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Michael D 02-02-2013 12:04 AM

Troubleshooting Furnace Control Board
 
Looking for some help with an on-again, off-again issue dating back to October. Thermostat calls for heat, but inducer doesn't come on but once in every great while (24-48 hours) when I power-cycle the unit, and when it does only for one cycle (heat works fine for 15-30 minutes). From October to now, the system worked in fits -- fine for 3-4 weeks then out for 3-5 days, always going out the night of a sharp temp drop and coming back unexpectedly.

Anyway, it's been out a week solid now (save the stray cycle every 2-3 days). We've tested the roll out and limiter, changed out the pressure switch (and checked the tubing & condenser box). We've also changed out the thermostat and checked the flue. The issue is somewhere else between the thermostat and inducer. On an outside recommendation based on a similar problem and having checked everything else, we're down to the board, which is, of course, the most expensive link in the chain by far. I don't mind the cost to be rid of the problem, but considering there's no return on this (or pretty much any) part, I'd like to have some sort of indication that this is the likely culprit.

There is no burning on any connections on the back of the board, or even any "haloing" of the solder joints. When it's powered, there's a little white ceramic block at R10 that gets pretty warm and a ceramic cylinder at R20 that does too -- this one shows some very slight yellow on the board around it. The board is working well enough to call for the A/C unit and even the blower, just not the heat (inducer). It's an Aire-Flo furnace, Honeywell ST9160B1076 board. Any advice on this would be greatly appreciated...

how 02-02-2013 02:43 AM

Make & model of the furnace/ photos with doors off get more action.

I assume you don't have a furnace diagnostic led flashing a code?

If you believe it's between the thermostat and the inducer.....
Have you confirmed that the wiring between the thermostat and the furnace is OK? Turn off power/ remove the stat wires from the furnace connection R & W/ pig tail between connection R & W/ turn power back on to see if the furnace the inducer starts up.

Is the furnace a mid efficiency or high efficiency...condensate pump with float switch?

Michael D 02-02-2013 05:38 AM

It's an Aire-Flo furnace, model AF90MPE050U3 -- eight or nine years old. It's 92.0 AFUE, which is high efficiency, I believe.

LED isn't giving any codes and I have jumpered the R & W terminals on the board -- still won't start, so I'm assuming the stat wiring is not the problem.

I'm not sure about condensate pump/float switch or how to confirm that...

Marty S. 02-02-2013 05:54 AM

Your jumping R and W on the board eliminated all external components as being the problem. No inducer with no fault codes sure sounds like a board problem.

Michael D 02-02-2013 06:41 AM

Yeah, looks like that will have to be the next step. Part of the delay will be determining a replacement for the board -- a Honeywell ST9160B1076 which is probably a name-brand knock-off (Lennox?) and may have been obsoleted...

Michael D 02-07-2013 10:10 PM

OK, replaced the timer board -- not the problem. So, I've replaced thermostat, pressure switch and board, tested rollout and limiter switches, ruled out thermostat wiring. Problem must be between the board and the inducer (thermostat will kick on blower and A/C, but not start heat sequence). The only thing I can think to look at next is the gas valve (Honeywell SV9520H8042) wired between the board and inducer. Any advice on troubleshooting one of these? There are still no error codes on the valve or the new board.

I know the inducer motor is good because if I unhook a lead from the limiter, it starts right up. Not sure if this tips anyone off to anything -- last HVAC guy I spoke with used this info to determine it was probably a bad board (which, of course, it wasn't). Looks like the valve runs twice as much as the board and these trial and error replacements are becoming a real headache...

Thanks in advance,
Michael

danpik 02-08-2013 06:35 AM

Is the pressure switch closing when the inducer comes on?

old_squid 02-08-2013 11:56 AM

Is the pressure switch closed before the inducer comes on?

Sequence in most HE furnaces is:
thermostat calls for heat
inducer motor spins up
pressure switch makes
ignitor glows
gas valve opens

The thermostat does nothing really but tell the inducer to run
The pressure switch closing due to the pressure change made by the inducer is the "switch" that turns the rest of the sequence on.

In most control cases, if the pressure switch is closed on a call from the thermostat the inducer will NOT run. The pressure switch being closed without the inducer being told to run is a serious safety concern and because of this the inducer will not operate which means nothing else does.

I'd suspect either an intermittent problem with the inducer motor or the wiring to it, because of all the parts you've already changed. Next time it won't run wiggle the connections and wire from the inducer to the board. This "high tech" way of troubleshooting might just find you your gremlin.

If wiggling wires doesn't work "tap" the inducer motor itself. :whistling2: Your definition of "tap" I'm sure is different than mine..... so I leave you to decide.

When all else fails, sometimes the lowest form of technical investigation/troubleshooting still is needed as a fallback.

ben's plumbing 02-08-2013 08:00 PM

1 Attachment(s)
replace the smart valve.. this chart may help

Michael D 02-09-2013 04:41 AM

Quote:

Originally Posted by danpik (Post 1112253)
Is the pressure switch closing when the inducer comes on?

Inducer isn't even coming on...pressure switch has been replaced, should not be an issue there.

Michael D 02-09-2013 04:46 AM

Quote:

Originally Posted by old_squid (Post 1112408)
Is the pressure switch closed before the inducer comes on?

...

I'd suspect either an intermittent problem with the inducer motor or the wiring to it, because of all the parts you've already changed.

How can I tell if the pressure switch is closed? The inducer seems to run fine, which I can get it to do by disconnecting the limiter switch. This is why I suspect something in the gas valve or switches wired between the board and the inducer. Both limiter and roll out are testing fine though...

Michael D 02-09-2013 04:52 AM

Quote:

Originally Posted by ben's plumbing (Post 1112769)
replace the smart valve.. this chart may help

It may come to that, but I was told the same thing about the timer board. I'd like to troubleshoot it if possible before another $300 "trial", especially since I'm already out the cost of the board, pressure switch and thermostat to no avail. Is there anyway to test the gas valve/inducer connection by jumpering?

Marty S. 02-09-2013 05:52 AM

Sorry dude,didn't know it had a smart valve. They are well known for electrical connections inside the valve failing. Often times pushing the connections to one side or the other will get it to briefly make contact and confirm it's the gas valve giving you problems.

ben's plumbing 02-09-2013 03:06 PM

Quote:

Originally Posted by Michael D (Post 1112923)
It may come to that, but I was told the same thing about the timer board. I'd like to troubleshoot it if possible before another $300 "trial", especially since I'm already out the cost of the board, pressure switch and thermostat to no avail. Is there anyway to test the gas valve/inducer connection by jumpering?

you were told by someone who did not know that smart valves are stupid....change the valve after following chart i gave you ...ben sr that will fix furnace..

Focused2 02-09-2013 04:32 PM

Here is a link to troubleshooting chart posted but much clearer.
http://www.forwardthinking.honeywell...ll/69_2012.pdf
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Joe


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