Furnace circuit board repair - How stupid should I feel? - Page 1
Forum section: electronics / repair

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 19, 2024, 04:43:50 pm
content: My A/C (aircon, air conditioner) stopped working earlier this week. The blower (air handler, really my furnace that I use as a blower in the summer) wasn't working. I found the furnace was pretty simple - one circuit board. I found the motor relay. I toyed with the idea of taking the board out and seeing if I could fix it.Instead, I decided that because I'm not an A/C technician and never have been, I'd call in a pro. The guy came out, and agreed with me that the board was bad. I had them order a new one. It was expensive, almost 600 USD. (I bought one from the furnace manufacturer, via an authorized dealer. I didn't want to chance an aftermarket, used, or "ebay new" board, although there was an e-bay seller selling one.)When he came back to install it, I asked if I could have the old board. He said the old one wasn't required, and I could keep it. It had a plastic bracket that I had to drill off to get to the trace side of the single sided board. The attached picture is what I found. It's not the best picture, but I hope you can see what looks like part of the trace that has broken off and still attached to the relay pin, and an annular air gap between that and the board. The edges are jagged and there is evidence of heat with some black residue on the board.However, when I removed the relay, I found that wasn't the case. It was just a solder ball. The board is fine. I could have fixed the thing with less than a minute of work. (desolder, clean, re-solder)I suspect that the solder joint was poor from the factory, causing a high resistance that heated the solder and caused it to drip, but I really don't know. The rest of the components in the blower motor circuit (board terminals, other component leads) are undamaged with no evidence of heating from over current. So, I'm really at a loss to explain what happened.At least I have what I believe to be a good spare. I won't know unless it fails again, as I have no intention of taking apart my working furnace. (yes, I know. I'm turning it on instead of taking it apart.)I suppose I should have been bolder.

author: IanB
author_rank: Super Contributor
timestamp: July 19, 2024, 05:00:20 pm
content: This failure mode is extremely common on all sorts of mains powered devices (dishwashers, washing machines, ovens, etc.).It is a symptom of cost cutting in design at the expense of the consumer.Robust design says that mains voltage parts of a circuit should use appropriate mechanical connections (flexible wires, crimped lugs and spade terminals, for example).What you don't do is run mains voltage on traces on a circuit board with solder connections. The reason why is illustrated in the photo. If low voltages run on a circuit board, then any weak joint causes a high resistance that reduces the current, but if the current is already small this may not escalate. If a mains voltage, high current path runs on a circuit board, then any weak joint will not reduce the current, it will simply get hot. This heat will further weaken the joint and will accelerate the problem towards total joint failure.It's always worth looking for such a failure on a control board before ponying up hundreds of dollars for a replacement.

author: Stray Electron
author_rank: Super Contributor
timestamp: July 19, 2024, 05:22:27 pm
content: Thefirstthing that I was taught about soldering is that you need to have a goodstrong(and clean) mechanical connection on any joint before you solder it. Pretty much every consumer grade gadget out there violates that rule! Just an example, when I installed CO equipment for the Tel Cos we had to make ONE wrap of the insulated wire around the terminals on the Frame Blocks and then exactly one and a half wraps of bare wire around them before soldering the joint. Joints made like that will NEVER come part on their own.

author: calzap
author_rank: Frequent Contributor
timestamp: July 19, 2024, 07:12:50 pm
content: First thing to do when an electrical device fails is visual inspection. I’ve found somewhere around half the time, something wrong can be spotted. Next thing is check and clean contacts and connections. Then bring out the multimeter, scope, etc.Mike

author: blue_lateral
author_rank: Contributor
timestamp: July 19, 2024, 08:09:25 pm
content: Often resistive contacts on a relay create the heat, which travels down the pin and unsolders it from the board. I would replace the relay, or alternatively if you can get the lid off, clean the contacts.

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2024, 08:57:58 pm
content: Sometimes it goes that way, but IIRC from years in the TV trade, we commonly saw such burnups on pins of inductors and transformers as well as relays. I think the critical factor is vibration, eventually cracking the solder joint, then due to high voltage being present at significant currents, there is enough energy released to cause localised melting, the solder retreating to the pin and pad leaving a gap, which may then arc over and char the PCB substrate resulting in a large area burnup. The fix was always remove all char by grinding back the PCB, replace the component if the insulator round the pin was charred, and make a solid copper wire link to reconstruct the track. If the component didn't have enough pins in good pads remaining for stability it might need to be glued into place. Better quality sets from before the Muntzing race to the bottom in the final years of CRT TVs typically had hollow brass rivets through the board on critical pads that the designers had determined to be susceptible to this issue, acting as a bridge between the pin and the pad forming a soldered lap joint to both.

author: wraper
author_rank: Unknown
timestamp: July 19, 2024, 09:02:46 pm
content: Circular solder cracking is very common on single layer boards and almost does not happen on boards with plated holes.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 19, 2024, 09:06:54 pm
content: Quote from: Ian.M on July 19, 2024, 08:57:58 pmIn this case, the board wasn't charred at all, and the trace copper was fine. The relay pin also is in "as new" condition.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 19, 2024, 09:08:18 pm
content: Quote from: blue_lateral on July 19, 2024, 08:09:25 pmHow effective is a DC resistance test of the relay contacts? The relay looks fine, and the contact resistance (when coil energized) is just the meter leads. (.1 or .2 using whatever Fluke meter was laying around)The relays are cheap, however. It might be best just to get another, now that I think about it.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 19, 2024, 09:12:51 pm
content: Quote from: calzap on July 19, 2024, 07:12:50 pmI was initially worried about disassembling my furnace to get to the board. I was afraid I'd cause another problem. It was impossible to see that it was a simple problem when installed. There is a plastic bracket "hot staked" to the board. The A/C technician did the swap for me, so I was handed the board. Once the new board was in, I was emboldened to drill the bracket off the old and take a look.Hindsight's 20-20, and obviously I should have done it. I'll say the heat affected my thinking - LOL. Seriously, I should have gone further.

author: floobydust
author_rank: Super Contributor
timestamp: July 19, 2024, 09:23:44 pm
content: Fractured solder joints on appliance control boards are commonplace, especially at the large through-hole power components like relays, connectors. Vibration seems to aggravate it so a dryer, furnace blower etc.In the case of Whirlpool/KitchenAid/Kenmore dishwashers, the boards caught on fire and a person's house burned down. It went to class action I think.https://www.consumerreports.org/dishwashers/whirlpool-settles-dishwasher-fire-lawsuit-with-offer-of-rebates-and-repairs/I remember seeing it as due to a bad PCB design, a high-current relay pin had a skinny neck and the connection fractured and overheated. Then the PCB burns and carbon tracks, not enough fault current to pop a fuse- but many appliances rarely use fuses anyhow because of the cost. A 15A circuit is fine with 10A of glowing smoke. Then the door catches fire etc. But a furnace is a metal enclosure so it would not spread.I would replace the relay because that pin getting hot can damage them. The furnace control boards failing during winter are a crisis situation and who can really afford that big expense. I keep spare relays for that day.

author: Paceguy
author_rank: Regular Contributor
timestamp: July 19, 2024, 10:03:48 pm
content: Same thing happened to a friend's furnace. The tech from the repair company charged him close to $600 for a new board plus another $180 in labour. The service tech left the board behind and my friend brought it to me to take a look at it. I changed the two relays and repaired the bad melted solder on the connector. Cost: $16 in parts, no labour charge because he's a good friend.Now he has a spare board for the next time his furnace craps out. Not too many service techs repair boards at the component level. It pays more to replace boards that most of the time are repairable.

author: floobydust
author_rank: Super Contributor
timestamp: July 19, 2024, 10:12:45 pm
content: Is that what they charge out at, $600 parts +$180 labour to replace a furnace control board? Engineering is cheap in comparison, should been a tradie.I know during winter they have you by the balls (house freezing cold) so $780 is no problem to charge.I've fixed so many furnaces for my friends for free, it's usually a dirty flame sensor rod or busted hot-wire igniter. Draft pressure switch is a bit more hassle, draft induction fan motors are easy to change and massive markup on the cheap chinese motor makes it crazy expensive (thanks Lennox!).

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 19, 2024, 10:18:06 pm
content: In my experience if you clean it up good, wick off all the mystery solder and flow in a good amount of quality 60/40 (or better, e.g. the silver bearing stuff or 63/37) those sort of joints tend to hold up a lot better.

author: Runco990
author_rank: Frequent Contributor
timestamp: July 19, 2024, 10:51:48 pm
content: That's a common failure. I add a piece of solder wick as a beefed up trace. My dishwasher was the latest casualty.... fixed and fine for years now.It's all made to fail.When I moved into my new house 2 years ago, I made sure to service the HVAC system before winter hit. A good cleaning, re-cap and check of the furnace board showed 1 bad cap, which caused the fan motor shut down delay to NOT work. Relays were re-soldered, etc. Runs like new, for a 35 year old furnace.I service EVERYTHING in my house. 2 weeks ago my garbage disposal froze up. Having nothing to loose.... I took it apart, DE-rusted the metal parts, oiled the motor bearings, cleaned EVERYTHING, put back together. Runs fine, NO leaks. My wallet stays closed at least a few more years.My take is, it's broken anyway. Why NOT try to fix it before you order or call for a repair? Can always do that if repair attempt fails.

author: rich t
author_rank: Contributor
timestamp: July 19, 2024, 11:33:12 pm
content: I think the lesson here is that if you disassemble appliances to the point of inspection, maybe half the time you find obvious discoloration near the failure, and the fix can be as simple as re-soldering, or often replacing a single component (connectors are notorious to fail as well, and often a clean and reseat and some dielectric grease to hold off further corrosion is all that is needed). Our furnace motor failed a few years ago, and it was a dollar ntc thermistor that was supposed to limit inrush current that had blackened the pcb... The dollar fix was much easier (and better for the environment) than replacing a $400 blower motor (and taking the old one to the dump)!

author: Excavatoree
author_rank: Frequent Contributor
timestamp: July 20, 2024, 01:23:05 am
content: Quote from: rich t on July 19, 2024, 11:33:12 pmI know what I've done in this case doesn't show it, but that's my usual attitude. Dishwasher, TV/computer monitor, the car, lawnmower, whatever, I usually fix it myself. I don't know why I was so afraid I'd really screw the furnace up. I honestly thought I might screw it up so bad it'd cost more to fix than if I didn't make an attempt. (how's that "rate chart" go? Double the price "if you tried fixing it yourself..." ) As I said in another post, I think it was the heat frying my brain. I honestly thought it might be some problem with the sealed system causing the blower not to come on or something. I definitely spooked myself.

author: Runco990
author_rank: Frequent Contributor
timestamp: July 21, 2024, 03:04:54 pm
content: Quote from: Excavatoree on July 20, 2024, 01:23:05 amActually, do some reading! HVAC is less complicated than you might think. I know manufacturers are falling all over themselves to insert "complexity" into them to make them un-repairable, but the basic systems are very simple. If nothing else, it's good to understand how these things actually work.

author: rsjsouza
author_rank: Super Contributor
timestamp: July 21, 2024, 05:38:36 pm
content: Excavatoree, I can relate. I fix everything at my house, including the furnace/HVAC system. Once I had a fan capacitor issue that prevented the external heat exchange unit from running. I replaced the caps (it was a dual capacitor), improved and cleaned the connections but the compressor wouldn't start on its own, unless I bypassed some protection switches on the heat exchanger using an alligator clip. I had a brainfart/paralysis moment and ended up calling a technician (on a Sunday no less!) as I was afraid I would somehow seize the compressor. He praised the work I had done on the capacitors but was in disbelief: his visit consisted in pressing a button of a gas trip breaker near the input of the compressor. I thanked him immensely for the information he taught me and chalked the US$179.00 emergency call fare as the cost of a training session. (He felt pity for me and ended up doing a major revision on the unit).

author: madires
author_rank: Super Contributor
timestamp: July 21, 2024, 06:40:05 pm
content: Yep, cracked solder joints are a common fault. And it's not related to mains voltage only. For example, failed 12V halogen lamp SMSPUs often have cracked solder joints at the secondary of the transformer.

author: MathWizard
author_rank: Super Contributor
timestamp: July 23, 2024, 01:27:18 am
content: Quote from: IanB on July 19, 2024, 05:00:20 pmYeah I fixed a stove PCB with a bad relay once, and I was surprised how small diameter, the pins of the relay's were. I bet most most stereo's of the 80's/90's would have more metal in their audio relay's.

author: floobydust
author_rank: Super Contributor
timestamp: July 23, 2024, 02:35:24 am
content: How is it due to cost cutting? A tiny annular ring and low grade wave soldering I see.It's worse because appliance mega-conglomerates have one division or contractor doing control board design.Bad PCB layout, DFM or even firmware errors gets copied into many different "brands" and models.So much e-waste from people giving up and buying a new appliance because the repair cost is outrageously high. Over a bad solder joint or relay etc.It's a big expense for a household and not everyone has a few grand lying around.When I've fixed furnaces, I'm shocked at the ripoff parts pricing. And it's needed in winter when it's -30C and a house quickly cooling off...Last one was on a oddball model and the generic part I realized would work fine for 1/10 the cost.Now chinese-built furnaces, appliances are coming in and the quality has dropped again.Furnace controls have endurance tests like 100,000 cycles in the safety standards.Governments should mandate a minimum product life on white goods or give us the right to repair.

author: IanB
author_rank: Super Contributor
timestamp: July 23, 2024, 03:27:23 am
content: Quote from: floobydust on July 23, 2024, 02:35:24 amI think it's cost cutting because they want to put everything onto one single board. Including mains power transfer and relays. But it would be more robust to keep mains power off the board and carry it through wires and spade terminals. The downside of this is that it would be more expensive to manufacture.