vs replace. My challenge is truly identifying the cause of the problem . I don't want to just throw parts at it. It is a simple system compared to today's new units. I'm hoping to get help problem solving that actual cause of the short cycling.
I'I've read that gas valves have a 100k cycle limit. Could the gas valve simple be cycling because of hitting some 'limit'?
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i'd use a test light or analog meter hooked to various points to see where the power is dropping. since 24v to the ignition control drops it is something like the pressure switch or some other safety that is dropping out. an analog meter or test light will respond faster to a transient change than a digital meter which only tales a couple readings every second.
if i am reading the schematic right, it is happening before it proves flame so it is some other control.
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it could even be something like the blower door moves around as it heats or when the blower comes on and makes a dirty/worn/misaligned blower door interlock switch pop open briefly
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The 100k cycle limit just means that if you ask the manufacture how long do you expect you part to last, they will not give you a time but an amount of on and off cycles. So if your furnace turns the gas on and off 1 time per day, then the part will last over 270 years. If your furnace makes the gas valve cycle off and on every 3 minutes the entire heating season, then the gas valve will last about 1 heating season. There is no magical counter inside the part that counts down and automatically stops when it reaches the 100,000th cycle.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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i can almost guarantee that heat exchanger has small cracks…
So it's time for a new furnace
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I believe this control is called a pilot relight control. I can't be 100% sure but that might mean that this unit has a standing pilot. If the pilot drops out then the flame sensor will cause the sparker to relight the pilot. If there is no flame signal measured at the pilot burner, for any reason, the main valve will not open. This was manufacturered in the beginning days of the Department of Energy, so there were all kinds of things being invented to reduce fuel usage like Auto Vent Dampers and a surge on clock thermostat sales. Until this time in our history, just about every residential gas heater had a standing pilot.
The retrofit kit to replace the gas valve and the ignition control is the Y8610U kit. The test to determine if the control or the gas valve are the problem would be to place a meter set to measure 24 VAC on the T1 and T2 terminals of the RobertShaw ST710A and watch it when the flame goes out. Do you see a drop in voltage to the ST710A indicating the power was interrupted? If not then the problem is with the obsolete RobertShaw control or gas valve. This would mean replacing the control with the upgrade kit.
If there is a momentary interruption of the 24 v. to the ST710A control then you need to locate that interruption one step at a time until you find it. If you place your meter leads on either side of a limit switch when there is power going through the closed switch, you will read zero (0) volts. If that limit opens during the burner operation the meter will read that open circuit as a voltage reading for the amount of time the limit switch is open. Here is the illustration from my one day seminal on Electric for HVAC three slides ask the question "Will we read voltage?"
Measuring voltage at the load with the switch closed (turned on) you will get a voltage reading. With the switch open (turned off) you will not get a voltage reading and the load will not operate. On the bottom illustration you get the opposite result. Look closely at the bottom right illustration to see that the load will conduct the electricity to the meter on one side of the switch that is open the the return path will conduct the electricity back to the source. That is why the meter will read voltage when the switch is open
To determine what limit or other switch is opening when the burner goes out you place your meter on each limit, one at a time, until you find the limit that opens when the failure happens. That is what is causing your issue. Here is an illustration of three of the switches you may want to check.
If the top meter reads voltage when the burner stops, then that relay that is hanging loose in the left side of the cabinet is what is causing the problem. If top meter does not read voltage during the failure then move to the middle meter. If the middle meter reads voltage when the flame goes out then the pressure switch is the reason for the main flame outage. If that does not read any voltage in the middle configuration then move to the next location and continue to follow this procedure until you locate the contacts that are opening.
Once you locate the contacts that are opening, now you need to determine if that set of contacts are doing their job properly or if there is a problem with the contacts when they should not. For example if you determine that the pressure switch is the culprit, you need to determine if the pressure switch is defective or if it is not defective and the hose connecting the switch to the combustion blower is clogged or damaged. You will need a manometer to see if the proper pressure is being exerted on the switch by reading the pressure rating on the pressure switch, and measuring the actual pressure
If the pressure switch is rated a 0.6” WC and the line only has 0.5” WC in the tube from the combustion fan to the switch, then the switch is not the problem, the fan is the problem. When was the last time you cleaned the fan? Is the line from the fan to the switch cracked or blocked?
That is how you determine the location of the problem. And solve the problem.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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Crystal Ball?
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When the burners go out, is the Fan/Limit control at Limit?
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Can you provide more details about this ?
" i measured the voltage on the blue wire between the control board and gas valve.
With pilot on = 24v
With burner on = 7.1v
With both off (no flames) = 1.2v (this is when inducer is still running) "
I see the blue wire in the picture but it is poorly documented on the wiring diagram. 7.1 Volts sounds like a poor connection in that vicinity assuming the 24 VAC power remains good.
The 7.1 Volts was measure between what two points ?
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Is the Blue wire even used with that gas valve ?
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
I measured between ground and the blue wire where it plugs into the control board at the bottom of the picture. That wire goes to the gas valve connector and then to the black disk on the bottom of the gas valve. (I don't know what that part is called)
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No. The limit switch doesn't click or rotate enough to open that circuit. I do see the voltage change briefly on both ssides of that switch though. One of those black wires goes down to the control board.
could it simply be the limit switch is bad/tripping before it should??? There have been times where the fan switch sticks and the fan doesn't turn off after everything has cooled down.
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I found this (below), it appears the three wire portion is the Pilot Valve 'Pick' and 'Hold' coils. Don't know which wire is which, maybe only the hold coil needs to be energized after the main valve is open. Although the 7.1 volts still seems odd.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
you have to measure between the 2 sides of the transformer secondary, not ground. it may or may not be referenced to ground.
the draft proving switch, the limit on the fan/limit, and the interlock on the blower door are the only safeties that could interrupt the 24vac to the gas valve. the flame proving and ignition is done by the module connected to the gas valve.
when the flame goes out, do you lose 24vac briefly right at the transformer secondary or does that stay powered?
er, i was reading the incomplete schematic slightly wrong. measure t1 and t2 in to the control board. see if you see a brief drop in the 24vac there.
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To start I'd simply measure between the two Red arrows to see if the 24 VAC to the Ignition Control module is steady when the burner drops out. This gives you troubleshooting direction.
One side of the transformer should be referenced to ground or to the units cabinet (Green circle), it should be verified.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Agree with @109A _5 about where to start. If the 24 VAC power does not waver at that location when the flame goes out, then you are finished. The problem is within the Ignition control and the gas valve system. Since those parts are no longer available, You need to decide if it is time to replace the parts with the Y8610U system or replace the furnace.
If you find that the power does fall out for a short second, then you can assume the ignition control system and the gas valve are operating properly and start looking at the controls that lead up to the two Red arrows in @109A _5's photo of the diagram. Once you have verified that the voltage drops out at that point you need to go to the rest of the circuit leading up to the Ignition control one step at a time as I indicated in this illustration.
Along with all the information that I posted with it.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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@109A_5 said: "Although the 7.1 volts still seems odd."
This is often the case with Pick and Hold circuits on the pilot solenoid. It is similar to the old series 10 thermostat circuits that will pull in a relay when W and B get power but W and R powered will not pull in the relay coil. The Hold circuit gets a lower voltage from the center tap of the transformer secondary coil while the Pick circuit will get the full 24 volt secondary coil to open the valve with the stronger solenoid. You would get it if you had the entire wiring diagram referenced in that info you found.
Sometimes the hold circuit has some other load added in series with the hold coil. If you measure from the second load to ground you may find a lower voltage. If you measure from the first load to ground you will get the full 24 VAC reading on your meter. There are several other explanations I can think of to explain 7.1 Volts on that blue wire at different stages of the cycle. I would not worry about it. I would verify that you are getting 24 V. uninterrupted at the Red arrows first. Once you have verified that is or is not the point of the flame drop out then move on from there.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
0 -
I'm wondering if the 7.1 Volts is just simple Transformer Action or Mutual induction of the two windings wound around the same core of the solenoid. This would act as a transformer, the voltage difference is due to the turns ratio. The Pick coil being less turns of heavier wire.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
I tested between these 2 wires. For 2 burner drops. The first time the voltage was quite steady and moved from 24.8v to 27.9v when the flame dropped.
the second time when the burner flamed out the voltage was at 24v, Spiked to 96.3v then dropped to 6.2v then stabilized back to 24v Again.
so does this imply that it ccould Be the limit switch since the black wire is coming from the limit switch?
Thanks,
Michael
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it means the ignition control is shutting the burner down for some reason. The spiking to 93 makes me wonder if the ground path from the pilot burner to the ignition control is good.
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I'd repeat the process a few more times looking for more consistent data. Are you it was not 96 mV or 0.96 VAC ? A sample of 2 with erratic data may not be helpful. Are you sure the test probes had a good connection ?
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
To possibly get better data; If your meter is auto ranging, disable the auto ranging and lock it down to an appropriate range. Also use alligator clip leads if you have them to make the connections, old metal can often makes poor connections to meter probes.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Yup
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OK a bit of an update. I see voltage drop at T1, across both sides of the fan limit switch, and at tthe pressure switch. After looking over the schematic with and electrical engineer, we jumped the black to brown wires on the pressure switch and the burner doesn't stop running after 2 minutes. It ran for 10+ min before I turned it off. I also hooked up a Manometer to the inducer. When the inducer first turns on I see .71 in of water. When the pilot turns on the pressure drops a little. Once the burner is on the pressure continues to drop until .51 Or even .46 When the flame goes out.
So I think it's this pressure drop that's causing the burner to turn off. What are the reasons the pressure would drop as the furnace gets hotter?
Thanks
Michael
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Pressure drop, bad inducer motor or its fan, inducer pressure port restricted with debris, blocked flue, cracked heat exchanger.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
the inducer motor does get quite warm. I already cleaning the pressure port, there didn't seem to be any blockage there.
Michael
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did yo make sure the inlet and out let vents are clear and that condensate isn't backing up somewhere?
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When the inducer runs and the flame starts either pilot or main flame the air pressure will drop. The inducer starts out exhausting just air. When the flame comes on you exhausting more than air.
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Low low of a pressure is 'acceptable'? I don't hear my pressure switch diaphrag drop out. I added a Pic of the pressure switch to show the sticker. It looks like it says .75 but there are no unit on the label or I don't know if that's psi or in of water.
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I believe it is in inches of Water Column.
Seems it is right on the edge.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Yes " of water
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