PLEASE DO NOT ATTEMPT THIS IF DO NOT KNOW WHAT YOU'RE DOING. Your furnace control system is a critical safety system. If you repair or install it incorrectly or damage the system, it can burn your home down or cause a gas leak with people inside, causing severe injury and/or death. I cannot emphasize this enough. Be careful, PLEASE! The following information is for informational purposes only.
For those people who would like to replace rather than repair their furnace board, I believe something like this should work: HK42FZ007
The non-condensing forced air gas furnace in our home had been having intermittent trouble starting up. The furnace brand seems to be "Night and Day" and the model is Plus 80. The normal startup sequence is something like this:
- Inducer fan starts
- Hot surface igniter glows
- Gas turns on
- House blower turns on
After doing some initial debugging and internet research, I learned that our furnace uses a Carrier HK42FZ007 Furnace Control Board. This board is similar to a number of control boards such as HK42FZ004, HK42FZ008, HK42FZ009, and HK42FZ011. From my internet research, it seems that all these boards except the newest HK42FZ011 have a design flaw that causes one or two of the power resistor solder points to crack, leading to an intermittent connection. The symptoms are intermitted inducer behavior much like what our furnace was doing.
I pulled the board and took a peek. Here are some photos from this project:
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To remove the board, there were two sheet metal screws holding the black case to the furnace. Make sure to record which wires go to which terminals before removing the board; I took detailed photos above.
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To remove the board from the case, gently pry the back off. There are clips all around the edge, and if you are careful, you can remove the back without damaging the case. Inside, you will find the circuit board:
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After some close inspection, I found cracks in the solder on R33 and R34, the large resistors in the board's cutout. Here's a closeup of the cracking. You can see the cracks on the two solder points on the left side of the image:
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To repair this problem, I used my soldering iron and reflowed the four solder points on those two resistors and added some more solder to the points also. Here's a photo of the repaired board. It's hard to see but the solder points are nice and solid now.
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I reinstalled the board and ran the control board's self-test. Everything seemed ok, so I fired up the furnace, and I have not heard the inducer hiccup for more than 6 hours now. I'm pretty certain the problem is fixed now.
I hope this is helpful for people out there facing a similar problem.
16 comments:
similar problem, same furnace, works sometimes, hear buzzing when it fails, I suspect bad IDR inducer relay K4.
Did your fix work?
Bob
bobbyfred@aol.com
This is very helpful, thanks! I have the same symptoms and replaced the inducer fan relay which did not improve things. Can you clarify where the cracks are? Are they where the resistors are supposed to be soldered onto the board? I couldn't see the cracks in your photos, and my board looks to be in better shape than yours. I guess I could try flowing more solder at the 4 points if that's what you mean. Thanks!
Thank you so much. I am on my second c. board and the same thing kept happening. Your fix was right on the mark. The cracks are hard to see, but this is an easy fix if your handy w/ a soldering iron.
Bob, my fix did work. Its been running great since I wrote this blog post.
Harris, Yes the cracks are where the large resistors are soldered to the board. So I reflowed the 4 spots where the large resistors connect to the board.
Michael, I'm glad this blog post helped!
I just found this -- Carrier Weathermaker 8000 with the HK42FZ011 module, same problem. The unit would cycle through the startup stages, stopping most often when the gas valve would open.
Only two heat cycles so far, but the fix seems to be working. I should note that the control board resistor solder points showed no obvious signs of cracking, but the amount of solder looked inadequate. I reflowed and added some additional solder.
Fingers crossed!
Great Scott!
I hope it works!
If I want to just replace the board myself, what tools do I need? In particular, the multimeter sensitivity needed? Do I need " low impedance"??
Hi Walter,
To replace the board, I think you need just a screwdriver. A multimeter is not necessary, so I wouldn't worry about it. Just make sure the power to the furnace is off before you begin working on it.
Wow, this fix just saved me a $99+ service call (during which they would probably try to sell me a whole new furnace) on a furnace that has worked flawlessly since I bought this 1957 house in 2002 but began to display intermittent interruption of the turn-on sequence at various stages this winter and generating various LED trouble codes. I wonder if it is original; if so, nothing you buy today is going to last 59 years. Knowing nothing, I threw a new thermostat at it - no help. I bypassed the blower compartment panel power switch - no help. Then I googled the control unit model number and bingo! I'm guessing that temperature cycling of those two large ceramic? resistors over time causes the problem. Anyway, even though no cracks were visible, I freshened up their solder joints and any others that looked cold. Everything is back to normal. Thank you SO much!
Wow, this fix just saved me a $99+ service call (during which they would probably try to sell me a whole new furnace) on a furnace that has worked flawlessly since I bought this 1957 house in 2002 but began to display intermittent interruption of the turn-on sequence at various stages this winter and generating various LED trouble codes. I wonder if it is original; if so, nothing you buy today is going to last 59 years. Knowing nothing, I threw a new thermostat at it - no help. I bypassed the blower compartment panel power switch - no help. Then I googled the control unit model number and bingo! I'm guessing that temperature cycling of those two large ceramic? resistors over time causes the problem. Anyway, even though no cracks were visible, I freshened up their solder joints and any others that looked cold. Everything is back to normal. Thank you SO much!
Such a nice article. It helps me a lot.
Heating and Air Conditioning Vaughan
Thanks for sharing these effective tips with us. It would help.
Heating and Air Conditioning Toronto
I really appreciate you for
all the valuable information that you are providing us through your blog.
Heating and Cooling Markham
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I have a different problem with the HK42FZ011 controller, specifically that the Hot Surface Ignitor (HSI) doesn't turn on. At first I thought it was the ignitor itself, since it is a common point of failure. Removing the ignitor and connecting it to 120 AC showed that it was still functional. I then concluded that it is likely the relay. All the other portions of the control board work fine - the furnace will start with no problem as long as I stand there with a barbeque starter and turn it on at the right moment in the sequence. So I figure it can't be the 24V transformer because if that failed, none of the relays would work. So I pulled the board, removed the relay - but when I tested it, it worked fine. At least when it was off the board. I'm thinking maybe it was stuck and desoldering it freed it, or maybe it has an issue under load. I bought a replacement relay, but while I'm waiting for it to arrive, I'm wondering if there are any other failure points on the board - a resistor or cap, for instance. I can't find an actual schematic for the board. Does anyone have one?
Thanks.
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