Continued Heating Woes!

[JohnB]

Originally Posted by mainah

Well there has to be an interrelation due to the time lag between the fan coming on and an ign cycle. I guess the bottom line if it fires and heats most of the precycle works the problem is it does not know how to turn off pulling the fuse will stop it but there will be no post purge cycle.

Yes, I agree. This "timing relay" by the way it heats the metal dic switch and the way it cools the metal, creates a unique on and off delay time delay relay. But it may not be creating both the "pre burn" purge and the "post burn" purge. For sure, it is controlling the post burn purge delay.

See if I have this right and point out what I may have mixed up. Again, this is for the older per 7/2001 furnaces with this time delay relay as being separate from the PC board.

On page 16 it states this for the when the T stat calls for heat.

The relay allows current to pass to the motor by closing a switch within the relay. Voltage from the thermostat activates the relay to turn the fan on. This takes 1-25 seconds.

While this is a delay from the time the T stat calls for heat, it is not really a "pre burn" time time delay. It is simple a byproduct of the way this heating time delay relay works. This is a "pause" from the time the T stat calls for heat until the blower starts to run. And it can change from 1 second to 25 seconds.

I now believe the ignition board even on the older furnaces is where the "pre burn" delay comes from. By the way the furnace is wired, T stat power (+12 vdc) is sent to the time delay relay to start the fan and at the same time, power is sent to the normally open sail switch where it stops for the moment. Once the fan is up, power from the sail switch flows to the normally closed high temp switch, and then onto the ignition PC board.

When the blower is up to speed enough, the sail switch closes. I see this happening fairly quickly. But we know we have a pre burn delay and the burner does not fire instantly. Once the sail switch allows the board to be powered up, it starts timing the pre burn purge (15 - 17 seconds) as the blower is already running. Once the board on delay completes, then the trial for ignition sequence starts.

Here is from page 16 in the service manual about the board powering up.

As power is applied to the circuit board, the system does the following:
1. A timing circuit allows the blower to purge the chamber (15-17 seconds)
2. The board supplies current to the gas valve and causes it to open.
3. As the valve opens, the board sends a high voltage spark to the electrode at the burner. The board detects the presence of a flame. If the flame is not
sensed after approximately six seconds, the board will lock out (three try for ignition, one hour lockout and then three retry), shutting off power to the
valve.

So this gets the blower up and going, the preburn purge done and the burner up and going. I do not know if we really have confirmation if the front end very first preburn delay has occurred or not. Bottom line, the very first time the fuse was plugged in, it should of given a pre burn delay before burner fired. This is even if there is a wiring problem keeping the T stat wire hot all the time. If there was never a preburn since powerup, then yes the PC board has a time delay issue but the ignition and flame sense seems to be working.

Now to the post purge delay. Again on the pre 7/2001 furnaces.

On page 24 it states this for the Time delay relay.

This component is commonly referred to as a time delay relay. The same relay is used on the 7900, 8500 and 8900 series furnaces.

FUNCTION - The relay has one primary function.
• to purge the plenum of heat and the chamber of any unburnt gases after each heating cycle.

OPERATION - The motor voltage path of relay is normally open. There should always be voltage from the circuit breaker to the relay terminal of the circuit breaker. There should always be continuity between the thermostat terminal connection and ground terminal connection or the relay.

Only when the thermostat contacts are closed is voltage supplied to the thermostat terminal of the relay. This voltage heats a coil in the relay body. In approximately 20 seconds this heated coil causes a bimetal disc to close. Voltage now passes through the relay and on to the motor, which in turn should allow the furnace to ignite and start a heating cycle.

When a heating cycle is complete, the contacts of the thermostat open and
voltage ceases to the heater coil of the relay. In approximately 45 - 90 seconds, the heater coil cools down, the bi-metal disc opens and voltage ceases to the motor as well.

AMP Draw - The relay should draw no more than 1 amp. If the relay should draw more than 1 amp, it will burn out the anticipator.

Once the T stat signal stops calling for heat, the wire feeding the timing relay and the power to the sail switch goes dead. The PC board is powered down and the timing relay starts cooling off and keeps the blower running until the disc switch cools enough to open the relay contacts. This is a off delay relay. That off delay is the post burn purge time.

With now thinking through all this in this much detail, if the furnace will not shut off until the fuse is pulled, this could also be a simple wiring issue on the T stat wire, somewhere. If that T stat wire is crossed to being hot all the time, there would only be 1 pre burn purge cycle, the blower would run all the time, the over temp switch would shut down the PC board and the burner when it was too hot. As the over temp switch cooled down, the over temp would close, power up the board and go back into trial for ignition. You would never get a power down of the timing relay, so you never get a post burn purge as the blower never stopped running until you pulled the fuse.

I think the PC board is OK. The only way it gets a signal to operate is through the sail switch and the over temp switch. And that wire originates at the input terminal of the timing relay which gets its power from the T stat wire.

This now all comes back to, why is the T stat wire at the furnace hot all the time when the fuse is plugged back in? The HVAC guy should be able to back track that with a voltmeter and unplugging the T stat wire off the timing relay. If he pulls that T stat wire, the the whole furnace may start the shutdown sequence.

I believe I now have this clearer in my head. I'm hoping we can get a report back what the guy finds out.

Thoughts?

Thanks

John

[mainah]

I'll ask the question again once the fuse is replaced does it go through the normal restart?It should do nothing then the fan should start for a bit then the gas valve should open with a "clunk" and burner should fire. Pulling the fuse will defeat any post heat procedure. If the pictures match yours the two small wires under the wire nuts are the T stat wires one white the other red, in the photo the red is landed with the larger blue wire.

[Tinstaafl]

Following up on mainah's question: Minor detail that hasn't really been addressed is whether the furnace is actively burning gas and heating during the entire time that the fuse is in place. Even with a shorted t-stat wire, eventually it should get hot enough for safeties to kick in and shut it down.

[JohnB]

Originally Posted by Tinstaafl

Following up on mainah's question: Minor detail that hasn't really been addressed is whether the furnace is actively burning gas and heating during the entire time that the fuse is in place. Even with a shorted t-stat wire, eventually it should get hot enough for safeties to kick in and shut it down.

Just clarifying for you, on the 85-III with local sense, the over temp switch (actually the high limit switch in Atwood's words) opens when the heat chamber gets to a certain temperature. On the 85-III model this is 190F. This limit switch when it opens, shuts down the PC board on this vintage furnace which shuts down the burner and gas. The blower keeps running blowing out heat.

Once the heat chamber cools enough, the high temp limit switch closes once switch differential temp is reached and again it will fire up the PC board, trial for ignition starts over, after a time period of pre burn purge, the gas burner starts back up again. It will do this as long as the T stat signal "wire" keeps sending voltage to the board and the sail switch is closed. Basically it will keep cycling the burner on and off with the high temp limit switch non stop until T stat power signal is gone or the blower shuts off. There is nothing in the controls to actually shut the entire furnace down once the high temp switch limit opens. It was not meant to be used that way.

Pending the BTU sizing of the of the furnace, the amount and size of the heat ducts, the T stat response time, along with how far away the room temp is from the T stat setting, it is common the high temp limit switch will cycle the burner on and off a few times before the room temp reaches T stat setpoint is satisfied and the T stat open up and allows the furnace to cycle down.

Hope this helps

John

[JohnB]

l will get a better pics tomorrow to show this. This is what I am seeing. See if you agree.

In the 85-III and the 85-IV, (both with local sense) there are 4 wires coming out of the furnace to interface with the camper. This is the plug on the side of the furnace that has a pigtail to connect to the camper

1. A blue wire that is + 12 volts sent from the furnace to the T stat. (furnace wants its own power to be sent to the T stat)

2. A blue wire that is a return from the T stat. (the T stat signal back to the furnace, which on the 85-III and the 85-IV, both with local sense is + 12 volt DC)

3. A yellow wire that is the DC - (ground) for the furnace

4. A red wire that is the 12 DC + (hot) power supply for the furnace.

[mainah]

Ok removing the small red wire from the blue will eliminate the wiring and the t stat if it continues to run then something is up with the board thinking there is still a demand for heat.

[JohnB]

Originally Posted by JohnB

l will get a better pics tomorrow to show this. This is what I am seeing. See if you agree.

In the 85-III and the 85-IV, (both with local sense) there are 4 wires coming out of the furnace to interface with the camper. This is the plug on the side of the furnace that has a pigtail to connect to the camper

1. A blue wire that is + 12 volts sent from the furnace to the T stat. (furnace wants its own power to be sent to the T stat)

2. A blue wire that is a return from the T stat. (the T stat signal back to the furnace, which on the 85-III and the 85-IV, both with local sense is + 12 volt DC)

3. A yellow wire that is the DC - (ground) for the furnace

4. A red wire that is the 12 DC + (hot) power supply for the furnace.

Here are the pics promised showing the furnace to camper interface.

These are from a 2004 T1950




This one is from a 2005 T2363


Pending the floor plan, getting to the wire nut connection may or may not be possible, easily. On some floor plans you can actually reach all the way to the wire nuts with your arm as nothing is directly next to the wire bundle.

But, like on the 2005 T2363, there is a cabinet side right next to the furnace. I could not reach all the way to the back to touch the wire enough to attempt getting a wire nut on or off. The furnace may have to be pulled out to do it in this case.

There are other options. Some are:

Pending the model of the furnace, if it is the one from this post, a 85-III vintage, you can pull the blue wire off the thermal timing relay. It is a spade push on connector. That blue wire is the 12 volt power return back from the T stat. See here on the top right.



When you pull that blue wire off the thermal timing relay, and you power up the furnace by plugging the fuse back in, and having the furnace on switch on, the furnace should "not" run. Meaning the blower will not start and the ignition board should not start, no gas valve and no ignition spark.

If it does start running the blower, then the thermal timing relay has it's contact welded closed and the relay is bad.

And if the board starts trying to fire the gas valve or the ignitor, then the board has internal issues as it is ignoring the sail switch contact as it should be open with no blower running. Unless the sail switch has issues too allowing power to pass to the ignition board.

Odds of a thermal timing relay going bad along with a PC board at the same time, I think would be low. Not impossible, but I would say low.

Odds are higher something is sending power through the T stat wire from upstream of the furnace from some crossed connection IF the blower does not run with the blue wire pulled from the timing relay but the blower does run with the blue wire attached.

Hope this helps

John

[JohnB]

Originally Posted by mainah

Ok removing the small red wire from the blue will eliminate the wiring and the t stat if it continues to run then something is up with the board thinking there is still a demand for heat.

I 100% agree about unhooking the T stat power return wire going back into the furnace. That is the place to start.

Think about this, the blower runs now in the state it is in. And the blower control is separate from the PC board on this older model. The blower gets its run power through the thermal timing relay contacts closing which the relay is activated directly by the T stat. Something is powering up the thermal relay to run the blower unless it's contact is welded closed. The blower running is the first tell tail of a problem with the T stat unhooked or off.

What are your thoughts thinking the board would have issues if the T stat wire is unhooked and the board fires up? I'm not following that. Since something is powering up the thermal relay to run the blower, the sail switch will be closed with the blower running. With the sail switch closed, the board will get signaled to powered up from the same T stat wiring issue and start the trial for ignition sequence going.

The way the wiring is, the blue T stat return wire lands on the timing relay and then blue wire proceeds onto the sail switch. Out of the sail switch wire it is now a white wire that connects to the high limit switch. And still a white wire out of the high limit switch goes to the PC board as a run signal. With the blower running and the sail sith closed, the boards gets signaled to run.

It's not to say the board and the timing relay both have issues, but the likely suspect seems to point to the "something" sending 12 volts + down from the T Stat wiring. That T stat wiring needs to be rung out first, then go from there.

Hopefully we can get some kind of feedback on this from Aariel. Curious on the issue found.

[Aariel]

Thank you for these instructions. Currently, my HVAC guy is reading this and I'm waiting for him to come back and we'll test the wires more. Just the clarify, the furnace blows full force without stopping the whole time the fuse it in. The longest I ran it is about 1 1/2 hrs. It does not cycle. It just starts and stops with the fuse.

[mainah]

Originally Posted by JohnB

I 100% agree about unhooking the T stat power return wire going back into the furnace. That is the place to start.

Think about this, the blower runs now in the state it is in. And the blower control is separate from the PC board on this older model. The blower gets its run power through the thermal timing relay contacts closing which the relay is activated directly by the T stat. Something is powering up the thermal relay to run the blower unless it's contact is welded closed. The blower running is the first tell tail of a problem with the T stat unhooked or off.

What are your thoughts thinking the board would have issues if the T stat wire is unhooked and the board fires up? I'm not following that. Since something is powering up the thermal relay to run the blower, the sail switch will be closed with the blower running. With the sail switch closed, the board will get signaled to powered up from the same T stat wiring issue and start the trial for ignition sequence going.

The way the wiring is, the blue T stat return wire lands on the timing relay and then blue wire proceeds onto the sail switch. Out of the sail switch wire it is now a white wire that connects to the high limit switch. And still a white wire out of the high limit switch goes to the PC board as a run signal. With the blower running and the sail sith closed, the boards gets signaled to run.

It's not to say the board and the timing relay both have issues, but the likely suspect seems to point to the "something" sending 12 volts + down from the T Stat wiring. That T stat wiring needs to be rung out first, then go from there.

Hopefully we can get some kind of feedback on this from Aariel. Curious on the issue found.

Yes if it does not go through a prepurge with the fuse replaced means the board is convinced the all is fine not only is it wrong it is dangerous.

[JohnB]

Originally Posted by mainah

Yes if it does not go through a prepurge with the fuse replaced means the board is convinced the all is fine not only is it wrong it is dangerous.

Agreed. I just do not know if we have it confirmed yet a prepurdge never happened upon fire up.

[JohnB]

Originally Posted by Aariel

Thank you for these instructions. Currently, my HVAC guy is reading this and I'm waiting for him to come back and we'll test the wires more. Just the clarify, the furnace blows full force without stopping the whole time the fuse it in. The longest I ran it is about 1 1/2 hrs. It does not cycle. It just starts and stops with the fuse.

Hi Aariel,

We may be confusing you on the sounds we are looking for. There are 2 kinds of sounds that come from the furnace. And since you may have just bought the camper and the furnace never worked right, you may not know what normal sounds like on these 2 sounds.

I'll try and describe the 2 sounds.

1. The blower running. This sound we believe you are hearing. It is the fan motor pumping air out of the vents. You can hear it and feel the air coming out. In your case the way the furnace is right now, the blower may run non stop once it starts until you pull the fuse. At the very start of putting the fuse in, the air coming out would be cold or room temperature.

2. The gas burner running. This sound is different then the blower. This sound can wait a few minutes after the blower is up and running the "first time" to hear it over the top of the blower running. Heat also now starts coming out of the ducts. The furnace running with a blower and the gas burner running at the same time is very different then just the blower by itself. I will attempt to explain the sound we are looking for and you to listen for.

You put the fuse in and the blower is now up and running. Blower noise and moving air is coming out of the ducts.

After a few minutes of putting the fuse in, a "whoosh" sound will start and be adding to the blower noise at the same time. You "might" also hear a "clunk" once when the whoosh starts. The whoosh was the gas burner starting and it will run for a long period of time with that woosh sound along with the blower. After many minutes, depending how cold the camper is, the whoosh may stop, you might hear a clunk or click again when the whoosh stops, but the blower keeps on running all the time.

The woosh is the gas burner. The clunk and click which you may or may not be able to hear unless you are trained to listen to for it, was the gas valve turning on and off.

Now armed with those sounds, can you tell us if you can hear the the gas burner starting at a different time then the blower when you first plugged the fuse in?

And does the burner sound comes and go at times with blower still on all the time? There would be many minutes between the burner being on and then off and then back on again.

Yes, the furnace is running all the time, but if it changes sounds while it running, then we can tell what is going on or not in the background.

Hope this helps

John

[Aariel]

Just an update on Heating Woes... I found a person who repairs campers who tested the wires, and he said the control board is bad. I am waiting for a new one to come in the mail; hopefully replacing the control board will fix the problem. Thank you for all the great info.

[Aariel]

And btw, I listened harder to the heater when I plug in the fuse. In fact, the blower comes on first and then the heater kicks in. I don't think the heater ever goes off, though, till I pull the fuse.