New HVAC Issue - Advice Needed

[onewhippedpuppy]

So with the help of the Pelican BBS, I managed to resolve my A/C issue. Now my heater downstairs is giving me trouble. It's a 19 year old Coleman 70, traditional natural gas forced air furnace. At some point it's had a Robertshaw ignition controller retrofitted, I assume probably to convert it from a pilot-light burner to automatic ignition.

Anyway, when commanded by the thermostat it fires up as it should, but once the fan kicks on the trouble starts. Typically, I can hear the relay in the Robertshaw ignition controller click, the burner will flicker, and the fan won't start. Sometimes this clicking and burner flicker will occur several times, then the burner will shut off and the fan will start. Other times, the ignition controller will click and everything will shut off. Sometimes the fan will start after a long delay (1 min or so). Sometimes the burner will re-fire while the fan is running, and work as usual. Other times the unit will shut-down entirely, and not re-fire until the power or thermostat is reset. This can lead to a chilly house.

Summary: the unit seems incapable of heating up and running the blower motor for an extended period of time, and can't seem to start the blower motor while the gas is still burning on initial start-up.

My first instinct was the gas valve, but it seems to be working as commanded by the ignition controller. So now my #1 suspect is the Robertshaw ignition controller. What say you Pelican braintrust?

[rick-l]

Have you run steel wool over the front of the flame detector? The burning gas acts as a diode.

[onewhippedpuppy]

What/where is the flame detector?

[rick-l]

It looks like a piece of copper wire that goes into the burner flame

[911boost]

Not sure if this applies or not, but those are the exact same symptons I had with my propane heater in my camper.

Long story short, there is a "sail" switch, that moves with the fan turns on, this switch tells the gas vavle to open. The switch wasn't moving far enough, so the burner would click, but it would never light since there wasn't gas. I messed with it for a long time, since it made me very nervous. I replaced the sail switch with a similar one that had a larger "sail" surface area, and now it works like a champ.

Like I said, not sure if it helps or not, and messing with natural gas / propane heaters can be very dangerous.

Bill

[billybek]

It is hard to say what is happening from your description.
Is the module in control of the fan functions as well as the burner control?
On older furnaces there is usually a fan and high limit combination switch. It sounds like from your description that the high limit may be weak.
The limit switches are typically 110 vac, so if you put a meter across these contacts and see a voltage, that would indicate that the switch is open.

[wilke3169]

Gas furnaces can have many different types of controls. Often on units this age there is a fan limit switch. It will turn the fan on when temperature is reached but will also shut the burner off if the temperature gets too high in the furnace. Others have a timer that brings the fan on after a set period of time.
The sail switch that Bsiple is talking about may be on a forced draft furnace and is used to prove there is indeed forced draft before it will allow the burner to ignite.
One thing to look for is a change in the flame when the blower comes on. This could indicate a bad heat exchanger which is reason to condemn the furnace and get a new one.
You can check this by starting the furnace and watching the flame as the fan starts. If the flame changes at all there is a problem.

[onewhippedpuppy]

Bill, I believe the ignition controller box is activating the fan, it clicks when the fan comes on. Are you saying a bad limit switch at room temperature will be open or closed?

At the root of the issue is this: the fan motor won't come on with the burner operating. The relay clicks, the gas fluctuates, but the fan does not come on. I'll try to break it down to make it more easily understood.

1. Gas burner ignites and heats
2. Relay clicks in ignition controller, gas fluctuates, fan DOES NOT operate (happens approximately 3 times)
3. Gas burner shuts off
4. After a brief pause, fan activates
5. Fan runs for a short period of time, then gas burner lights
6. Heater runs for a while, shuts off, then does not come back on

That's another point, after it runs through one cycle, it does not come back on. I can normally tell it's not running because the house starts getting cold. Would the heater not coming back on also tend to implicate the ignition controller?

[Brian 162]

It sounds like either it's the motor or the control that brings on the motor.

If there is a fan switch on the thermostat try putting the fan switch from auto to on so the fan runs all the time.

Then see what happens on a call for heat.

[onewhippedpuppy]

Brian, good suggestion. I gave it a shot, the burner fired up as it should, ran for a while, the relay clicked a few times and the gas flickered, then the burner shut off. After the fan ran for a few minutes the burner came back on and the cycle repeated itself. Bad upper limit switch?

[rick-l]

http://www.youtube.com/watch?v=NjOsXv4Z-lg&feature=related

[billybek]

If you could remove the front panel and take a picture of your gas valve, the ignition module and also the front face of the furnace above the burner rack, it would be helpful.
Without knowing exactly what you have this is a wag.
Burner comes on, heats for a period of time, fan limit may or may not reach temp to start fan, high limit cuts out power to gas valve. Having the fan running from the t'stat switch gets you more heat time but burners still go off before heat cycle is complete.
After 20 years of service, it doesn't owe you anything, I would consider a new furnace before dropping too much money on this one. They are not cars and an old Colman was never the Porsche of the furnace world. Just a thought...

[Heel n Toe]

Matt, there is one more thing that could be occurring. If this is happening it is because of a safeguard built into the system to protect you from getting put to sleep permanently with carbon monoxide.

This happened to me and my first wife in our first house, and the blower kept shutting off. Some rust on one of the sheet metal panels (this panel kept the carbon monoxide separated so it could go up the flue) had, over time, thinned the metal just enough so that when it heated up, it would warp just enough to pull away from a joint or something, and the blower would have blown the CO through our ducts into our home.

This is probably something you want to have a competent serviceman look at... as you know, you don't screw around with CO.

[onewhippedpuppy]

Bill, I don't disagree with you that a new furnace would be a good idea, but financially this isn't the best time for us to drop $4k or so. I can easily replace a controller, limit switch, or gas valve, and can buy through from our local wholesale HVAC suppliers thanks to a family business account.

Should a limit switch be open or closed at room temp? I'm leaning towards the upper limit switch after Brian's suggested experiment. Pics are below, excuse the crappy camera phone quality I was in a hurry this morning.

John, it's always a valid concern with a furnace. However, in this case it's the burner that does not operate long enough, not the blower. Regardless, I do have a CO detector plugged in adjacent to the furnace, so we should be notified if there's a leak.

[rick-l]

Is it doing this?
http://www.youtube.com/watch?v=z28HCVRTVpA

Another video I found looking for a picture of a flame sensor.

[onewhippedpuppy]

Rick, I can't view youtube at work. Something about productivity or something........ I'll check it out tonight.

[dhoward]

I might suspect the limit switch. I think it's one of those bi-metallic jobbies...
Sounds like when the burner shuts off because it thinks it's overheated, it kicks the blower on to cool down the combustion chamber/heat exchanger?

[lm6y]

Matt, my old furnace did the same thing.

It was a combination of a bad igniton control unit, a small pinhole in the heat exchanger, and an intermittent flame sensor.

1. Gas burner ignites and heats (it's supposed to)

2. Relay clicks in ignition controller, gas fluctuates, fan DOES NOT operate (happens approximately 3 times) ( Flame sensor is not detecting a flame, shutting off the gas to prevent an explosion)

3. Gas burner shuts off ( Supposed to if no flame)

4. After a brief pause, fan activates ( To help clear the gas, and, or, cool the heat exchanger if there was a flame, and a bad flame sensor)

5. Fan runs for a short period of time, then gas burner lights ( can't explain this one)

6. Heater runs for a while, shuts off, then does not come back on ( The control unit hasn't gotten a response from the flame sensor, and it has entered a safety shutdown mode. if you kill the power it will start the cycle all over)

I found the hole in the heat exchanger AFTER I replaced the other parts. I had that same EXACT ignition unit until this spring. I tossed it in the trash. I would have gladly given it to you. They are right at $100.

Instead of waking up dead from CO poisoning, I just got a new furnace, and installed it myself. It paid for itself in energy savings alone in two years. I actually went from a 100,000 BTU unit to an 85,000 BTU unit, and it keeps a more even temp than the old furnace.

That was 10 years ago, so the savings would be even better now.

The limit switch should probably be closed until tripped, so it can shut the whole system down.

[billybek]

If you look in the box with the printed circuit diagram on it (it looks like a control box in your pictures...) the fan and high limit should reside in the box on your furnace. More than likely it is a combination limit. One side does turns the fan on and off. It is normally open at room temperature. The other side is the high limit. It provides overheat protection and cuts off power to the burner if it gets too hot. This switch is normally closed at room temperature.
So, if you have the power off to the furnace, a ohm meter reading across the fan limit will show open or infinite ohm value indicating the switch is open. The high limit will show 0 ohms indicating that the switch is closed.
Now, power down the furnace and remove the cover with the diagram on it and have a look inside... Take another picture if you can on the guts inside the box.

[rick-l]

Originally Posted by lm6y

4. After a brief pause, fan activates ( To help clear the gas, and, or, cool the heat exchanger if there was a flame, and a bad flame sensor)

In all these cases are we talking about the exhaust fan or the fan that circulates air in the house?