Gas and Oil Home Heating Furnaces - Rudd/Rheem Gas HA Furnace Locks Out on Long Run

I have a 1993 horizontal furnace with integrated ignition & blower controller (Honeywell/Hamilton board w/2 LED) and hot wire ignition. You wouldnt know there was a problem if the thermostat was left at one temp. System goes thru all proper startup, run & shutdown sequences to heat house. But if you setback at night, you have trouble after the 1st morning cycle. If furnace needs to run more than ~ 20min, it will go off before thermostat is satisfied. (Stat LED is still on and 24VAC still on R&W.) System will immediately restart normal cycle if Stat is cycled to off then on.
I have the install manuals and wiring diags. It could be one of the limits in the safety chain. That should cause a lockout according to the op-description. Might also be that HALC (heat assisted limit control). I dont know how that affects the controller.
Since controller is inside fan box in attic full of blown fiberglas insulation, I cant easily run with door off. How do you check limit and LED status? How do you catch intermittent limit issues? I cant see that the controller is bad.

The most common cause of the symptoms you describe is a limit switch opening because of plugged furnace filters, an AC evaporator coil plugged by crud from not having a filter installed, the return air vents covered over or warm air vents closed.

Anything restricting thew flow of air through the furnace. Verifying that the blower motor is operating is worthwhile too.


Check that out and see what happens.

In keeping with the original problem...
(horizontal electric start w/IDB in attic)
Filters are clean. Both blower and IDB fans are working ok. Hot surface ignitor lights gas and must be ok as flame sensor as flame remains lit.

Diagnostics: The control board with the 2 LEDs is in the blower box with the interlock switch. You cannot observe them with the cover in place. You cannot operate the furnace with cover off even if you bypass the interlock cause the fan will suck in fiberglass. How does a service tech observe the LEDs?

Burner limits: (in series 24VAC) 3 thermals at burner (roll-out), 1 at IDB (vent safety) & 1 at heat exchanger (main limit).
IDB pressure limit: 1 static diff sensor 24VAC
I understand the operation of these fully.

Heat assisted limit: Mounted on main blower housing in blower compartment. It is in series with the main gas valve 24VAC.
It contains a heating element and thermal limit.
I dont understand its purpose. Can someone explain?

I tightened all grounds and re-seated fast-on connectors on limits. I can monitor limits with DMM while running. I looked to see if 24VAC ever appeared across a limit or string of limits, which would indicate one opened. I assume they have some hysteresis, but maybe not. Here is what I get now.

With thermostat (TS) set for long run:
Furnace starts a it should in this seq: IDB, ignitor, main gas, burner lights, ignitor off, main blower on, heat to house.
Runs 5-6 minutes, burner goes out, IDB stops, main blower stays on. I detect no change in burner limits. Heat assisted limit is hard to monitor. It looses 24VAC on both sides simultaneously. This could just be the PCB sensing the loss-of-flame and removing the source 24VAC.
Anyhow, ~1-2 minutes after flame out the IDB starts and the cycle repeats until the TS is satisfied. (This is with the burner unit cover off...)

Question now is... Previously it would lockout and require a reset to restart. Now the burner is going out and restarting. Is this how a gas furnace is suppose to work? My old house oil burner remained lit until the TS was satisfied or the tank went dry.
My gas water heater runs til satisfied also.

So... Do I still have a latent problem....?

If anything 24 volts shuts off upstream of what you tested, where you lost both sides of the 24 volt, then every safety feature downstream will go off. Maybe your test was that downstream test. You obviously have to test further back up the wire.

Usually the IDB will continue to run if say a pressure switch shuts off. But if the flame shuts down due to a thermostat or low voltage wire shutdown (possibly a short), or the high limit is tripped, the IDB will shut down. And soley in the case of high limiting, that said AND the blower motor will come on, to cool down the heat exchanger.

I'd be simply following the 24 volt circuit all the way back and see where the very first thing that is being lost; before or after the control board. Your wiring schematic (often found on the inside of the blower door itself) should be helpful to see the order. It will show L1 and you follow that all the way to see how it gets to neutral, especially through all those 24v safeties. Also observe how it enters the control board, and where a good place would be to test before, as it leads to the transformer, and at/after the control board.

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Diagnostics: The control board with the 2 LEDs is in the blower box with the interlock switch. You cannot observe them with the cover in place. You cannot operate the furnace with cover off even if you bypass the interlock cause the fan will suck in fiberglass. How does a service tech observe the LEDs?




I've never encountered that kind of defective design, and it obviously shouldn't happen. I don't suppose the cover could be on upside down?

If all else failed, I might be inclined to drill a 1" or so hole in the sheet metal so that the LEDs could be observed and then glue a piece of plastic or glass on the inside of the cover to provide visibility and prevent air from getting into the fan compartment through the hole.

Heat assisted limit: Mounted on main blower housing in blower compartment. It is in series with the main gas valve 24VAC.
It contains a heating element and thermal limit.
I don't understand its purpose. Can someone explain?

?

With this description of the wiring of this switch, it's clear that my earlier theory that it was a fan switch is wrong.

A limit switch in the fan compartment isn't unusual. It would commonly be used as a way of turning off the burner if the furnace overheats for any reason, such as a failed blower motor.

Why it would have a heater I can't imagine. I've never seen such a limit switch.

Is the heater coil in parallel with the limit switch and does it then go to the neutral/ground, allowing the heater coil to operate with 24 VAC across it when the gas valve is energized?

I would presume that the limit switch is then directly in series with the gas valve. But perhaps you can describe that in more detail.



Is this a limit switch that will reset itself or is it a manual reset with a push button?


It would be helpful if you would post the model of the furnace from the rating plate in the furnace and the model of the Honeywell ignition control.

System is Ruud Silhouette MOD UGLG-12EARJR
SER CE1D307 F1092-3113 135Kbtu 200 deg F Limit
45deg to 75deg rise.
The control PCB is a Hamilton 1012-83-800B with 2 green LEDs
and 3-rocker switches to set blower timing.

I have done the following tests:

1) Operated with blower door off and held in the interlock switch to run. I could observe the LEDs this way. System did the odd burner-off cycling. LEDs (power/OK) remained on and green always. Obviously PCB doesn't think something is wrong.

2)THe burner has that string of 5 24VAC limits in series. I bypassed all but the "main limit" attached to the heat exchanger. It is different than the "disc type". It is a square phenolic plate screwed into the heat exchanger. This bypass attempt made no difference. I was reluctant to bypass the main limit.

None of the limits in this system appear to be manually resettable. I am assuming they are all self-resetting.


The limit on the main blower (heat assisted) is not manually resettable either. It is a combination of a disc type 2-terminal limit and a 2-terminal heater apparently integrated into the body. The circuit is 24VAC. The limit switch is in series in one wire to the gas valve from the control voltage terminal on the controller PCB. The other wire goes straight to the gas valve. The heater is wired across the 24VAC on the controller side of the limit switch. It would be heating as long as 24VAC is sent to the gas valve.

I checked the condition of both sides of the evaporator and it is spotless on both sides. Not likely to be a restriction problem. I removed all filters with no change. I measured the differential across the evaporator to be 0.3 in water regardless of the presence of filters.

thanks for the help.

I found a diagram of a Ruud furnace circuit diagram that looks similar to what you have:

http://forum.doityourself.com/humidifiers-dehumidifiers/376664-he365-ruud-wiring-question.html

You have to page down a bit to find it.

It's not the best diagram, or even the right one. And there's a second circuit diagram to the right that would be more helpful.

But with that as some kind of guide, it looks like the HALC and the pressure switch are somehow in series, and the main limit is on a separate circuit with the flame rollout switches.

You were saying in your post that the wiring went directly to the gas valve, not through the pressure switch (?).

Very likely one of those switches is turning off the burners. You need to test each one to identity which one and verify that one is, in fact, shutting off.

If you don't want to bypass the main limit, you need to measure the voltage on both sides when the flame shuts off to seem if it is opening.

It would be helpful to post some pictures of the circuit diagram if you can.

I should have indicated what material I have in hand. Sorry. I have the complete install manual from the original house owmer. He was a bit anal in keeping every piece of paper on the house. (But to my advantage now...).

The unit you referenced is a CB5D302. 75K btu
The CB indicates it is a standing pilot type with a Honeywell
ST9101A control board. That circuit is somewhat different from mine.
Mine has that same HALC on the blower in series with the gas
valve, but the pressure switch has an independent circuit to the
control PCB.

I made some more measurements of operation this morning with
outside air temp of 30 deg & room temp of 60 deg with furnace
off for the night. I monitored the burner and blower operation
while measuring the outlet temp at one high flow register.
Furnace heated air to 85-93 deg in 3-4 min then burner goes out and fan remains on. After 2-4 min the burner re-ignites.
It does this ~5 times then shuts down & locks out with the thermostat still
demanding heat. Max register temp never gets above 93 with 65 in. This furnace is rated at 45-75 rise. I should get
~110 - 140 deg outlet temp I think.
Does this point to a bad hi-limit?
I measure the blower box to be 0.65 in. water (Magnahelic)
which manual says is 1300 CFM & ok.
Ill try to get pics from the manual.

Here is the installation manual for the new circuit board that replaced the one on your furnace. It might have some use in figuring out what is going on, but I'll be interested to see the circuit diagram for your furnace.


http://www.forwardthinking.honeywell.com/related_links/combustion/st9120u/install/69_0644.pdf

That board seems to be for standing pilot units. I think the current replacement "hot surface" board is a 62-24084-82. But it requires the addition of a flame sensor in my installation because my 2-LED control PCB uses the ignitor as the flame sensor.
If I did this correct, here is my wiring diagram.

http://www.hollywoodcontrols.com/ebay/Hamilton-1012-800_IBC.pdf

Well, I tried bypassing every limit but burner still short cycles.

1) Should a gas furnace ever cycle the burner under normal operation if the thermostat is in constant heat demand?

2)What is that HALC switch for?

This IBC board uses the ignitor to sense the flame. There is no separate flame sensor.
How should the ignitor be positioned in the flame?

I suspect the problem is the ignitor/sensor system now.
I did an experiment. The ignitor was positioned in the edge of the flame cone.

I moved it more into the flame. The burner lit and went out in 6 seconds. (That is the IBC PCB sensing timeout according to the manual.) So in this position the IBC didnt sense the flame.

I then moved the ignitor further out that the original position in the flame cone edge (brighter blue). Burner lit and remained lit until thermostat was satisfied. (~5 min)

I am now noticing a pattern. The burner cycle is shorter at night when the combustion air is cold and longer during the day when the temp in the attic is warmer. (Say 40 at night & 60-70 day).

I think the flame sensing is marginal and sensitive to the combustion air temperature. I think, with the ignitor/sensor just in front of the burner venturi outlet, there is incomplete combustion yet, and combustion air temp has an effect.

I need to wait for the house to cool to see if the cycling is gone with the adjusted position of the ignitor.

If I wanted to replace the ignitor, where might I start for this furnace?:wall:

Oh! Nothing quite like having the actual circuit diagram all of a sudden.

1) No.

2) The HALC is there to turn off the burner if the furnace overheats. Why it has the 24 VAC resistor or heater isn't clear to me.

In theory, the flame could touch any portion of the electrical part of the ignitor and the flame should be sensed by the circuit board.

Usually there's a way to measure the DC flame rectification current to find out whether there is sufficient current for the circuit board to detect, but I don't see any way to do that with this board.

You might look through the installation manual for a way to test the flame rectification circuit and see if something like that is listed.

It is fairly common for metal flame sensors to become coated with oxides so that they don't function as flame sensors until they are cleaned or replaced. I don't know if that can happen to ignitors, but installing a new ignitor to see if that solves the problem might be worth considering.

Having a replacement for the cold night when the one you have burns out is a good practice in any case.

Your temperature theory doesn't appeal to me. In an earlier post you were noting that the burner was shutting off when the burner was running continuously for 5-6 minutes or so.

The idea that it is functioning marginally and therefore erratically is a typical pattern observed when parts are slowly failing.

Since you have ruled out a limit switchs or pressure switch shutting off the burner, that leaves you with the hot surface ignitor as a likely cause of the problem or a bad circuit board.

Bad grounds can cause problems too, as can reversed power polarity.

Thanks for all the help.
I think this windy thread is close to a conclusion.
I see no way to test the flame sensing function with this setup and the manual is quite on it also. But it is clearly very sensitive to flame location. Maybe it is just getting old. With the tweeked position, the burner now stays on for ~10 min. Long enough for the heat exchanger to reach a stable temp and give ~120 deg air out. House heats fast. Burner cycles once for thermostat demand of 10 deg rise. I can live with that till I get a new ignitor.

Lesson learned: Study & inspect system and list what you think is not working and what is working. Fault will eventually be found in second list!

THanks...Beer 4U2

Does this point to a bad hi-limit?


Not if you are not getting 24 volts to the incoming side of that phenolic plate type hi-limit (these have a small disc limit mounted on shaft extended into furnace between sheetmetal and heat exchanger.) I just did one, and my length was 7 inches. Mine WAS bad. I had power in but no power coming out.

If you have no power in, check upstream, as I mentioned in earlier post. DOWNstream would head to the other 4 safeties. So take the OTHER wire and follow it.

I'll be interested to hear if a new ignitor solves the problem, which would imply that the ignitor can indeed get covered with oxides that prevent the flame sensing function from working.

Moving it around might just be exposing a small area that's not yet contaminated. At least that's a plausible guess.

Another thing to try when you get the new ignitor would be to remove the old ignitor and GENTLY use a fine sandpaper on the flat, easily accessible surfaces of the ignitor --- then reinstall it and see if it works.

A little cleaning might get it to work if my theory is correct, but those ignitors are very brittle and easy to break, so I can't recommend you try cleaning it until you have a replacement in hand.

If it happened once, it might happen again. Finding out if cleaning makes it work would give you a spare against problems in the future.