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My mother finally told me her furnace (from 1995) has been acting up for a few weeks, and she had bought a new thermostat. I drove down to put it in for her, which was a simple job, but the furnace is still squirrelly.

It will only light if the power is shut off at the main fusebox for a few minutes. If that is done, the ignitor glows and the furnace fires normally. However, the flame has a lot of dancing orange around the perimeter, which doesn't seem right, and after a varying amount of time - anywhere from 30 seconds to 5 minutes - the flame cuts off, the blower continues to blow (even after the air is no longer warm), and the trouble code indicator flashes a code four - limit cycling. Even when the furnace is operating normally during the time before it shuts off, the heated air is not terribly hot; warmish, at best.

There is sometimes a clicking noise coming from the induced draft blower motor, but it is not consistent. (I'm not a furnace guy, but I was a mechanic; I can tell when a motor is making an unusual noise ) It is not always clicking when the burner shuts off.

The ignitor will not glow again, although the furnace will repeatedly attempt to operate at short intervals, unless the main breaker is pulled again.

From the behavior of the flame, I thought I'd better pull out the burner and check for blockage, but I'm unsure if that alone would cause the burner to shut off. Any other ideas, or symptoms I should look for?

Check burner tube for blockage, burner orifice for blockage (spider-web-bugs), flue/roof jack for separation/leakage, booster motor/wheel for damage-debris-blockage.


Also check all limits to see if any have opened.


Thanks,
Robert

After pulling the cover plate off to check voltages, I found corrosion at a couple points on the board: across resistors r28, r36, and r13, and also at the other side of the board surrounding the resistor to the left of r12.

The furnace was actually running for quite a while until I touched the corrosion over the three resistors with a plastic probe, at which point the flame instantly shut off and the trouble code started flashing four times again.

While I might be able to clean the corrosion off enough to provide functionality for awhile, I think I'll just play safe and order a new board.

I still am going to pull the burner out to check for blockage, as the flame was quite yellow/orange except at the center (where it was blue) - looks more like a fireplace than a gas burner in there.

We did manage to keep the furnace running long enough to kick the temperature up to 65 degrees, so at least it will be comfortable in here for a few hours! I also brought her another space heater, and the insulation is quite good, so she won't freeze to death before parts arrive, hopefully. The weather isn't supposed to get too extreme any time real soon, anyway.

I'll update when I get the burner out.

Resistor r36 was corroded completely through one end; just enough contact was being made through the corrosion to allow intermittent operation. After cleaning off the corrosion, of course, the furnace won't operate at all

Removed the gas valve and looked for any blockages, found nothing. Removed the orifice, but it was shiny clean, with a bright clean wire mesh down at the bottom of the tube.

We just re-roofed the trailer this summer, and saw nothing odd at any of the vents at the time. The booster motor is clean and operating smoothly.

Could inappropriate pressure be causing the flame issue? Or should I just wait till I have the new board, and see what happens then?

Go here to see and order Integrated Control Board # 7990 - 319P :

http://www.aberdeenhomerepair.com/store ... esults.php



Thanks,
Robert

In this thread https://www.mobilehomerepair.com/phpbb/v ... f=1&t=7908, some discussion was made regarding orifice size. Rather than continue to hijack that thread, I thought I'd ask this question here...

How will elevation affect the performance, relative to the orifice size? In the Gas Heater Maintenance and Troubleshooting article, there is a link to a .pdf that contains a selection chart. I've simplified it drastically, since I can't format it properly, but the .082 orifice (for propane) is listed for 8000 feet altitude, while the furnace I've been looking at is only at 2600 feet. Could that cause the heated air to be cooler than it should be? Lower altitudes call for larger orifice size; would that imply higher flame temperatures?

GAS ORIFICE SELECTION CHART
Elevation
0 .0935
2,000 .0935
3,000 .0890
4,000 .0890
5,000 .0890
6,000 .0860
7,000 .0860
8,000 .0820
9,000 .0810
10,000 .0785

That was used as an example and is not for your furnace. LP 0.082 is correct for your furnace.


Thanks,
Robert

Installed the new control board, furnace operating normally. I still don't think the air temp is terribly warm, but I think I'll wait till spring before fiddling with it any more. The trailer heats, and that's good news.

I do have a really dumb question, though.

There are two removable panels, one sits above the other. One panel has a single vent area, at the bottom of the panel, the other panel has two vent areas, one at the top and one at the bottom.

Which one is installed on top? I've got the panel with two vents up there now, as the blower had so much suction it seemed reasonable that it was supposed to pull air in through the top vent, but it would be nice to know for certain. I can't even be sure that they were installed correctly before I took them off, as my mother had taken them off at some point and put them back on, and she can't remember which way they were either...

Double louvered door goes on top in front of filter. Single louver door goes on bottom.


Thanks,
Robert