Help with Burnt Resistor and Diode Identification on Furnace ECU Motor

[Generalquark]

Hi,
 I had my board smoke on the furnace fan motor. Took it all apart and found a diode and resistor pretty burnt. There is also a component H1 (Maybe a Hall Effect IC?) has some burn plastic, but I am hoping that was just burnt from the diode and will still function. I was looking to get help on the identification of the diode and if someone is really good, the resistor. I believe the Diode is Taiwan Semiconductor. I think I have run across a part marked 8D in the past and maybe it was a TVS? The resistor was burnt, but not completely open. It measured around 2.6 Meg Ohms. It was not solid starting at around 2.4 Meg and slowly walking up to 2.6 Meg. That does not match any color code I can decipher. Any help is always appreciated. I'm quite thankful for everyone out there who lends their expertise to help others out.

Thanks

[fzabkar]

Here's one possibility:

http://pdf.datasheet.live/datasheets-1/taiwan_semiconductor/SMAJ58CA.pdf

BK -> SMAJ14A, 400W, 14V, Surface Mount Transient Voltage Suppressor, DO-214AC

A TVS diode is meant for protection. It is never meant to draw any current at its rated voltage, so any sustained voltage over 15V or 16V (breakdown voltage) will burn it up. I would examine other components in the circuit, including the motor itself.

The manufacture date on the motor's label appears to be 09/02/2009. The YW (Year/Week) date code on the diode is "8D", so I'm guessing 2008.

This is a good resource for smd code searches:
http://www.smdmark.com/en-US/search/code?id=bk

I also use Google's advanced search:
https://www.google.com/search?q=filetype:pdf+diode+"bk"+"absolute+maximum"+"taiwan+semiconductor"

[WattsThat]

I wouldn’t be optimistic for a simple fix. Dead EC motors are usually failed motor driver ic’s with mfg part numbers that are almost impossible cross. Yes, there is obvious damage but it is usually damage you cannot see that caused the visible failures.

That motor is available on eBay used for about $200. A entire new draft inducer assembly from the online hvac houses is about $650-$700. I’ve had very good experiences with supplyhouse dot com for boiler and furnace parts. A local distributor could supply perhaps source a new old stock motor as the original part appears to be obsolete.

[Generalquark]

I am not sure if it was caused by a noisy ultrasonic cleaner I had running near it. I have had 3 problems with the cleaner and will no longer use it. It takes out my oven display while running when running from another level and across the house. This board started smoking while it was running. It has also tripped my GFIC breaker when it was not even on the same circuit. Im hoping that is what caused this tvs to go rather than the motor.

[perieanuo]

trace the schematic, it may be the diode is just half wave rectifier in series with the burnt resistor and the cap afterwards, that one may be Kohms (i had some industrial temperature regulator, the resistor was 18kohm for example)
see https://www.deeptronic.com/electronic-circuit-design/types-of-power-supply-circuits-how-to-design-the-right-ac-dc-adapter/

[WattsThat]

I cannot understand how an ultrasonic cleaner could cause problems but obviously you’ve seen the effects. Perhaps you’re in a much older house without modern grounding?

[Generalquark]

Oddly enough it is a 80's home with good grounding.

I believe the ‎SMAJ14A is the correct part.

I am looking at the resistor under the microscope and it looks like it might be a brown, grey, black, gold part which would be 18 ohms 5%. I am hoping someone can tell me if these seems logical. I did find a post here

https://www.eevblog.com/forum/projects/help-understanding-and-using-tvs-diodes/

If I am reading this right "Any resistance is better than none, but 10-100 ohms for digital or non-precision analog outputs, or 100-10k for inputs, is usually good." I suppose 18 ohms could be a possibility?

[james_s]

I think I would buy a new motor if it's available somewhere, maybe keep an eye on ebay and then take a picture of the board in that and take some notes so you have a reference in case that one ever blows up. If you then manage to fix this one you'll have a spare, which could come in handy if it fails in the winter.

[perieanuo]

hi, do you read what i pointed?
transient voltage suppresors are mounted parallel with protected voltage, no resistor in series. you got also a burnt resistor, doesn't this point to what i said, it can be rectifier?
have you traced the schematics or we discuss theory of semiconductors?

[Generalquark]

I don't have schematics. I ohmed out almost everything on the board and do not see any shorts. It could be part of a rectifier circuit, but I am not sure. If I had to guess I would say it is the power line to that hall effect IC (if that is what the part is)

[Smoky]

I've installed a few furnace and air conditioning systems in my life and here's some advice to those who are looking to avoid some of these ECM motor issues in these newer "Energy Star" furnaces and air handlers.

#1, measure your external static pressure and stay below what's rated on the furnace's or air handler's data plate.

#2, avoid pairing a high efficiency furnace/air handler with a high efficiency air conditioning coil. The combination of these "dense" coils are too restrictive to allow good/clear airflow.

Also, these ECM motors cannot tolerate being operated in a HVAC system where the ductwork is too restrictive either on the return or the supply side causing the ESP to rise beyond the manufacturers limit. Manufacturers are now denying warranty repair work if the static pressure is too high.

The motor is going to die early because it's trying/struggling to maintain its programmed cfm's in a HVAC system that has too much static pressure.

[james_s]

Interesting, I could swear I read somewhere that ECM motors could typically tolerate a higher TESP than the standard PSC motors. Also when I last did some HVAC stuff "high efficiency" coils were just physically larger, for example I have a 3.5 ton coil paired with a 3 ton condenser, are the new coils more dense? I'm a little out of the loop with this stuff, everything was R-22 back then.

It's disappointing that these motors are not more robust. There's no fundamental reason that a ECM motor needs to be fragile, it would be technically feasible to make it very well protected against mechanical overload, insufficient airflow, supply voltage, etc. I suppose like any consumer stuff they probably cut every corner they can and hope it lasts through the warranty. I've been contemplating replacing my furnace with a higher efficiency unit and I like the idea of having a variable speed blower but I'm hoping to avoid having something that breaks down regularly. I installed the one I have 16 years ago, it had cosmetic damage from shipping so I got it for a song, and in that time it has never given me any trouble at all.

[Smoky]

I keep an eye on a few elderly folks in my neighborhood making sure their home's are in tip-top shape. I found one where their nephew came over and swappped-out the MERV 6 filters to extremely high MERV (13+) 3M sponges

But the design of some of the units are just wacky. I inspected a horizontal furnace/horizontal "A" coil install where there wasn't very good air flow out of the vents. It turned out that when I separated the furnace from the coil, the flow of air through the center gap between the tubes of the heat exchanger was being obstructed by the top of the "A" of the attached horizontal evaporator coil. The blockage was enough to overheat the heater box and occasionally trip a sensor.

I personally don't like any of this "efficiency" crap especially when most of the older homes I work in lose 50% of the energy through leaks in their ductwork buried behind plaster walls that have no insulation

[james_s]

Do you mean the A-coil was installed with the top of the A facing the business end of the furnace? I thought they were always supposed to have air blowing in from the wide end of the A? I've never seen a horizontal installation around here, every house I've peeked at the HVAC had a vertical furnace, probably 60% upflow 40% downflow. At some point I came across some pictures of horizontal furnaces and it looked a bit strange to me.

It's been too long since I took measurements off of my system but I do remember determining that the ducting in my house leaves much to be desired, and that's before you factor in a bit of monkey work done by the previous owner. It's the ubiquitous split level house that they built zillions of in the 70s as cheaply as possible. There's an upflow furnace in the garage and a single return air vent in the wall of the stairwell on the other side of the wall where the furnace is. There's a goofy setup with a supply duct that runs parallel to the central beam under the house with a pair of round jumpers to another supply duct parallel to the other side of the beam. It actually works pretty well with the 3 ton AC, probably a bit oversized but I was poor at the time and it was another damaged unit I got cheap. I'm financially in a much better situation now so next time around I'll go with new equipment and I can choose exactly what I want. I need to go through and measure everything again at some point but I suspect without some major surgery to the house I'm going to be stuck with higher static pressure than would be ideal. Since it's a split level on a slab all of the ducting is in the ceiling of the lower floor and there's a huge beam and firestops between the two supply ducts.

[Smoky]

Do you mean the A-coil was installed with the top of the A facing the business end of the furnace?

Yes. Some horizontal coils can be installed in either direction depending upon which side the lineset comes in.

I'm a big fan of adding a mini-split to the second floor or to a distant area of a house where the main HVAC system falls short. The mini-split just picks up where the main system leaves-off.

Not many basements in North Carolina, mostly crawlspaces. Horizontal systems get stuffed into wet, buggy crawspaces or in attics (which are ovens in the Summer) around here

California is now requiring static pressure tests for new installations.

[james_s]

Yes. Some horizontal coils can be installed in either direction depending upon which side the lineset comes in.

I'm a big fan of adding a mini-split to the second floor or to a distant area of a house where the main HVAC system falls short. The mini-split just picks up where the main system leaves-off.

Not many basements in North Carolina, mostly crawlspaces. Horizontal systems get stuffed into wet, buggy crawspaces or in attics (which are ovens in the Summer) around here

California is now requiring static pressure tests for new installations.

I'm not a fan of that, it just seems wrong to run the coil "backwards" even if it's designed to work that way, seems like it's asking for exactly the sort of problem you encountered.

I've thought about putting a mini split in my rec room downstairs, possibly a second one in the guest room to replace the baseboard heater in there but I don't want the outside to get crowded with outdoor units. Cooling is not a problem year round but the downstairs is very difficult to heat in the winter, the heat all just flows upstairs right away. Years ago I put an insert in the fireplace in the rec room and that works great but requires some effort to get it going.

Not many real basements here either, but the classic 70s split level house is the exception. These typically are built in hilly neighborhoods and have a daylight basement where one side of the house is built into the hill. They were commonly sold with the basement unfinished or partially finished as starter homes, the idea being you'd live on the upper floor and then finish the basement to your liking later on once you can afford to. I've never seen a furnace in a crawlspace or attic out here and I hope I never have to deal with one, sounds like a real pain. My mom's place has a crawlspace and I don't enjoy being down there, her furnace is in a closet, the supply plenum is in the crawlspace and the return is in the attic, probably 90% of houses here have it in the garage and the rest are like hers. Regional differences are interesting.

Static pressure test seems prudent anywhere whether it's required or not, it's not like the tools required are particularly exotic or expensive. I only paid around 40 bucks used for my manometer, it was worth it even just for one job and I've found other uses for it. I like having data and I don't mind an excuse to buy tools.

[Smoky]

The newer furnaces are really incredible when it comes to how you can custom-tailor the system using the onboard menu.

I'm a fan of Goodman. Their cabinets may need to be tweaked due to hasty assembly but all in all they work well and I stick to what works.

For instance, I like the "circ" setting found on new thermostats. It's like the "auto" setting where it turns the fan on, but when the "circ" mode is selected, it turns on the fan to run ~30% of the time when your HVAC system is not in the heating or cooling mode. What I do next is I go to the furnace's control board and set the fan to run at ~40% of its max "cfm" setting. So when your furnace isn't heating or cooling, the fan is lightly blending the air throughout your house to even the temperature.

So many options are available like trimming, ramp profiles, and such. Too many to discuss but all geared to make your home more comfortable and to lower the stress on the system

[james_s]

See that's one of the things that has me wanting to upgrade to something newer, the ability to tweak and tune appeals to the engineer in me. My current furnace is a Goodman and it has been fine. I know they have a less than stellar reputation but I haven't really seen any serious faults in the design, they're made in the USA and generally use reasonable quality components. I suspect it largely comes down to the fact that they'll sell the units to any random goober that comes along and not everyone has the skills or attention to detail that I do. I'm still glad they're available though, I'm fascinated by HVAC and refrigeration in particular, I took chemistry and physics in college, my dad was a mechanical engineer and taught me carpentry and plumbing, my uncle was a master electrician and taught me all the tricks of that trade when I was a kid so most of the individual aspects of HVAC were familiar right from the start. I get a kick out of tinkering with that stuff but I wouldn't want to do it for a living. I have no desire to spend my days sweating or freezing my butt off trying to repair or replace systems in the peak of summer or the dead of winter when most people are likely to call for help. Especially not when I can make twice the money sitting at a desk in a climate controlled room.