Furnace controller board repair

[dlminehart]

I have a 6 year old Trane XL80 furnace that's gone flaky on me. I found that the 24V fan relay on the circuit board works only when I tap on it, emitting a nice spark as the main fan comes on. When the heat cycles off later, the relay won't switch back on without another tap.

So I looked at having my HVAC guy come out and replace it for about $500 for parts and labor. Or at replacing the whole controller board myself, for about $175 with shipping. Or replacing the 24V relay (desoldering and removing flaky one, soldering on a good one) for about $10.

Which would you do, and why?

[linear]

Which would you do, and why?

Relay R&R without a doubt. It's the only moving parts. If you can get the replacement, try the cheap fix before you replace the board at least.

[LCHIEN]

if you won't freeze to death while waiting and you can replace the relay by unsoldering and not destroying the old board, the by all means replace the relay and if that doesn't work, go for the board.

YOu can also go deeper and before removing the old relay just hit the solder connections to the board with a soldering iron, intermittent operation may be due to poor solder connections of the relay to the board. Or you can check the relay coil contacts with a DVM and see if there is driving voltage present before you tap on it and close the relay.

Most likely the relay is weak or sticky but there could be a bad connection and tapping the relay in turn transmits the mechanical stimulus to the board causing the solder contact to conduct... you don't know for sure.

[parnelli]

You might be able to get a board cheaper than $175. See my post from last year:

http://www.bt3central.com/showthread.php?t=16184

I tried to track down a relay- couldn't- and when I found out that I could get the board for 60 when the relay would've been 20- I figured I'd just get the board.

Also, I had this bookmarked- don't know if it's a helpful site:
http://toad.net/~jsmeenen/blower.html

[cgallery]

Is there enough access to the relay that you can shoot some relay/switch contact cleaner at it? And touch-up the solder joints like Loring said.

Desolding large relays can result in the invention of entirely new four-letter words.

[dlminehart]

I thought there might be switch cleaner. I'll try that, and resoldering. I found that http://www.patriot-supply.com sells White-Rodgers OEM parts, including an updated one compatible with that on the Trane XL80, but that includes a nitride rather than carbide ignitor, less sensitive to environmental degradation (from salt, solvents, etc. in the air). Only $110 plus shipping. So, if my soldering fails, I'll order that one. Thanks for the help!

[scorrpio]

I'd definitely replace the relay. The main thing is to get down the relay specs, and order the same. That would be 24VAC coil, and at least an SPST switch rated for (my guess) at least 15A/120VAC. Could be a more complex switch configuration. My first place to go to would be Jameco - their selection is huge, though locating a proper relay in that multitude can be tricky.

P.S. I'd say it would be something like this:
http://www.jameco.com/webapp/wcs/sto...oductId=552471

[LCHIEN]

my 2 cents, I assume this is a relay in a case (as opposed to open frame).
It's not that easy for them to get dirty in the case. Whatever caused it to fail and the contacts to be sticky will still be at work, so cleaning, in my opinion will only delay for a short while anotehr failure. I'd just replace the relay and be done rather than trying to clean and or repair the relay.

You know when it fails again it will be the coldest possible day.

[dlminehart]

Well, I resoldered the existing flaky relay, and sprayed around it (it's in a black plastic housing, but maybe there are some openings under the edge of it?). For now, it's working! Has gone through a few cycles, anyway. Meanwhile, I've received the spare relays (exact same part, quite hard to find, Allied Electronics ended up the best retail source at $2.50 . . . plus $30(!) overnight shipping). If necessary, I'll swap the old one out.

[HarmsWay]

Very similar experience as mine last week except with a hot tub. The heater relay was stuck on. Did a quick search on-line and a replacement controller board was US$400. Looked up the relay in the DigiKey catalog and it was a whopping C$3.48 and would require about 5 minutes to R&R. I was going to order a few until my wife reminded me that it was still under warranty. Then of course by the time the service guy came around it started working again. I will order a few spare parts for when it's out of warranty though.

Mine was clearly a relay problem as the (normally open) contacts were closed with the power off. Yours sounds like contact problems as well but not necessarily. Without any additional troubleshooting, it's the cheapest and easiest thing to replace though.

Bob

[Kristofor]

<snip>
I'd just replace the relay and be done rather than trying to clean and or repair the relay.

You know when it fails again it will be the coldest possible day.

I agree with just fixing it and being done with it, on general principle!

But the saving grace there is that the coldest possible day (ever) in San Jose was only down to 17 degrees, and the average low in January is 42. Assuming they put at least a little insulation in those houses going an evening without heat should only make it a bit cool inside until someone went and tapped it again.

[dlminehart]

Actually, the house had only a few inches of insulation in the attic, none in the walls, and still has some single-pane windows, including two sliders downstairs. Poor sealing against air infiltration in walls and over the crawl space. Tracks outside temperature too closely for my taste, even after bringing attic insulation up to R30! Someday I hope to redo the siding and install insulation while I'm at it.

My resoldering has kept the furnace going now for 3 full days . . .